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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
7/28/2019 HAM Magazine 2006
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2006, Vol1 ON THE AIR
For internal circulation amongst members only.
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
7/28/2019 HAM Magazine 2006
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For internal circulation amongst members only.
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
7/28/2019 HAM Magazine 2006
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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
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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.
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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
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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.html7/28/2019 HAM Magazine 2006
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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
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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
7/28/2019 HAM Magazine 2006
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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.
7/28/2019 HAM Magazine 2006
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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,
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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
7/28/2019 HAM Magazine 2006
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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.
7/28/2019 HAM Magazine 2006
14/15
2006, Vol1 ON THE AIR
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BITX PCB Artwork andcomponent layout
7/28/2019 HAM Magazine 2006
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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 : info@calcuttahams.com
Editor: Nilanjan MajumdarVU2HFR
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