[AMRadio] shunt

Rick Brashear rickbras at airmail.net
Fri May 5 12:13:44 EDT 2006

Good info, John and something to think about.  Shrader (Electronic 
Communications 3rd Edition) seems to think a shunt is  doable.  However, 
there is no mention in the test of how the shunt (coil of wire) would be 
affected by radiation.  A shunt would certainly limit the current 
available to the thermocouple, but would the results be contaminated by 
external RF?  I think you're right, it would be a risky setup.

I want to measure the antenna current from a broadcast transmitter (1 
KW) into a dummy load, of course.  I realize a 5 amp thermocouple meter 
should suffice, but I wanted a little headroom in the event of a load 
disaster that could send the current soaring and destroy the meter.  
Your suggestion of a sampling device would certainly do the trick and 
probably with much better accuracy, however, I want to set it up as it 
would have been in a broadcast station situation.  Since I'll be running 
much less than 1000 watts I'm sure, after rethinking it, the 5 amp RF 
meter will do fine.

Thanks for the info and comments.


John Lawson wrote:

>   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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