[Collins] Meter sensitivity-312B-4 etc

[Gerald]

On Fri, 2005-04-22 at 20:04 +1200, Dave Brown wrote:
> Finally got a few spare minutes and measured one of the meter 
> movements in the 312B-4- as expected, 200 uA.
> So no problems using the 100 uA movements in the JRC cross-needle 
> meter. Think I need to use an order or two of magnitude scaling down, 
> though- forward power is scaled at 10 kW fsd!  Nothing a pot won't fix.
> 
>  Comments on CTs etc noted- yes, quite familiar with RF CTs now, 
> Gerry.  I spent quite a few nights working on accurately measuring LF 
> loop antenna current (~180 kHz) some year back- uses a CT with full 
> scale on the meter at 50 amps- hadn't ever played around with RF CTs 
> before then.  Didn't have the Tek current probe then either-would have 
> made life a bit easier calibrating the darn thing!
> 73
> Dave, ZL3FJ
The key to the good directivity of the Collins wattmeter is the
electrostatic shielding of the CT. I have a prototype that didn't get
the directivity they wanted, but its worked on my antenna tuner for
about 40 years and the rigs have all been happy with the match. I was in
on making one for the 250KW VOA radios too. Didn't use a toroid for the
CT there, didn't have any problems getting enough sample, the problem
was keeping the sample coil inductance down to cover the entire
frequency range accurately.

You may have to change the resistors inside the coupler to get the
sensitivity up to 1 KW or 100 Watts full scale, but so long as they are
above roughly 50K, the meter load shouldn't upset the diodes seriously.
And so long as you don't insist on fitting a given nonlinear meter
scale, lower R shouldn't upset the directivity.

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