Message-ID: <20090515183514.mt168ada0ug4k4c4@webmail4.kuleuven.be>
Date: Fri, 15 May 2009 18:35:14 +0200
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: rsgb_lf_group@blacksheep.org
References: <4A0D91E3.9040602@btconnect.com>
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Hello Richard,

is you antenna matched to 50 Ohm ?
If you just bring it to resonance (loading coild adjustment) the =20
antenna impedance can be anything between 10 and 100 Ohm.
And some 10m's length of coax can cause some odd impedance =20
transformations, even on 500kHz.

As I started on 500 I just tuned the loading coil to bring the antenna =20
to resonance at 502.5kHz. At the antenna feeding point I measured 40 =20
Ohm what comes down to a SWR of 1.25. Fair enough I thought, my PA =20
would handle such an SWR without a problem. But when I fired the PA I =20
got less output current than expected and far more input power than =20
expected, so the PA seemed not so happy.
Next I measured the impedance at the TX end of the coax (40m long) and =20
found out that resonance was at 490kHz instead of 502.5kHz.
I went back outside to measure the impedance at the antenna feeding =20
point: still 40 Ohm at 502.5kHz resonance.
It is common knowledge: if the antenna impedance is different from 50 =20
Ohm then the coax act as an impedance transformer. But I did not =20
expect such significant effect.

73, Rik  ON7YD - OR7T

Quoting Richard Newstead <g3cwi@btconnect.com>:

> 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 wi=
re?
>
> 73
>
> Richard
>
> PS - beacon on again 501.6kHz.