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
> One is - for most 'regular' meter shunts, they are simply a very
> 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
> heated by a small resistance wire through which the RF passes -
> there is
> no 'drop' measured electrically, rather the temperature rise of
> 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
> 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
> ammeter has, and use a DC opamp driver to provide full-scale
> readings in
> the ammeter of your choice.
> Just my 200 millidollar...
> John KB6SCO
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