Hello Hugh,
The way I calibrated mine :
Connect the TX to a 50 Ohm dummyload, current meter between TX and dummyload.
Measure the voltage over the the dummyload, either by using a suitable RF
voltmeter or a detector and DC voltmeter.
The detector is no more that a diode that rectifies the AC, followed by a
RC parallel network to ground (I used 100kOhm/1nF) . The diode should be
fast enough (not a big problem at LF, MF and lower HF) and be able to
handle the voltage (that can be a bit of a problem for higher power, for
10W the Vpp is 63V, for 100W it is 200V and for 1 kW it is 630V).
As I wanted to use a scotky diode out of the junkbox (that could withstand
80V = max. Vpp) it meant that Veff was limited to 28V. But my 450W PA would
give 150Veff (= 425Vpp), so over 5 times too high. I solved that by putting
a 10/1 divider in parallel with the dummyload and rectified the divided
voltage, that way I was save up to 280Veff (= 1.5kW). The resistive divider
was 5kOhm (made of 3 x 15kOhm/2W) and 560 Ohm/1W.
I wanted to calibrate my meter in the range of 0.5A - 3A, that is 25 -150V
over the dummyload (or 12.5 - 450W into the dummyload). so at the divider
output I had 2.5V - 15V and 3.5V - 21V at the detector output. To be exact
one shoud add about 0.4V for the drop over the diode and calculate the
current through the dummyload 'all the way back'.
eg : 7.2Vdc measured = 7.6Vp = 5.37Veff at the divider output = 53.7Veff
over the dummyload = 1.07A
73, Rik ON7YD
At 10:33 15/11/2002 +0000, you wrote:
Hi All,
I built the little antenna current meter shown in the LF Experimenters
Handbook last night (Page 81).
And it works. I measured the current from my HF rig into a dummy load at
1.8MHz, and it gave sensible results ...
But how can I calibrate it - how do I know that an indicated Amp really is
an Amp.
My DVMs, being intended for mains current measurement, don't have the
frequency response.
How have other people done it ? Any ideas ?
73
Hugh M0WYE
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