[600MRG] Southern Avionics SWR Bridge

[Murray Greenman]

Frank and others,

R1 is 20 Ohm Non-inductive. Its the long black resistor at the edge of 
the board in the attached photo. I imagine it has two windings in 
opposite directions. The rating is 10 W. You could replicate it easily 
enough with a bunch of metal film resistors in parallel.

I agree, it would be best if the two transformers were at right angles 
for minimum coupling, but I'm sure you will see that they are offset by 
half the coil diameter, and this also provides a point of minimum (or at 
least cancelled) coupling. Besides, since the two primaries are in 
series, and thereby coupled anyway, it would not matter greatly. Any 
imbalance is compensated for in calibration.

The bridge drives a 1 mA meter via two unity gain buffer amplifiers and 
a 3-way switch, labelled OFF/REFL/FWD (see drawing). The meter is scaled 
0 - 200 %, with graduations 0-10-50-100-150-200 and a non-linear scale 
('100' is mid-scale). I found that up to at least 500 W you could read 
the scale as Watts with reasonable accuracy. The same scale is used for 
FWD and REV. The second scale (MOD %) is not used in this model, since 
it has no AM capability. There is a separate PA meter with switches for 
Voltage and Current and another to select the PA. This transmitter has 
two 500 W PA stages, and a combiner. The two PA stages have independent 
switch-mode power supplies 0 - 150 V which track the power level 
control. I rarely had enough working parts to achieve more than 500 W.

BTW, the transmitter is for sale, but I doubt anyone could afford the 
shipping cost!

73,
Murray ZL1BPU


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