You don't say if the SWR of this antenna changes when this happens. I
wouldn't look towards the antenna as the only cause - some amps have
coupling caps that can't take the current on lower frequencies and drift in
a manner much like you've described.
73 Don
VE6JY
On Sun, Dec 7, 2008 at 11:20 PM, Tod -ID <tod@k0to.us> wrote:
> In the ARRL 160 meter Contest this year I used an antenna that I have
> experimented with off and on during 2008. It has a vertical element 57 feet
> high insulated from ground. The feed point is at the base of this vertical
> element. At the 57 foot point a 55 foot horizontal wire is extended as a
> 'top hat'. Very much like an inverted L you might say.
>
> I added an 80 meter trap in the horizontal wire -- at a point 22 feet from
> the connection to the vertical element. The trap is made from a 30 uH coil
> and a 60 pF capacitor in parallel. This combination provides excellent
> matching for the 3500-3660 kHz part of the band and good matching for the
> 1815 to 1845 or so.
>
> I found an interesting effect when I used the system with high power [1.0
> to
> 1.5 kW]. After a very active operating period [lots of answers to CQ's] the
> power output from the amp began to drop. Eventually, the power dropped to
> about 500 watts. At the 500 watt level there was some noticeable mismatch
> which I presume caused the setting of the plate tuning circuit to be
> incorrect and as a result the power output dropped. If I paused a bit the
> power level when I resumed was close to the initial power.
>
>
> After fighting this on Friday night I removed the 80 meter trap and
> replaced
> it with a large AirDux coil with the same inductance. I figured that maybe
> the capacitor in the trap [it was not mica or vacuum or air dielectric] was
> experiencing dielectric heating and that might affect the input impedance
> of
> the antenna.
>
> Saturday night I saw the same effect as I did on Friday night. At this
> point
> I am speculating that the coil is heating sufficiently to cause changes.
> The
> coils are about 3 inches long, 2 inches in diameter and use bare #15 wire.
> The trap coil is wound on an epoxy type grooved form. The AirDux coil is
> essentially air wound with small spacers.
>
> Anyone else have a similar problem historically? Anyone feel they know with
> certainty that a resistance change in a coil will cause this problem? Is
> there a chance that the inter-turn capacitance is changing significantly
> and
> detuning the coil? Any other ideas that might help understand and solve
> this?
>
> Clearly I can discard the trap and do the matching at the base, but I would
> rather use the trap because of the simplicity.
>
> The "good" news is that the temperature is dropping and by the Stew Perry I
> can expect -10 C which should give longer CQ runs before the power drops.
> There is no similar problem on 80 meters.
>
> Tod, K0TO
>
>
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