maybe hes loading a sheet of copper..... ----- Original Message ----- From: John Lawson <[EMAIL PROTECTED]> Date: Friday, May 5, 2006 10:42 am Subject: Re: [AMRadio] shunt To: Discussion of AM Radio <amradio@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 > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb >
