HAM Magazine 2006

7/28/2019 HAM Magazine 2006

1/15

For internal circulation amongst members only.

2006, Vol . 1

Inside this Issue

Fox Hunt SpecialButterfly Handi-finder

3 element 2M yagi

BITX20 SSB XVR

What is Sporadic E ?

Published by :

Calcutta VHF AmateurRadio Society

PresidentDeepak Mitra VU2DPM

Vice PresidentJ ayanta R. Mukherjee VU2J M

SecretaryNikhilesh CH. Sinha, VU2NKI

TreasurerSurojit Kr. Dey, VU2SKD

EditorNilanjan Majumdar VU2HFR

website :www.calcuttahams.com

Locating Hidden Transmitters - Radio Direction FindingA "fox", which is a low power transmitter set to transmit a tone or ID for a set

length of time, and then goes silent until its time to transmit again. By using

various radios and antennae and attenuating systems, the teams start out from a set

start line to locate the "fox". The first team to locate the "fox" is the winner. (The

"fox hole", which is the location of the transmitter and the "fox howl", which isthe tone or id transmitted from the transmitter).

Objective: To develop radio direction finding skills, foster the construction ofradio direction finding tools, and have a good time through competition and social

interaction. Designed to be a fun event for all.

General rules: The location, the game frequency and the call frequency should bedetermined before the start of the fox hunt. The fox howl should be copied at the

starting point by the majority of the hunters present. The fox howl will be at a

regular interval of every 5 minutes duration, in omni direction for one minute. The

fox transmitter should maintain constant power throughout the hunt. The antenna

may not be changed during the course of the hunt. The fox antenna and

transmitter shall be within 500 feet of access by standard passenger cars in a

publicly accessible area with no charge for admission. Reasonable care must be

taken to ensure that hunters can safely get to the hidden fox. The fox may not

simultaneously transmit another signal to interfere with the fox. The fox must alsomonitor the talk-in (Call) frequency to collect drop outs and announce if he is

having technical difficulties. A good map of this area should be available among

the hunters. The hunters can ask the first clue one hour before the end of the

game, from the game controller who will be available in the call frequency and

also the second clue 30 minutes before the end of the game. At the end of the fox

hunt, the game controller will announce the fox hole and the winners. There

should not be any QSO at the game frequency. The fox hunt may be on vehicle or

on foot.

ON THE AIRJournal of the Calcutta VHF Amateur RadioSociety


2/15

2006, Vol1 ON THE AIR


The first and only fox hunt in Calcutta

was held in 1995 and was organised by

CVARS. VU2EM won the event using

this butterfly handi-finder that was built

by VU2KFR and VU2HFR the Fox

Brothers who had been the fox during

the hunt. An article on this handi-finder

based on the design by Bob Leskovec,

K8DTS and the article published in the

May 1993 issue of QST was published

in the society's journal "On The Air". It is

high time we had another fox hunt in the

city to home in our Dfing skills and have

a field day. In our fox hunt special are

two articles on the handi-finder and a

portable plumber's delight 3 element

VHF yagi which are quite easy to

construct and give excellentperformance.

The HANDI-Finder is a hand heldDirection Finder which can be used to

localize both AM and FM sources using

only a single connection to the antenna

input of a VHF-FM receiver tuned to

the frequency of interest.

The HANDI-Finder is a good example

of integrated simplicity, wherein one

simplification contributes to another.

First, it uses a single CD4047B CMOS

IC, which contains both an oscillator

and a divider flip-flop to automatically

provide complementary symmetrical

square wave outputs without special

adjustments. Only a single resistor and

capacitor are needed to set the

frequency. Second, very little current is

used to bias the switching diodes so the

total current draw is only 1.7mA at 9V.

Good service can therefore be provided

by a common alkaline transistor radio

battery. Supply voltage for the

CD4047 can be anywhere from 3-18

volts. Finally, since all the parts,including the battery are mounted on a

single circuit board, the board is

designed to also serve as the mounting

base for the two open-loop antenna

elements easily made out of bent wire.

until later if you really want to!

HOW IT WORKS: An electronicswitch alternately connects two

antennas to the coax cable downlead

going to the antenna input of an FM

radio receiver tuned to the frequency of

interest. First one antenna is connected,

then the other, etc., back and forth with

equal intervals. This is done at an audio

rate, well within the audio bandpass of

the receiver, and usually in the range of

400 to 1500 Hz. A good frequency is

1000Hz. Of the two antennas, if one is

slightly closer to the source, it receives

the wave front slightly earlier in time

(phase) than the other. There is a phase

difference in the signal received by one

antenna compared to the other. Since

the receiver is being switched between

the two antennas, the switching action

imposes phase modulation on the

incoming signal. This is detected in the

FM receiver and is heard at the audio

output as a tone equal to the switching

frequency. The amplitude of the audio

The Butterfly Handi-finder


3/15



signal corresponds to the deviation,

which depends on the physical

separation of the two antenna elements,

up to wavelength. In other words, if

the antennas are farther apart the circuit

will impose a higher percentage of

modulation or a larger deviation,

producing a louder tone, but the

modulating frequency will stay the

same. If the antenna is rotated so that

the plane of the two elements isperpendicular or broadside to the

direction of the signal, both elements

receive the signal at the same time

(phase) and there is no longer a

difference in phase. Hence, the audio

tone disappears. This is perceived as a

rather sharp null in the audio as the

antenna array is rotated into position

perpendicular to the direction of the

signal. This type of direction finder has

the disadvantage that it exhibits 180-

degree ambiguity. However, it has

several advantages:

1) It works on a nulling principle rather

than a peaking principle. The null is

sharp and much easier to detect than the

peak from a directional or beam

antenna.

2) When you null the superimposedaudio, you are not nulling the carrier.

This is unlike a conventional loop

antenna or cardioid array, which nulls

out the carrier. The problem with

carrier-null, is that as you get closer to

the null, the signal you are trying to hear

in order to null out, is getting harder to

hear! Also, when you null the

superimposed audio, you can still hear

the audio coming from the source.

3) Since audio is being nulled, the

operator does not have to watch a field-

strength meter. He only needs to listen,

which is something he can do while

driving, riding a bike or walking.

4) Since this method uses phase

information, it works well with strong

signals, so no attenuator is required. (By

comparison, the signal from directionalgain antennas must be progressively

attenuated to keep the receiver RF

within the range of the S-meter.)

The values of C1 and R1 determine the

switching frequency and hence the

frequency of the audio tone. The inset

table shows the values of C1 and R1 to


4/15



be used for a particular frequency and

corresponding tone. Very little current

to bias the switching diodes D1 and D2,

so the total circuit drain is 1.7 mA at

9V. No power on LED is used as even

an LED uses 10 mA or more. The 9-volt

battery holder is fastened at one end by

soldering the two brass terminal tabs

that pass through the board. The switchS1 is a DPDT slide switch (on-off-on).

Centre is off, up for DFing and down is

standby mode for straight receiving.

DONOT TRANSMIT THROUGH THE

UNIT EVEN ON STANDBY MODE

AS THIS COULD BLOW THE

SWITCHING DIODES AND THE

SWITCHING SYSTEM DOES NOT

MAINTAIN A 50 OHM IMPEDANCE.

As the antennas have no path to circuit

there is no need for using DC blocking

capacitors. I used two simple copperwire antennas having the appearance of

butterfly wings soldered directly to the

board. Take equal lengths of stiff wire

about 19 inches long and bend each of

them into a neat U shape. The bottom of

the U should be 6 inches across. One

end of the U is soldered to the junction

of D1 and R4 at one end of the board

and the other end soldered to the other

extremity of the board 6 inches from the

previously soldered end of the U. This

end of the U is electrically isolated and

floating and should not be grounded.

Grounding the other end creates a

closed loop that results in a carrier null

in the direction of the signal. Brass or

brazing rod may be used. It is very

important that both the loops and the

associated circuitry should be identical

and symmetrical and the values of all

resistances and capacitances on both

sides should be the same. The active

element is the vertical portion of the

loop. The longer the vertical section, the

more signals it receives and hence better

carrier strength and better quieting. You

can experiment with better and highergain antennas such as two identical

dipoles or yagis connected to both sides

with equal lengths of coaxial cable.

The handle should be attached to the

bottom area of the circuit board ( I had

used a plastic scale). You can also fix a

compass on the handle. The unit works

best when 1/4 wavelength of the

feedline is kept vertical and parallel to

the centre of the board. Measure the 1/4

wavelength feedline from the floating

antenna terminal. The feedline should

be taped securely to the handle.

TRYING IT OUT: It is best to start off

with a test situation where you knowthe location of the source, and

experiment with getting a feel for the

null. The null itself is fairly sharp, but it

does not always manifest itself as a total

null in the audio tone. Sometimes, you

will observe instead, a jump in tone one

octave up or down. At other times, you

may hear a buzz or a fast twiddle.

This is usually due to multipath, so

moving just a few feet may help clear

up the null. Calibrate the unit in an open

field by walking in a circle around a

central signal source. Never test orcalibrate the unit indoors. Happy DFing

and Fox Hunting. One last tip:

DURING A HUNT NEVER TRUST

WHAT THE OTHER HUNTER SAYS.

Sense The Right Way To GoWith The HANDI-Finder

by Joe Moell KOVARRL Technical AdvisorThe following was submitted to theTechnical Correspondence column in

QST Magazine for October 1993 inresponse to the HANDI-Finderconstruction project in a previousQST issue. The original HANDI-Finder was created by Bob LeskovecK8DTS. The KOV unidirectional

pattern modification and antennaimprovements described below areapplicable to all of these bow-tie sets.

Add a Sense Mode

By adding time delay to the signal from

one of the two HANDI-Finder antennas,

the peak/null tone pattern of the unit is

modified. If you add just the rightamount of delay, the pattern changes

from bidirectional with nulls

perpendicular to the antenna plane to

cardioid (heart-shaped) with a single

null off one end. This two-pattern

technique forms the basis of the Handy

Tracker, a RDF unit that I designed in

1989. The cardioid pattern concept

came from an unpublished 1980 project

by Russ Andrews K6BMG. The Handy

Tracker sense mode scheme can be

applied to the K8DTS HANDI-Finder

with the simple addition of a switch anda length of coaxial cable to provide

delay. As K8DTS points out, the

bidirectional HANDI-Finder indication

is independent of frequency. This is not

true of the added sense mode. A delay

line of a given length produces a precise

cardioid pattern at only one frequency.

A useful pattern is obtained over a

frequency range of several per cent,

however, so one delay line will provide

ambiguity resolution over one full VHF

amateur band.

Modification Details

The modification adds a DPDT micro-

mini toggle switch without center-off


5/15



and a precisely trimmed coax delay line.

Mount the switch in the etch-free area

of the circuit board between the two

antennas.

On the rear side, cut the traces where C6

and C7 connect to the antenna terminalsper the drawing. On the Antenna #2

side, wire the antenna connection

through the switch per the schematic.

On the Antenna #1 side, install an

unshielded wire jumper 1-3/4 inches

long, to compensate for the added

switch wiring on the Antenna #2 side.

This jumper is important, since one inch

of difference in feed length to one

antenna will cause 6.5 degrees bearing

error in the figure-8 bidirectional mode.Cut the delay line according to the

following formula:l = (11808-df)v/f

where l = length of coax in inches, v =

velocity factor of coax as a fraction, f =

frequency in MHz, and d = spacing

between the vertical antennas in inches.

For example, assume we are adding the

sense mode to a HANDI-Finder with

"bow tie" open loop antennas for two

meters. The vertical sections of the two

loops are 15 inches apart. We are using

RG-174 coax, which has velocity factor

of 65.9 per cent. Computed for the

center of the band (146 MHz), delay

line length is(11808-(15)(146))(0.659)/146

= 43.4 inches.

Coil up the coax and lace it to the board.

Miniature coax such as RG-174 is best

for the delay line because it makes a

compact coil. Whatever coax you use,

make sure to determine its characteristic

velocity factor. It is different for foam

and non-foam type cables.

Because of environmental multipath and

stray coupling, a perfect cardioid tone

pattern may not be achieved. The end

null may not be complete or there maybe a double null on one end. This

doesn't matter, because the purpose of

the sense mode is merely to determine

which of the two figure-8 nulls to

follow. There is plenty of peak/null

amplitude difference for that.

Getting RDF bearings with the modified

HANDI-Finder is a simple two-step

process. In a clear location, with S2 set

for normal (bidirectional) mode, listen

for the tone nulls in the receiver to

obtain a line of bearing as described by

K8DTS. Next, switch S2 to the cardioidpattern mode and turn the unit 90

degrees so that one end points along

your line of bearing. Note the tone level.

Rotate the unit 180 degrees and note the

tone level again. It should be distinctly

different. The lower amplitude tone will

occur when Antenna #1 is closest to the

signal source. To help you remember,

mark the peak/null directions on the

board.

Other Observations

In the K8DTS QST article photo, the

receiver feedline passes over the open

end of one antenna loop. This

unbalances the antenna pair and can

cause major bearing errors on two

meters and above. The coax should be

routed away from the loop.

The horizontal sections of the bow tie

antennas severely degrade RDF

performance. Properly spaced coax-fed

vertical dipoles are far better for serious

use with this type of RDF set. Such an

antenna set is easy to make with

inexpensive whip antennas and PVC

plumbing parts.

The two antennas should be spaced as

widely as possible for maximum audible

df tone, but spacing must not exceedone half free-space wavelength at the

receiving frequency. Approximately 24

inch spacing is practical for both two

meters and 125 cm. Coax cables from

the HANDI-Finder board to each of the

two vertical dipoles must be exactly

equal in length.

Wide antenna spacing gives better

performance, but it may increase the

received tone deviation to the point that

the receiver "squelches out." If this

occurs, reduce the tone oscillator

frequency to 400 Hz or lower byincreasing the value of R1 and/or C1.


6/15



The following design was published in

the April, 1993 issue of QST. It can also

be found in the 1996 issue of the ARRL

Handbook.

This 3 element yagi is nothing more

than a half-wave dipole antenna (the

driven element) mounted between the

other two elements which are the

reflector and the director. Typically the

reflector is 5% longer and the director

5% shorter than the driven element. By

spacing elements about 0.15 to 0.25

wavelengths from each other you end

up with a beam antenna with about 7dB

of gain. To figure out the correct length

of the driven element the following

simple formula for a half-wave antennaabove 30 MHz is used :

Length in feet = 475 / Frequency (MHz)

Construction : 3/4 inch PVCplumber's pipe is recommended for the

supports. Two 18 inch pieces become

the boom where the elements are

installed. A 36 inch piece is used for the

mast. The boom and mast pieces are

held together with a PVC T joint. Three

small holes are drilled in the T as well

as the boom and mast pieces. The boom

and mast are inserted into the T androtated until the holes are aligned.

Screws are used to secure everything in

place. The antenna is vertically

polarized which is best for FM work.

Inexpensive elements can be made from

steel oxyacetylene welding rods or brass

rods. The director and reflector

elements are cut as according to the

lengths in the diagram. Director and

reflector holes are drilled in the boom

around 1/4 or 1/2 inch from each end.

A straight pencil line should be drawn

along the boom with an X mark for eachhole to keep the holes aligned at the

same plane. The hole size chosen should

provide a snug fit for the rods when

pushed through. After drilling the

reflector and director elements are slid

through the holes and adjusted for equal

length on each side of the boom. Solder

or epoxy cement is used to hold them in

place. To construct the driven element

the boom is measured and marked at adistance of about 16 inches from the

reflector. Two holes are drilled 1/8 to

1/4 inches apart. Two rods each around

20 inches in length are pushed through

the holes until 1/4 inch protrudes from

each side of the boom. THE DRIVEN

ELEMENT AND MATCHING

DEVICE HAS JUST BEEN MADE.

The coaxial cable is soldered directly to

the protruding stubs. Solder or epoxy is

used to secure the rods to the boom.

Electrical tape or a silicon compound is

used to weather proof the solder joints.It is better to keep a little excess lengths

for the driven elements. The antenna

should now be hoisted to an elevation of

at least 5 feet from the ground and kept

away from nearby metal objects. With

the aid of an accurate SWR meter

placed in the line between the

transceiver and the antenna the driven

element lengths should now be trimmed

equally for the lowest SWR at the

desired frequency.

2M 3element yagi designed by Nathan Loucks, WB0CMT


7/15



BITX20 is a bidirectional SSBtransceiver for the 20m amateur radioband, designed and built by AshharFarhan, 2004. The project details are onhttp://www.phonestack.com/farhan/bitx.html

You may also join the BITX Yahoogroup at :http://groups.yahoo.com/group/BITX20This group exists for constructors of theBITX project, to discuss all aspects ofthe design and construction includingproblems, components, modificationsand experiences using the rig. TheBITX20 transceiver design has gainedtremendous popularity amongst Indianas well as international Amateur Radiooperators. The circuit boards for thesame are even available with FarCircuits, USA. Joining the yahoo groupgives access to a wealth of informationon the BITX20 including all themodifications made by VU and DX hamsincluding artwork for double sided PCB,overlay and solder mask. This is anexcellent project for both the novice aswell as the experienced avidhomebrewer.

BITX is an easily assembled transceiver

for the beginner with very cleanperformance. Using ordinary electronic

components and improvising where

specific components like toroids are not

available, It has a minimum number of

coils to be wound. All alignment is non-

critical and easily achieved even

without sophisticated equipment. The

entire instructions to assemble the rig

are given here along with relevant

theory. The Indian hams have often

been handicapped by a lack of low cost

equipment to get them on air. A mono-

band, bidirectional design usingordinary NPN transistors was developed

to cater to this demand. The design can

be adapted to any particular ham band

by changing the RF section coils and

capacitors and the VFO frequency.

BITX evolved over one year from the

excellent S7C receiver described in the

new ARRL book Experimental

Methods in RF Design ( an ARRL

publication ) into a bi-directional

transceiver. Several hams across the

globe contributed to its design. A series

of emails were exchanged with OM

Wes Hayward (W7ZOI) during theevolution of this design. His

contributions have been invaluable. He

urged me to strive for higher

performance from the simple design.

The resultant rig has sensitive receiver

capable of strong signal handling, a

stable and clean transmitter capable of

enough power to make contacts across

the World. All the parts used in BITX

are ordinary electronic spares

components. Instead of expensive and

hard-to-get toroids, we have used

ordinary tap washers. Broad-bandtransformers have used TV balun cores.

The entire transceiver can be assembled

in India for less than Rs.300. I have

designed a single side PCB with large

tracks that can be easily etched at home

or by any PCB shop. Modificationsand improved compact PCB artwork,component placement guide andoverlays are available at the BITX20

yahoo group. You may join thisgroup at :http://groups.yahoo.com/group/BITX20for a wealth of information.

For those who don't readlong articles ...

There are a couple of things you should

know before you start assembling the

circuit:

The same amplifier block isused throughout. But the

emitter resistors vary in some

of the places. Double check the

values. If you swap values, the

circuit wont stop working. It

will work terribly. That might

be a little difficult to diagnosein the end. Check the emitter

values and the resistors that go

between the base and collector.

The receiving IF amplifierbetween the filter and theproduct detector is coupledto the product detectorusing a 100pf (not 0.1uf).

BITX20An easy to build 6 watt 20M Bi-directional SSB Transceiver

by Ashhar Farhan
http://www.phonestack.com/farhan/bitx.htmlhttp://www.phonestack.com/farhan/bitx.html


8/15



The crystal filter worked forme, I used crystals from the

local market marked as KDS.

These are the cheapest and

they work with the capacitor

values given in the filter. Your

crystals might require a

different set of capacitors. Try

the values given here, if youfind the bandwidth too narrow,

decrease the capacitances, if

you find it too open then

increase the capacitances.

The microphone is directlycoupled to the amplifier as my

headset microphone needs 5V

bias. If your microphone works

without bias, then insert a 1uf

in series with the microphone.

The pictures show myprototype on two boards. Dont

do that, split up the VFO into aseparate box.

The pre-driver is built onto themain board. The driver and the

PA are on a separate board.

Keep the same layout to keep

the PA stable.

There is a 50uf on the powerline soldered near the BFO,

don't forget it. It cleans up the

audio noise which would

otherwise get into the receiver.

On the PCB, there are jumpers

between T lines and R linesacross the ladder filter. There

is a jumper from the BFO

supply to the VFO supply.

Development Notes

Bi-directional SSB transceivers have

been quite common in amateur

literature. A transceiver was described

in the ARRL SSB Handbook using

bipolar transistors. W7UDMs design of

bidirectional amplifier (as the basis of

bidirectional transceiver) is referred to

by Hayward and DeMaw in their bookSolid State Design. The bidirectional

circuitry is often complex and not

approachable by the experimenter with

modest capability (like me).

The broad band bi-directionalamplifierMy current interest in bidirectional

transceivers arose after looking at an

RC coupled bidirectional amplifier in

the book Experimental Methods in RF

Design (p. 6.61). An easily analyzed

circuit that was simple and robust was

required. It began its life as an ordinary

broad-band amplifier:

In any bipolar transistor, the current

flowing from the collector to emitter is a

multiple of the current flowing from the

base to the emitter. Thus, if there is a

small change in the current flowing into

the base, there is a bigger change in the

current flowing into the collector. What

follows is a highly simplified

explanation of working of the above

amplifier.

In the above circuit, imagine that a

small RF signal is applied through Rin to

the base of Q1. Also imagine that the Rfvoltage is swinging up. The transistor

will accordingly amplify and increase

collector current causing more current

to flow through the Rl (220 ohms)

collector load. This will in turn drop thevoltage at the collector. The drop in

voltage across the collector will also

result in a drop at the base (base voltage

is a fraction of the collector voltage due

to the way the base is biased). This

circuit will finally find balance when

the increase in base current flowing

from Rin is balanced by the decrease in

base current due to the voltage drop

across Rl. In effect the RF current

entering from Rin flows out through the

feedback resistance (Rf). The impedance

seen at the base is effectively very lowand the signal source will see an

approximate input impedance of Rin.

Thus, Vin/Rin = Vout/Rf(Eq.1)

Another factor to consider is that that

emitter is not at ground. At radio

frequencies, it looks like there is a 10

ohms resistor between the emitter and

the ground. Thus, when the base voltage

swings, the emitter will follow it. The

AC voltage variations across the Re (10

ohms) will be more or less the same as

that across the base. The current flowing

into the emitter will mostly consist of

collector current (and very little base

current). Thus, if the emitter current

almost equals collector current,

Ie = Vin / Re = Vout / Rl (Eq. 2)We can combine these two equations to

arrive at:

Vout / Vin = Rf / Rin = Rl / Re. (Eq. 3)

This is an important equation. It means

several things. Especially if you just

consider this part:

Rf/ Rin = Rl / Re. (Eq 4)

Lets look at some interesting things:

1. The voltage gain, and the input and

output impedances are all related to

resistor values and do not depend upon

individual transistor characteristics. We

only assume that the transistor gain issufficiently high throughout the

frequencies of our interest. The precise

value of the transistor characteristics

will only limit the upper frequency of

usable bandwidth of such an amplifier.

This is a useful property and it means

that we can substitute one transistor for

another.

2. The power gain is not a function of a

particular transistor type. We use much

lower gain than possible if the transistor

was running flat out. But the gain is

controlled at all frequencies for this

amplifier. This means that this amplifier

will be unconditionally stable (it wont

exhibit unusual gain at difference

frequencies).

3. You can restate the eq 3 as Rf * Re =

Rl * Rin . That would mean that for a

given fixed value of Rf and Re, the

output impedance and input impedances

are interdependent. Increasing one

decreases the other and vice versa! For

instance, in figure 1, Rf = 1000, Re = 10,

if we have Rin of 50 ohms, the output

impedance will be (1000 * 10)/50 = 200

ohms. Conversely, if we have an Rin of200 ohms, the output impedance will be

50 ohms!

In order to make bidirectional

amplifiers, we strap two such amplifiers

together, back to back. By applying

power to either of amplifiers, we can

control the direction of amplification.

This is the topology used in the signal


9/15



chain of this transceiver. The diodes in

the collectors prevent the switched-off

transistors collector resistor (220 ohms)

from loading the input of the other

transistor. A close look will reveal that

the AC feedback resistance consists of

two 2.2K resistors in parallel, bringing

the effective feedback resistance to1.1K. Thus, the above analysis holds

true for all the three stages of

bidirectional amplification.

Diode mixersThe diode mixers are inherently

broadband and bidirectional in nature.

This is good and bad. It is good because

the design is non-critical and putting 8

turns or 20 turns on the mixer

transformer will not make much of a

difference to the performance except at

the edges of the entire spectrum of

operation.The badness is a little tougher to

explain. Imagine that the output of a

hypothetical mixer is being fed to the

next stage that is not properly tuned to

the output frequency. In such a case, the

output of the mixer cannot be

transferred to the next stage and it

remains in the mixer. Ordinarily, if the

mixer was a FET or a bipolar device, it

usually just heats up the output coils. In

case of diode ring mixers, you should

remember that these devices are capable

of taking input and outputs from any

port (and these inputs and outputs can

be from a large piece of HF spectrum),

hence the mixer output at non-IF

frequencies stays back in the mixer and

mixes up once more creating a terrible

mess in terms of generating whistles,

weird signals and distorting the original

signal by stamping all over it.

A simple LC band pass filter that

immediately follows the diode ring

mixer will do a good job only at the

frequencies it is tuned to. At other

frequencies, it will offer reactive

impedance that can cause the abovementioned problems. It is requirement

that the diode mixers inputs and

outputs see the required 50 ohms

termination at all the frequencies. In

other words, they require proper

broadband termination. Using broad-

band amplifiers is a good and modest

way of ensuring that. A diplexer and a

hybrid coupling network is a better way,

but it would be too complex for this

design.

Circuit Description

Although simple, every effort was made

to coax as much performance as was

possible given the limitations of keepingthe circuit simple and affordable.

The ReceiverThe RF front-end uses a triple band-

pass filter for strong image and IF

rejection. The three poles of filtering are

quite adequate and the out-of-band

response of the receiver is only limited

by external shielding and stray pickups.

An RF amplifier follows the RF band

pass filter (Q1) biased for modest

current. More current would have

required a costlier transistor. There is

8mAs through the RF amplifier and thepost-mix amplifiers to keep the signal

handling capacity of the circuit above

average. The Post-mix amplifier (Q2)

does the job of keeping the crystal filter

as well as the diode mixer properly

terminated. The crispness of the

receiver is more due to this stage than

anything else. An improper post-mix

amplifier easily degrades the crystal

filters shape and introduces spurious

signals and whistles from the diode

mixer. Note that the mixer is singly

balanced to null out the VFOcomponent and not the RF port and in

the absence of proper pre-selection,

10MHz signals can easily break into the

IF strip.

The VFO is fed via a broad-band

amplifier into the singly balanced

mixer. We used the simplest VFO

possible with a two-knob tuning

mechanism. It works really well and for

those (like me) used to quick tuning, it

offers best of both worlds, slow tuning

through the varactor and fast tuning

through the capacitor without any slow

motion drive. Getting a slow motion

drive is an increasingly difficult

problem and this is an electrical

substitute for slow motion drives.

A word about the VFO: depending upon

component availability, skills and

preferences, everybody has a favourite

VFO circuit. Feel free to use what you

have. Just keep the output of the

collector of Q7 to less than 1.5 volts (it

will appear clipped on the oscilloscope

trace, that is okay). For 20 Meters

operation, you will need a VFO that

covers 4 to 4.4MHz. The given VFO

has low noise though it does drift a

little, but I have had no problems with

ordinary QSOs. After 10 minutes ofwarm up, the drift is not noticeable,

even on PSK31 QSOs.

A Hartley oscillator using a FET like

BFW10 or U310 would work much

better. You can substitute this VFO with

any other design that you might want to

use. If you are using the PCB layout,

then skip the VFO on board if you want

to use a different VFO and build it

externally in a separate box.

The simple IF amplifier has a fixed

gain. Earlier it was noted that IF amp

was contributing noise at audiofrequencies. It was later traced to noise

from the power supply and placing a

50uf on the transceiver power line has

cured it. The IF amplifier has a 100pf

output coupling to provide roll-off at

audio frequencies.

The BFO is a plain RC coupled crystal

oscillator with an emitter follower. The

emitter follower has been biased to 6V

to prevent limiting.

The detector also doubles up as the

modulator during transmit mode; hence

it is properly terminated with an

attenuator pad. It has no impact on the

overall noise figure as there is enough

gain before the detector. The audio pre-

amplifier is a single stage audio

amplifier. The 220pf capacitor across

the base and collector provides for low

frequency response.

The receiver does not have an AGC.

This is not a major short-coming.

Manual gain control allows you to

control the noise floor of the receiver

and I personally find it very useful when

searching for weak signals or turning it

down to enjoy the local ragchew.TransmitterThe microphone amplifier is DC

coupled to the microphone. This was

done to steal some DC bias that is

required when using a Personal

Computer type of headset. If your

microphone does not require any bias,

then insert a 1uF in series with the


10/15



microphone. The microphone amplifier

is a simple single stage audio amplifier.It does not have any band pass shaping

components as the SSB filter ahead will

take care of it all. One 0.001uf at the

microphone input and another at the

modulator output provide bypass for

any stray RF pickup.

The two diode balanced modulator uses

resistive as well as reactive balancing. A

fixed 10pf on one side of the modulator

is balanced precisely by a variable 22pf

on the other side. A 100 ohms mini

preset allows for resistive carrier

balance. The attenuator pad at the

output was found necessary to properly

terminate the diode modulator and keep

the carrier leakage around the IF

amplifier to a minimum. While this may

seem excessive, it produces a clean

DSB with carrier nearly 50db down

with careful adjustments on the

oscilloscope.

Rest of the transmission circuitry is

exactly the same as the receiver. Thereis an extra stage of amplification (Q14)

to boost the very low level 14MHz SSB

signal from output of the microphone

tip to driver input level.

The output amplifier boosts the SSB

signal to 300mV level, enough to

directly drive a driver stage.

The Power ChainA simple power chain consisting of a

low-cost medium power NPN transistor

(2N2218) driving an IRF510 for 6 watts

of power at 14MHz. The output of

IRF510 uses a tap washer as an output

transformer. The output transformer has

40 turns of bifilar winding; these can

lead to enough stray capacitance to

affect proper performance as a

transformer. The half-wave filter that

follows the transformer absorbs these

capacitances as a part of the matching

network.

I used this power chain because it works

for me and delivers 6 watts on 14MHz. Idont use more power because I neither

require more nor do I have a power

supply that can source more. If you

need more power, there are a number of

things that you can do, you can simply

increase the supply voltage on the

IRF510 up to 30 volts and extract nearly

15 watts of power from the same

configuration. At 30 volts, the drain

output will be at 30 ohms impedance

and the pi-network will have to be

designed to directly match the drain to a

50 ohms antenna load. Alternatively,

you could try two IRF510s in push-pull.

These are variations that you can play

with. A word of warning though, The

RF energy at these levels can give you a

serious RF burn. RF burns can be more

painful than fire or steam burns. QRP is

not only fun, it is also safe.


11/15



ConstructionI would highly recommend that you

construct it over a plain copper clad

board by soldering the grounded end of

the components to the copper and the

other ends of components to each other.

Look at the pictures to see how it has

been done. If you dont know about this

method of assembling RF circuitry, then

you should read about it, there are quite

a few write ups on the Internet about

this method of RF experimentation. It

does not require any PCB, it is quite

robust and very stable.

Assembling the PCBFor those who feel intimidated by this

ugly method, I have designed a PCB.

The PCB layout (component side) is

provided with this article. It is a single

sided PCB with wide tracks that can be

easily made in the home lab. I am

making a run of these PCBs but

shipping them abroad (outside India)maybe a problem. Drop a mail to me if

you are planning to make some PCBs, I

can put your contact information on the

website. There are no copyrights over

either the PCB, the circuit or even this

article, feel free to copy and distribute.

The PCB is laid out in a long line.It is 8-

1/2 inch long and 2-1/2 inch wide. The

circuit board is big for the circuit that

goes onto it. This was done so that the

board is non-critical and it works well.

All the bidirectional amplifiers are

similarly laid out.When you get your PCBs, inspect them

thoroughly, preferable in the Sun.

Check for small cracks in the tracks.

Check for tracks that might be touching

each other or touching the ground plane.

The PCB layout was done to minimize

this, but check it anyway. Especially

check for the tracks that run diagonally

to the base of each transistor in the

bidirectional circuitry. These are laid

out very closely and they are candidatesfor shorting.

Almost all assembly instructions ask

you to solder the transistors in the end. I

would highly recommend that you

solder the transistors and the diodes

first. You are most alert when you start

a project and if you place the transistors

correctly, the rest of the circuit can be

soldered around it. Be very careful

about the orientation of each transistor.

The microphone amplifier transistor

(Q10) faces in a direction opposite to

the rest of the transistors and thetransistor pairs in bidirectional

amplifiers face each other. The diodes

have a ring to indicate which way their

arrow is pointing.

After the transistors are soldered, finish

the BFO. If you are assembling this for

14MHz and above, the BFO will need a

coil in series with the crystal (USB

mode), if you are need LSB operation,


12/15



you will need a trimmer instead (see the

schematic). Apply power to the BFO

and you should be able to hear it on

your Short wave broadcast radio around

31 meter band. It will sound like a silent

radio station. It should be quite strong.

Switching the BFO power supply onand off will help you identify your BFO

signal on the radio. If you have an RF

probe, or an oscilloscope, you should be

able to see the oscillations. Expect RF

of 2 volts or more.

Next, assemble the VFO. Winding 150

turns of the VFO coil is one of the most

tedious jobs while assembling this rig. It

has to be done, so just dig in and do it.

You dont have to attach the 365 pf

tuning capacitor yet. Check the

oscillations on a receiver or a frequency

counter. You may have to decrease thenumber of turns. Without the 365 pf, the

22pf trimmer should be able to set the

VFO to 4.3MHz or so. If the VFO is

oscillating at a lower frequency, then

remove some turns from the coil. If the

VFO is at a higher frequency, add 22pf

in across the 22pf trimmer (if you are

using the PCB, solder in from the foil

side). You will require a wire jumper to

carry power supply between the VFO

and the BFO. They are the only stages

that remain switched on during both

transmit and receive.

Assemble the audio pre-amplifier and

the audio power amplifier and attach the

volume control. When power is applied

to the audio stages, touching a finger to

the base of Q4 should produce static in

the speaker to move even the most die-

hard trash metal rockers. Next, assemble

all the three bi-directional stages! This

involves lot of soldering. But all the six

stages are exactly the same. Finish one

stage at a time. The capacitors are

symmetrically laid out and all of them

are 0.1uF with one exception (100pf at

the output of Q3). Remember that the

emitter bias resistors are 100 ohms, 220

ohms or 470 ohms. If you mix up thevalues, the rig will still work but it will

under perform in the presence of strong

signals and the transmission will be

splattered. There are jumpers for T and

R line across the crystal filter. Solder

them up and power on the R line and

then the T line alternatively. The

emitters of bidirectional stages should

show 2 volts approximately and the

collectors should show around 8 volts

and the switched-off transistor should

show zero voltage on all the three leads.

For the moment of truth, solder thethree coils, trimmers and capacitors of

the RF filter, attach an antenna and

switch it on! Check that the stages are

working starting from audio end. If you

touch the volume controls control pin,

you should hear AC hum and static. If

you touch the base of Q4, there should

be a pretty loud static. Take a lead from

your VOM and touch Q3, you should

get very loud static, probably mixed

with local AM broadcast. Touch the

base of Q2 with the test lead and you

should get lesser static as the filter

allows only 3 KHz of 10MHz through.

Finally, connect the antenna properly at

the input of the RF band-pass filter and

peak up the three trimmers for

maximum atmospheric noise. Attach the

365 pf and start tuning around the band,

peak the RF front-end on a strong signal

and then tune in a weaker signal and

peak for maximum clarity (not

maximum sound).

An important note: Be sure that youhave connected a proper 50 ohms

antenna load. The RF filter performs

correctly only at 50 ohms. If you use a

long wire to do the initial testing, youwill have to touch up the trimmers again

for the proper antenna.

Take a break, spend the evening

listening to your new homebrew. If the

CW signals tune to dead beat and rise

on the other side again, your BFO has to

move its frequency. For USB, add more

turns to the coil to the BFO coil, for

LSB, tweak the trimmer. You should

have a perfect single signal reception. If

you tune past the dead-beat of a CW

signal, the signal should drop out

completely.Assembling the microphone amplifier

(Q10) and the output amplifier (Q14)

will complete the exciter portion of the

transceiver. To put the transceiver in

transmit mode, ground the R line and

apply 12V on the T line. Attach the

output of Q14 to an oscilloscope but

dont attach the microphone yet. Null

the carrier with the 100 ohms preset and

the 22pf trimmer. Each affects the other

so you might have to go back and forth

between the two controls.

Now plug-in the microphone and speak

into it. You should be able to see clean

SSB of between 200 and 300 mV on the

scope at the output of Q14. Instead of

the oscilloscope you can use another

14MHz receiver to test your

transmission quality. Switch off the

AGC of the other receiver while setting

the carrier null. A soft whistle (if you

can manage) into the microphone is


13/15



should result in a full carrier at the

output.

Next, assemble the power chain. At this

point, you will need a suitable chassis to

house your project. Any metal box will

do. If you dont have any, you can

solder pieces of copper clad together

(like I did) and make a U shapedchassis. Keeping the VFO in open air

makes it drift a bit. A closed box is

really very useful.

A big cookie (or chocolate) box of tin is

really ideal. With a hand drill, you can

easily make holes to fit the two PCBs

inside it. Tin is easily soldered on. Use

the biggest knob you can find for the

main tuning. The plastic broadcast

capacitors usually have a very short stub

that cannot take a big knob. It takes on a

small plastic drum that is held onto the

capacitor spindle with a retaining screw.Clip on the drum onto the tuning

capacitor, tighten the retaining screw

well and with epoxy glue, stick a big

knob over the drum. This will make

your main tuning mechanism.

I use a simple double pole triple throw

switch for Transmit/Receive switch-

over. If you prefer PTT operation, you

can easily substitute the switch for a

relay. Be sure to solder a reverse biased

diode across the relay coil to prevent

reverse voltage from entering into the

transceiver power line.

Use shielded cable for all the

connections between the power

amplifier and the main board.

Tune-up and Operation

Set the VFO to correctly cover 4.0 to

4.4MHz. If you can, take your rig over

to a ham friends shack, you can

monitor your VFO on his rig at the edge

of 80 meters band at 4.0MHz. Set the

trimmer so that you can hear the VFO

when the friends receiver is tuned to

4.0MHz and your tuning capacitor is

fully closed (as much as it will go anti-

clockwise). After this, connect the

antenna and peak the RF coils for

maximum noise in the speaker. If you

can tune it to a weak signal, then peak

the RF coils for best reception.

You might find that although you are

able to tune in CW stations, you are

unable to hear the SSB stations

properly. This indicates that your BFO

is not properly set. We will take that up

next.

On amateur bands above 10MHz, SSB

is transmitted on upper sideband and on

bands below 10 MHz, it is transmitted

on lower sideband. To tune a upper

side-band signal, your BFO has to be atthe lower edge of the crystal pass-band.

You will require either the inductor (for

USB) or the capacitor (for LSB) in

series with the BFO crystal. If your

BFO is set to proper frequency then the

signals will tune in and as you continue

tuning across the signal, they will drop

in pitch and disappear. If the signals

appear muffled, then the BFO is set in

the crystal filters center, add more turns

to the coil (USB), or tweak the trimmer

(LSB). If the signals appear shrill and

you are unable to zero-beat them, thenthe BFO is too far away from the filters

frequency - Decrease the coils turns

(for USB) or tweak the trimmer (LSB).

The transmitter tune-up essentially

involves setting the carrier null. It is

best to tune up the transmitter on a

dummy load. I use 8 220 ohms, 2 watts

resistors in parallel as my dummy load.

It is worth the few bucks to have a

proper dummy load. Attach the dummy

load on the transmitter, and attach an

RF probe to the dummy load (or an

oscilloscope). As you speak, you should

get 20 volts or more peak voltage on the

dummy load when you whistle or just

go haaaaallow. On another receiver in

the same room, connect a short piece of

wire as an antenna and monitor your

own signal. You will probably be able

to hear your own carrier as well. Null it

by tweaking the 100 ohms preset and

the 22pf balance trimmer. They both

interact, so you might have to go back

and forth between the two controls.

A word of caution, the diode mixers are

prone to generating odd harmonics. The

third harmonic of 4 MHz is at 12MHz.So, if you simply peak the coils for

maximum output on transmit, you might

wrongly peak the RF front-end to 12

MHz (I did that). The RF band-pass

filter is best tuned in receive mode over

a weak signal at 14.150MHz or so and

left at that.

Conclusion

There might be a kit (components and

the PCB in a bag) soon. I personally

dont have the time to put kits together.

If somebody is interested in doing so,

just go ahead and do it. The design is

free, you dont need to ask my oranybody elses permission. The purpose

is to address the need among Indian

hams in particular for an SSB rig that is

easily and cheaply built. My original

aim was to keep the price under Rs.

1000. The current design brings the cost

to well under Rs.300.

6 Watt Linear PA PCB artwork.

Modifications / improved compactPCB artwork, component placementguide and overlays are available atthe BITX20 yahoo group. You may

join this group at :http://groups.yahoo.com/group/BITX20

for a wealth of information.


14/15



BITX PCB Artwork andcomponent layout


15/15



This article has been prompted by the

DX we often encounter on the VHF

bands for an understanding of how

and why the S21 stations come

blasting on our beloved 2M band. The

article is based on the information

available at

http://www.g4xgt.co.uk/what-is-

sporadic-e.htm

What is sporadic E ?Irregular scattered patches of relatively

dense ionization that develop seasonally

within the E region and that reflect and

scatter radio frequencies up to 150

MHz. Sporadic E is a regular daytime

occurrence over the equatorial regions

and is common in the temperate

latitudes in late spring ,early summerand, to a lesser degree, in early winter.

At high, i.e., polar, latitudes, Sporadic E

can accompany Auroras and associated

disturbed magnetic conditions. It can

sometimes support reflections for

distances up to 2,400 km. Sporadic E is

a form of propagation that can arise

with little warning, and enable radio

frequencies of 150 MHz and more to

travel over distances of a thousand

kilometres and more. Many people

experienced it in the days of the old

VHF television transmissions. Whensporadic E propagation arose, it would

result in severe interference to the

signals. Even now VHF FM broadcasts

in the 88 - 108 MHz band can be

affected. In many instances the arrival

of Sporadic E can cause unwanted

interference as signals that are normally

too distant to be heard appear. However

for radio amateurs it offers the chance to

make contacts over much greater

distances than are normally possible.

Sporadic E arises when clouds ofintense ionisation form in the region of

the E layer. These clouds can have very

high levels of ionisation, allowing

frequencies up to about 150 MHz to be

reflected on some occasions. The clouds

are usually comparatively small,

measuring only about 50 to 150

kilometres in diameter. Their shape is

irregular. Sometimes they may be

almost circular, whereas others may be

long and thin. They are also surprisingly

thin, often only measuring a few

hundred metres in depth.

These clouds appear almost at random,

although there are times when they aremore likely to occur. They form in the

day, and dissipate within a few hours.

They are also far more common in

summer, peaking approximately in mid

summer. As they form the level of

ionisation gradually builds up, affecting

first the lower frequencies, and later

higher frequencies as the level of

ionisation increases.

Propagation via sporadic E occurs in the

same way as normal ionospheric

propagation. Signals from thetransmitter leave the earth as a sky-

wave, travelling towards the ionosphere.

Here they are reflected (or more

correctly refracted) back to earth where

they are heard at a considerable distance

from the transmitter. Like normal

ionospheric propagation it is the free

electrons that affect the signals, causing

them to bend back towards the earth. In

view of the fact that the sporadic E

clouds occur at around the same height

as the E layer, similar distances are

achieved. Typically the maximum

distances are about 2000 km.

It is found that the sporadic E ionisation

clouds move. Being in the upper

atmosphere they are blown by the winds

in these areas and can drift at speeds of

up to 300 kilometres per hour. This

means that when sporadic E is being

experienced, the area from which

stations are heard will change over the

life of the cloud.

Theories

There are many theories about sporadicE and how it occurs. Some believe that

it may be related to thunderstorms,

others think it results from the winds in

the upper atmosphere. None of these

theories have been established, leaving

the reasons behind sporadic E a

mystery, and predictions of when it will

occur have to be left to statistics.

However even though the mechanism

behind the formation of sporadic E is

not fully known it is still possible for

radio amateurs to utilise them to enable

them to make contacts over long

distances

What is Sporadic E ?

Wouldn't it be lovely to haveanother Fox Hunt in the city.The last and only one that wehave had so far was held wayback in 1995. Time foranother one !! This edition of"On The Air" carries simpleDF gear designs that anyonecan brew. The society has

received permission to changeits registered name from"Calcutta VHF AmateurRadio Society" to "BengalAmateur Radio Society." Ifthe resolution for the same ispassed at the EGM on 5thMarch, 2006 the society'sname will be changedpermanently. There are plansto have an annual event in the

form of a Fox Hunt this year.So gear yourselves and letshave a gala time at the 2006Fox Hunt.

"On The Air" published by :Calcutta VHF Amateur RadioSociety9, Mandeville Gardens

Apt - 5FKolkata - 700 019www.calcuttahams.comEmail : [email protected]

Editor: Nilanjan MajumdarVU2HFR

Documents

HAM Magazine 2006