Dear Dick, LF Group,
There are some odd problems you can get with an RF ammeter connected at a
high voltage point in the circuit.
One is stray capacitance to ground. With several kV of RF, even 1pF of stray
capacitance will allow several mA to flow. This can cause large errors,
especially if you are using the current transformer/load resistor/diode
voltmeter type RF ammeter, where the AC currents in the diode voltmeter part
are small. The solution is to enclose all the RF ammeter circuit in a metal
screen, connected to one end of the single turn primary of the current
transformer. The "ground" side of the secondary is also connected to the
screen. In this way, there are only a few volts of RF between components
inside the screen, and the stray capacitance has little effect.
The high electric field can also have funny effects on the meter movement
itself, with the resulting electrostatic forces causing the needle to be
deflected, or to stick. This is more likely to be a problem with plastic
cased meters, especially if the internal metal parts of the meter movement
are not connected to one of the meter terminals. Such a meter will also
effectively be a large hole in any screen that is added - I suppose you
could cover the meter face with wire mesh.
I suppose that if the meter is very close to the loading coil, the magnetic
field of the coil could induce significant EMF in the wiring of the meter
circuit, giving rise to the same sort of errors as stray capacitance. A
screening enclosure should also cure this problem.
If the meter is close enough to the coil to significantly change the
resonant frequency of the system, or gets significantly heated by the RF
field, that is probably too close...
You can put the ammeter in the low-voltage end of the circuit - take care to
connect it at a point where all the earth return current will flow through
the meter (this might not be easy). The meter will read high, due to the
additional current to ground via the stray capacitance of the coil. With a
big antenna, this arrangement will give little error. With a small antenna,
and a big coil, this error can be quite large. The magnetic field of the
coil is much the same at the ground end as at the "hot" end, so this might
still be a problem.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: Alan Melia <[email protected]>
To: <[email protected]>
Sent: Monday, August 14, 2006 9:40 PM
Subject: LF: Re: antenna current meter troubles
> Hi Dick I am not sure what the mechanism is. Jim seems to think it is RF
> getting into the diode and meter circuitry of the secondary, but I am not
> sure. I have not had enough power to experiment with this, and compare
with
> a thermoammeter. You might try sticking the meter at the bottom of the
coil
> to ground the same current should flow there. Otherwise assumings its a
> magnetic effect on the ferrite.....what about 2 coil diameters away at
least
> ??
>
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