1)
Slight Saturation in the core causing a loss in inductance at high power
perhaps?
Assuming the air wound and cored inductors were adjusted to exactly the same
values initially.
Or
2)
Direct coupling in the near field from your original air wound coils
influencing the coupling into the scope of your V and I pickoff, that is not
present in the closed torroidal cores.
Can you wind the power down and measure the phase difference which would
show up 1) but not 2)?
Andy G4JNT
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of J. Allen
Sent: 13 November 2006 12:57
To: [email protected]; LowFER at LW Cafe
Subject: LF: LPF coils and VY1JA to 1W ERP
Gentlemen,
During the amp repairs at VY1JA, the coils in the LPF were changed from air
wound to torroid core at the suggestion of one from our group as a possible
way of removing ringing from the transmitter. There was no difference in
the operation of the two, except that now there is a very slight phase shift
with the current waveform lagging the voltage waveform. The bottom line is
that the air wound coils produced a better LPF only because of this very
slight lag which is not apparent in the LPF using them. The inductances of
each coil was measured using an MFJ 257 analyser and was wound to the same
value. The current and voltage traces were picked up for the scope using
the same scope and scopematch coils, and measurements were made at the same
power level with the same transmitter, coax and dummy load connection.
Have any others seen this very slight lag on LPFs with torroid cores? Do I
now begin tuning the antenna slightly to correct for the phase shift which
comes from the LPF, so that the transmitter sees a purely resistive load?
Repairs to the interim transmitter at VY1JA are complete and the station is
transmitting now at license limit 1W ERP on ~137778.8 QRSS60. Again,
reports and captures are appreciated.
Thanks,
J.
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