Analyser-DDS a 500Khz up to 71Mhz antenna-analyser Update ver2:

Don’t you power supply exceeding 9,6 V ( 8 x AA-cell NiCd or NiMh ) , if not insert a 7809

voltage regulator .

Extension up to 71 Mhz instead of 65,536 Mhz

Software update for calibration bug and extension to 71 Mhz

Software PC program Analyser_RX to extend to 71 Mhz

Optional use of anti-rebound capacitors ( 100 nF ) over switches

Update of alignement procedure

Introduction

Analyser-DDS is based on the same principle as the ” AN EXPERIMENTAL HF AERIAL

ANALYSER from Jim Tregellas VK5JST ” .

This principle is measuring unknown impedances adding a pur ohmic resistor in series feeded with

a sinusoidal RF-source .

My specific goal was to make an antenna analyser covering at least 160 up to 6 meters , with ability

to view measurement results as graphs . Thereby it needs to be reproducible by the average HAM .

The main specifications of this analyser are frequency coverage from 500Khz up to 65536Khz

Further more the measured units are displayed on a 4 lines x 20 characters LCD and reflect the

frequency in Khz , the step in Khz , the impedance Z in Ohm , the resistive part in Ohm , the

reactive part in Ohm and the SWR as a ratio to 1 .

Tuning is possible in step by step mode ore as a scan of 80 times the step-value starting upwards

from the tuned frequency .

The scan can be transmitted via RS232 to a PC , where the data will be plotted in a graph using the

program Analyser-RX .

The RF-source is a DDS60 dds-vfo .

Imedances from 5 Ohm up to 1000 Ohm can be measured within a usable grade of accuracy .

Measure principle

Graphic representation of measured units:

If the antenna impedance is not pure ohmic , we get following vectors .

Pythagorean theorem :

The sum of the areas of the two squares on the legs (Vx and V50+VR) equals the area of the square

on the hypotenuse (Vin) .

Thus:

Vin² = VX² + (V50+VR)²

Of Vin² = VX² + V50² + VR² +( 2xV50xVR)

Of 2 x V50 x VR = Vin² - VX² - V50² - VR² and as VX² + VR² = VOUT² (triangle

VOUT,VR,VX)

thus: VR = (Vin² - V50² - VOUT²) / (2 x V50)

and VX² = VOUT² - VR²

so VX = SQRT ( VOUT² - VR²)

the current through the circuit is I = U/R

thus I = V50/50

as well X = VX / I

and R = VR / I

out of X and R we calculate Z = SQRT ( X² + R²)

what remains is to deduct the SWR from what was above

knowing that

SWR = (Vin + Vref) / (Vin – Vref)

Vin is known by measuring .

How can we get Vref:

In fig A Vref is the hypotenuse of triangle Vref,Vx,V50-Vr

thus Vref^2 = (V50-VR)^2 + VX^2

or Vref = SQRT (V50-VR)^2 + (VX^2)

In fig B it doesn’t matter if V50-Vr is either positive or negative .

In fig C , V50 – VR = 0 which means that R equals 50 Ohm.

Thus Vref = VX

In fig D the perfect match : V50-VR=0 en VX=0

Thus Vref = 0

So to determine the SWR :

Vin as measured unit calculate Vref = SQRT (V50-VR)^2 + (VX^2)

calculate SWR = (Vin + Vref) / (Vin – Vref)

Exceptions on this approach:

1° With a pure Ohmic antenna we have following vector diagram

thus we don’t have to calculate Vr and VX = 0

2° When a pure capacitor is connected

thus VX becomes a measured unit

So thats it. It just remains to build a measure circuit and have a calculator... or use a PIC16F876A

…

Schematics

PCB 3D-view and X-ray view

PICTURES of DDS60 position and PCB component side

Construction and alignment .

Preferably use an insulated enclose box .

While soldering don’t forget to make 2-layer connections where indicated (see PCB) .

Alignment is quite simple .

Let the antenna connector open without anywhat .

The power supply will be 9,6 V

The frequency will be set to 10.000 Khz

Turn trimpots anti-clock wise to minimise outputs to PIC .

Trim DDS60 output to obtain 1 V dc at pin 12 of LM324 .

Terminate antenna connector with 50 Ohm terminator without prolongator .

Increase RV1( Vi) until you read Vi=116 on LCD reading .

Increase RV2(V50) and RV3(Vo) to obtain Vin/2 reading on the LCD

Normally you would read Z=50 R=50 and X=0 . End of procedure .

Would you ever want to redo this procedure , then first turn trimpots anti-clock wise to

minimise outputs to PIC .

Analyser-RX program

Analyser-RX is a PC-program , receiving the data from the PIC within the SCAN2PC menu after

validation by the DO-key . 976 characters are transmitted . The first bytes define STEP and

START-frequency , followed by 80 times 3 bytes as Vin, V50 and Vout .

The Timer-function will renew the graph whenever new data are available .

The drawn graph include following measurements Z , X , R , and SWR .

On frequency overflow above 65536 Khz the graph will be clipped .

If V50=0 then graphvalues will be maximized .

If Vout=0 then graphvalues will be minimized with exception of SWR which will be maximized .

Have a look to the VB6-kernel of this program :

ReDim Meting(1 To Aantal, 1 To 3) As Variant

‘index 1=Vin

‘index 2=V50

‘index 3=Vout

ReDim Result(0 To Aantal, 1 To 6)

Result(0, 1) = "Z Ohm"

Result(0, 2) = "X Ohm"

Result(0, 3) = "R Ohm"

Result(0, 4) = "SWR"

Result(0, 5) = "50 Ohm"

Result(0, 6) = "SWR 1/1"

For A = 1 To Aantal

J = 0

If Meting(A, 2) = 0 Then

R = 9999

Z = 9999

X = 9999

SWR = 11

J = 1

End If

If Meting(A, 3) = 0 Then

R = 0

Z = 0

X = 0

SWR = 11

J = 1

End If

If J = 1 Then GoTo SetRes

Vout = Meting(A, 3)

V50 = Meting(A, 2)

If V50 + Vout < Meting(A, 1) Then

Vin = V50 + Vout

Meting(A, 1) = Vin

End If

Vin = Meting(A, 1)

VR = (Vin ^ 2 - V50 ^ 2 - Vout ^ 2) / (2 * V50)

If VR < 0 Then VR = 0 'test

VX = Sqr(Abs(Vout ^ 2 - VR ^ 2))

i = V50 / 50

X = VX / i

R = VR / i

Z = Sqr(X ^ 2 + R ^ 2)

Vref = Sqr((V50 - VR) ^ 2 + (VX ^ 2))

If Vin - Vref = 0 Then

SWR = 11

J = 1

End If

If J = 1 Then GoTo SetRes

SWR = (Vin + Vref) / Abs(Vin - Vref)

Debug.Print A, "SWR ", SWR

If SWR > 11 Then SWR = 11

SetRes:

If Z > 1999 Then Z = 1999

If X > 1999 Then X = 1999

If R > 1999 Then R = 1999

Result(A, 1) = Z

Result(A, 2) = X

Result(A, 3) = R

Result(A, 4) = SWR

Next A

The graph will be scaled automaticly in accordance with the max-values .

Z , X , R are clipped at 2000 Ohm or above . SWR will be clipped at 11/1 or above .

The graph will include a date and time title corresponding to the moment of transmission to PC and

it will determine the file-name of the graph when saved .

The program allows scaling of the graph to different screen and printer resolutions .

On start the program will list all RS232 ports .

The user will select the connected port by clicking it .

Have a lot of fun with building and using this analyser .

Willy ON5KN

Incuded some real graphs with comments:

A 55 Ohm resistor straight on the analyser , Step=1000Khz,Fstart=0 Khz

The start-zone is wrong due to limitations of the output bandpass filter of DDS60 .

Observe less then 80 points as 65000 Khz is reached .

55 Ohm straight on analyser Step=100Khz , Fstart=0Khz

Look at start zone of DDS60 .

Valid measures from +/- 500Khz

Limiting to 65536Khz was not yet functional !

EH-antenne measure , Step 500Khz , Fstart 10000Kihz

Observe SWR-dip near 14000Khz

Resonance zone of EH-antenna, Step=50Khz , Fstart 13000Khz

Observe automatic scaling of Z-X-R

Allowing maximal SWR of 1,5/1 we can reach 14000 tot 14150Khz

Reactive component (X) would be insignificant while impedance would vary from 65 tot 33 Ohm

Measure of 55 Ohm resistance on 17 m 50 Ohm coax , Step=500Khz , Fstart 2000Khz

Observe the minor mismatch and adaptor function of the coax depending from frequency (or coax

length) .Mismatch remains pure resistive (X remains 0) .

Measure of 17m open coax 50 Ohm Step=500Khz, Fstart=2000Khz

Observe the quarter wave effect around 5500Khz , High Z (X) but R=0

Measure of 6m dipole , Step=500Khz , Fstart=20000Khz

This dipole was quick made of wire and cutted down until good match .

With SWR 1,5/1 we can handle 50000Khz tot 52500Khz .

The reactive component is insignificant and the impedance Z=R between 30 and 70 Ohm .

Identical 6m dipole with zoom as Step=100Khz