Hello Richard,
is you antenna matched to 50 Ohm ?
If you just bring it to resonance (loading coild adjustment) the
antenna impedance can be anything between 10 and 100 Ohm.
And some 10m's length of coax can cause some odd impedance
transformations, even on 500kHz.
As I started on 500 I just tuned the loading coil to bring the antenna
to resonance at 502.5kHz. At the antenna feeding point I measured 40
Ohm what comes down to a SWR of 1.25. Fair enough I thought, my PA
would handle such an SWR without a problem. But when I fired the PA I
got less output current than expected and far more input power than
expected, so the PA seemed not so happy.
Next I measured the impedance at the TX end of the coax (40m long) and
found out that resonance was at 490kHz instead of 502.5kHz.
I went back outside to measure the impedance at the antenna feeding
point: still 40 Ohm at 502.5kHz resonance.
It is common knowledge: if the antenna impedance is different from 50
Ohm then the coax act as an impedance transformer. But I did not
expect such significant effect.
73, Rik ON7YD - OR7T
Quoting Richard Newstead <[email protected]>:
After my success in getting more current into the aerial yesterday I
decided to measure the RF current in the link (link coupled ATU) with a
proper RF ammeter. I was somewhat surprised that the current pinned my
1 Amp RF meter. How can that be? I can think of some possible reasons:
1) The meter is wrong (it must be nearly 70 years old)
2) The impedance is not 50 Ohms which is odd because I assume the TX is
designed to have a 50 Ohm output and I carefully adjusted the link for
maximum power transfer last night).
3) The transmitter is giving out far more power than expected (seems
unlikely).
Any ideas?
Incidentally, am I best to measure current in the link or in the aerail wire?
73
Richard
PS - beacon on again 501.6kHz.
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