From: "James Moritz" <J.R.Moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 19 Feb 2004 10:45:50 -0000
Organization: University of Hertfordshire
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Subject: LF: RE: RF Current meter
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Dear Gary, LF Group

>"When I have looked at designs in amateur radio magazines or internet
for a
>clip on RF Ammeter the designs seem to be limited to 2 ranges (100mA &
1A
>FSD), is there a limit to the range of this type RF Ammeter or can the
>ranges be extented to say 5 Amps maybe? is there a calculation to
extend >the
>range ?"
>My LF antenna seems to be showing in excess of one Amp so I need an RF
>Ammeter with a greater range than 1A.

For ammeters using current transformers, you can increase the FSD by
INCREASING the number of turns on the secondary of the transformer
(current transformation ratio for a transformer being the inverse of the
voltage ratio). So if you want to increase meter FSD by a factor N, just
increase the number of turns on the secondary (meter) side by N. This
divides the current going through the rectifier and meter by a factor of
N also, achieving the desired result. There are a number of second-order
effects that may alter the calibration somewhat, but experience shows
these are usually pretty small.

Shunting a thermocouple meter can be problematic, partly because the
thermocouple resistance is usually pretty low to start with, and partly
because it changes as the heater element heats up with increasing
current. A more reliable way of extending the range up or down is to use
a current transformer again. So using a 1:N transformer with the "N"
winding connected to the ammeter will increase the FSD by a factor of N.
Provided the impedance of the winding inductance is large compared to
the meter impedance, and primary and secondary are closely coupled, this
is an accurate way of changing the meter range - so don't reject
thermocouple meters just because they don't have the range you require!


Cheers, Jim Moritz
73 de M0BMU