[AMRadio] shunt


KB2WIG at twcny.rr.com KB2WIG at twcny.rr.com
Fri May 5 16:46:08 EDT 2006


maybe hes loading a sheet of copper.....

----- Original Message -----
From: John Lawson <jpl15 at panix.com>
Date: Friday, May 5, 2006 10:42 am
Subject: Re: [AMRadio] shunt
To: Discussion of AM Radio <amradio at mailman.qth.net>

> 
> 
> On Fri, 5 May 2006, Rick Brashear wrote:
> 
> > Has anyone made a meter shunt for an RF ammeter?  I see no 
> reason it would be 
> > different than a regular AC or DC meter shunt, is that correct? 
> I have a 5 
> > amp RF thermocouple meter and would like to extend it to about 
> 25 amps.
> 
>   I think there might be a couple of things to consider in such a 
> scenario.
> 
>  One is - for most 'regular' meter shunts, they are simply a very 
> low-ohm 
> calibrated power resistor. And generally the drop accross them at 
> full-rated current (50 amps, say) is measured in the millivolt 
> range: 25 - 
> 20 - 100 - 300 etc.
> 
>   RF Ameters, on the other hand, use a thermocouple element which 
> is 
> heated by a small resistance wire through which the RF passes - 
> there is 
> no 'drop' measured electrically, rather the temperature rise of 
> the 
> resistance element is used - so the 'cuurent' meter is actually 
> reading a 
> few MVDC coming from the TC junction in the "shunt" (which it's not).
> 
>   If you insert another actual resistance-type shunt around a 
> thermocouple 
> unit, it would have to still drop enough RF to provide the full-
> scale EMF 
> accross the heating wire.  So if the the meter is 5 Arf full-
> scale, your 
> shunt must drop a 5th at 25 Arf.
> 
> 
> (HEY!!!  Just what are you doing on the air that need 25 fargin' 
> AMPS of 
> antenna current...?)    ;}
> 
> 
>  The other thing I'm wondering is - what effect all that Stuff 
> (in the 
> feedline, basically) is going to have on SWR - production of 
> harmonics and 
> spurs - etc.
> 
>   I'd be tempted to just build a simple pickup like the MFJ 
> Balanced-line 
> ammeter has, and use a DC opamp driver to provide full-scale 
> readings in 
> the ammeter of your choice.
> 
> 
>   Just my 200 millidollar...
> 
> 
> Cheers
> 
> John  KB6SCO
> 
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