[Collins] Re: {Collins} Re: KWM-2 Low RF Output-PROBLEM SOLVED

[Dr.Gerald Johnson]

You said a SWR meter fed from 100 ohm source seeing 100 ohm load would 
show a 1:1 SWR. That's not true. It would be true for a slotted line (and I don't 
know what became of Art's 40" long slotted line) with probe, but not for the 
coupled samplers like the one in the Collins console or a Bird. Not even close 
for a monimatch type (typical of CB SWR meters). These meters use inductive 
coupling (in the Collins wattmeter its a shielded toroidal current transformer) 
for the current and a capacitive divider for the voltage, then take the vector 
product of current an voltage. The ratios of the voltage and current samples 
are adjusted so that when the load is 50 ohms, the reflected power indication 
is zero, independent of source impedance.

All these indicators (the monimatch more so) are upset by RF not IN the coax. 
Their indicator circuits aren't often very well filtered for RF and so that nearby 
RF from a close antenna or a broken braid connection gets to the diodes and 
odd results can be displayed. A hint of that problem can be detected when 
moving the observer, the antenna, or the coax changes the display.

There is at least one VHF laboratory bridge made by HP in the past that uses 
slotted line technology and when used in the vicinity of an antenna it gets 
unusual results because off the RF coupling to the probes from the antenna.

Gerald J.

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
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