Geri DK8KW recently asked me how much current I get into my antenna. I
am ashamed to admit that, in all my 30 years as a radio amateur, I have
never sought to measure antenna current.
So the following describes how I set about calibrating a thermocouple RF
ammeter and the initial results when used to measure the current into my
loop antenna. (Sorry Geri, measuring the current into my experimental
vertical will have to wait a few more days!)
The 3 ampere meter used for the measurement was re-discovered in my junk
box about 3 months ago, and is marked:
PAT. FEB.21,'22.
THERMOCOUPLE TYPE
AMPERES R.F.
ULTRA HIGH FREQUENCY TYPE
Weston Electrical Instrument Corp.,Newark, N.J.,U.S.A.
MODEL 425
I calibrated the RF ammeter at 23 degrees C using a variable DC source
against a digital multimeter. I initially found the RF ammeter to be
reading 'high', but was pleased to find that a meter shunt of 0.33 ohm
resulted in a readout error of not more than 0.1 A over the range 1 to 3
A (relative to my digital meter). Even in the range 0 to 1 A (at the
compressed end of the meter scale), the meter gives a very useful
indication of current.
I then used the RF ammeter to measure the current in my loop antenna.
For details of the loop see:
http://www.cableol.net/alg/radio/136/loop.htm
The 65 m single-turn loop has a DC resistance of 1.7 ohm, but the loss
resistance of the ATU components is unknown. When running 18 W from my
transverter, I find that the RF ammeter reads 2.3 A when inserted in one
'leg' of the loop (at the ATU end). Although I don't know the actual
phase difference between the current and the voltage at that point, I
believe the figure of 2.3 A to be a reasonable approximation.
This initial result indicates the high currents involved when using loop
antennas on transmit - even at quite low power levels. It also
emphasises the importance of using low resistance conductors for the
loop. Indeed, my loop resistance of 1.7 ohm (DC) is rather high for an
antenna of this sort, and I will seek to reduce this significantly over
the next few weeks. For example, the loop resistance of the loop
antenna used by Dave G3YMC is less than 0.1 ohm. (For more details of
Dave's loop antenna, see:
http://ourworld.compuserve.com/homepages/sergeantd/loops.htm
In practice, I regularly run 400 W into the loop and so the current will
be significantly greater than 2.3 A under those conditions. Certainly
enough to fry the thermocouple!
Regards to all,
Steve GW4ALG
[email protected]
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