Hi Jay as the antenna is now "tuned" using the lower powered rig, there
should be nothing in circuit to affect the scopemeter that was not present
at lower power. There is the possibility of saturation in the matching
transformer at the end of the coax, but I dont think that is likely to be
significant. Laurence's thoughts are interesting and ground loops with the
scope could be the culprit with the higher field strength despite the
greater distance to the antenna.
There is one other effect that will come in only as you increase power and
could affect the waveforms on peaks..... that is insulator loss, corona
discharge. At the higher powers you are now running, you will need to be
certain that you do not have any tight bends in the antenna wires and that
all end insulators are fitted with anti-corona rings. Sudden bursts of
discharge current at the peak voltage point on the waveform could possibly
give some of the effect you see. Jim who has run 1kW is more likely to able
able to comment on that possibility, and provide suggestions on anti-corona
rings.
Cheers de Alan G3NYK
----- Original Message -----
From: J. Allen <[email protected]>
To: LF (RSGB) <[email protected]>
Sent: 31 July 2006 07:57
Subject: LF: Antenna or transmitter tuning
> Hello again Friends,
>
> The single FET amp which Steve lent me has been working fine, but if you
> remember, during tuning the antenna system would come up with some weird
> wave shapes. The amp uses an air core output coil. The antenna loading
> coil , its matching coil and both coils in the transmitter's Low Pass
Filter
> are air core as well. So there is no iron in the working system.
>
> Remember that when the antenna was tuned, one suggestion was to place a
> matching transformer with TV Horizontal Ferrite core in the circuit. When
> that was done, the waveform on the scope turned to garbage. (high in odd
> order harmonics)
>
> Today, the two FET amp which Scott lent me was put on the air. It has a
> ferrite TV output core which I wound and even though it was placed at the
> exact length of coax from the antenna which Steve's amp had been, it 's
> output waveform is badly distorted.
>
> It is important that I run the station at the maximum allowed IERP for the
> sake of proving non-interference.
>
> On Scott's amp, the current and voltage waveforms are identical and in
> phase, the amp runs very stable at over 300 Watts input, but the waveforms
> look awful. I have decided to switch back to Steve's transmitter and
> revisit both the output of Scott's amp and the antenna system. I am
> considering winding a air core transformer for it, but am unconvinced
doing
> so will solve the real problem.
>
> I have a scopematch, and scope, an rf source capable of 18 kHz and up.
>
> Any ideas on how to approach this waveform problem?
>
> J.
>
>
>
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