[Lowfer] RF Milliammeter

[ToddRoberts2001 at aol.com]

In a message dated 9/28/2008 11:15:06 P.M. Eastern Daylight Time,  
ku4xr at yahoo.com writes:

Hey Todd:
Yes I have, unfortunately I don't have one at my disposal  right
now. All I have are Higher current Ammeters that just barely  move.

73, Andy - KU4XR
 
___________________________________________________________
 
Hi Andy,
Like Charlie W5COV mentioned the clamp-on RF ammeter by Kyle K0LR
is supposed to be a good one that you can build.
 
Thermocouple RF milliammeters are pretty rare these days. The one I used is  
the RF Ammeter from the ART-13 transmitter. It shows a scale of 0-5 Amps  but 
the actual meter movement is around .075 A or 75 milliamps I think but  don't 
remember exactly. It has a self-contained thermocouple. The ART-13 RF  ammeter 
is still available from Fair Radio Sales for $25 each. 
 
I built a test setup to calibrate the meter by using a variac and two  6.3vac 
small filament transformers in series, the 6.3v secondary of one  going to 
the primary of the second one to double step down the input to a  very low AC 
voltage. This is safe enough to connect directly across the  ART-13 meter if you 
make sure to start at zero voltage first and slowly  bring up the variac.  I 
calibrated the one I have by running the very low  level 60Hz AC thru it with 
a Fluke DVM in series set to a low AC amps scale and  varying the voltage thru 
it with the variac and calibrating the scale with  the Fluke DVM.  60Hz AC 
will calibrate an RF
ammeter nicely as the thermocouple reading will change very little when  
reading 
60Hz to LF frequencies. This setup can be used to calibrate other low-scale  
reading RF milliammeters.
 
 A good one-watt Lowfer setup should easily put more than 100  milliamps of 
RF current into the antenna/ground system. But starting out with a  less than 
optimum setup you will probably be well under 100 milliamps RF. You  can always 
put a shunt resistor across the RF milliammeter to increase the  full-scale 
reading to your liking.
 
John W1TAG has a good article on RF Ammeters for LF use :
 
_http://www.w1tag.com/RFA.htm_ (http://www.w1tag.com/RFA.htm) 
 
 
73 Todd WD4NGG
 
 



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