Difference in how these two VSWR bridges work?

[Chris Wilson]

Both seem to do a similar job in providing metering of forward and reflected power, but one which I am using has a trim cap to tune for zero reflected power into a 50 Ohm dummy load, the other, which I have not tried, seems to do it with a variable resistor. What are the pros and cons of the two types and how does the "resistive" one work for zeroing reflected SWR into a known 50 Ohm load please? Is the "resistive" one just dependent on zeroing for reflected on the value of the fixed capacitors of C15 and C16? And the other allows some adjustment of the tuning via the trim cap?

[dmills]

They are essentially the same circuit, just one has adjustable zero for reflected power and one does not.

The operating principle is to sum and difference the voltage developed by the capacitive divider with half the voltage developed across the current transformers load resistor, it makes for a poor sort of power meter, but the ratio gives a rather non linear indication of SWR.

73 Dan.