[Collins] 302C-3 wattmeter

[Dr. Gerald N. Johnson]

On Wed, 2007-12-12 at 23:47 +1300, Dave Brown wrote:
> I understand these wattmeters were originally supplied with a length 
> of cable (4 wire screened) exiting through the hole in the back of the 
> indicator unit case to connect to the coupler unit.  The 302C-3 manual 
> mentions that the coupler and indicator should be separated by no more 
> than 5 feet, so presumably the supplied 4 wire screened cable was 5 
> feet long. Can anyone confirm this?

The 302C-1 segment of my Yellow Book doesn't mention this limitation.
> 
> I don't see the reason for this seemingly very short distance 
> limitation-is anyone successfully using one of these wattmeters with 
> significantly more than five feet separation?

I can think of two possibilities. One is that the shielding isn't
perfect, and the bypasses while good are not perfect and that RF pickup
from the antenna can get to the diodes and prevent there being the 30 dB
directivity that Collins specified.

The other is that the shielded cable viewed as an unterminated line on
the capacitors of the coupler if long enough to go 1/4 wave resonant can
change the RF impedance of the feed through capacitors and change the
calibration of the directional coupler.

When 100 watts produces only 100 microamps at the meter, only 10
microwatts DC, a bit of RF coupling could easily negate trying to get
the directivity down to 30 dB where the power at the meter would be .01
microwatts DC.

With longer leads you may never detect the errors.

I have a prototype coupler that was rejected because it didn't meet the
30 dB directivity they wanted. Its been a part of my main antenna tuner
for more than 40 years and no transmitter has complained about the load
presented when the reflected power is minimized by its somewhat
erroneous indication caused by the imperfect directivity.
> 
> I'm putting one of these meters back together (mil version- 
> OD4/GRC159(V)) after having the case repainted. Was surprised to find 
> the case is actually fibreglass and not steel, as I had assumed.   In 
> this one, the coupler was mounted directly on the back of the 
> indicator unit and the connecting cable is only  2 or 3 inches long.
> 
> So, I'm considering whether I should mount a suitable multiway 
> connector on the rear panel (or maybe use a cable mounted connector) 
> to permit use of a plug/socket arrangement. This will allow the coupler 
> to be be easily separated from the indicator and located some distance 
> away if required. Any suggestions as to what plug/socket type I should 
> use to retain some degree of consideration for the original design?

Shielded. Gold pins to handle the low voltage and low current signal.
Perhaps something like a 4 or 5 pin Cannon XLR microphone connector with
gold pins, not the alternatives with less than gold pins.
> 
> Dave, ZL3FJ
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer