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Try to change load impedance.
The SWR Meter consists in two sense elements, line current sense and load voltage sense.
The voltage applied on the first transformer is the voltage from the transmission line - the reflected voltage, so we call it V=Vf-Vr.
Same thing with line current, I=If-Ir.
You know that I=V/Z , replace and obtain I=(Vf-Vr)/Zo , where Zo is the characteristic impedance.
We call the left unrectified voltage Vfwd, and the right unrectifer voltage Vref.
To obtain Vfwd and Vref, apply the method " sum of effects".
Sensing only the line current, Vfwd=Vref thats equal to (Vf-Vr)/2N where N is the transformer winding ratio.
Sensing only load voltage, Vfwd = (Vf+Vr)2N and Vref = -Vfwd.
Sum the two effects and obtain:
Vfwd = (Vf-Vr+Vf+Vr)/2N = Vf/N
Vref = (Vf-Vr-Vf-Vr)/2N = -Vr/N
What's SWR ? In poor words, if the signal trasmitted into a line is not 100% absorbed, the signal is reflected back, creating standing waves inside the line, if SWR=1 all the transmitted power is absorbed 50% by the load and 50% by the transmission line.
Introducing the reflection coefficient:
p=(Zl-Zo)/(Zl+Zo)
SWR= (1+|p|)/(1-|p|)
p range is from -1 to 1
SWR range is from 1 to +infinite.
With maximum power transfer(Zline=Zload*), p = 0 and SWR = 1
Doing some math, |p| = |Vref| / |Vfwd|
Where the module of Vref and Vfwd are the rectified voltages in this circuit.
How to measure the rectified voltage?
Use a 100uA ammeter and a 100k potentiometer, first off put the switch in left position to measure rectified Vfwd, than set the current with the potentiometer to 100uA. Setting the switch position to right you are now measuring rectified Vref, and his value can go from 0uA(if |p| is 0) to 100uA(if |p| is 1).
For example, if you are reading 25uA while measuring rectified Vref, it mean:
|p| is equal to 0.25;
your SWR is 1.25/0.75 = 1.67
A well matched antenna has SWR<2.
Use toroidal ferrite cores, germanium diodes, ceramic capacitors and carbon film resistors.
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