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