TopBand: Shunt-fed tower - HELP !!!

[K3BU]

In a message dated 98-01-22 10:15:42 EST, GW3YDX writes:

<< I got some excellent reports yesterday from the East coast on my
 repaired shunt fed tower. But every now and then the amp would trip out. 
 Never done that on the dipole at 120ft (which is 2 s-units down on the
 vert). 
Also something very odd was happening. If I sent a string of dots, the
reflected power would creep up (and sometimes the "resonant frequency"
would creep up too) to the point where it would go up to amp cut-off
point in about ten minutes.>>

Looks like case of ampheataitis. Problem might not be with antenna (maybe
different loading settings on PA from your inv V) but with capacitors in your
PI network in the amp heating up and changing value. (Have that happening with
Ten Tec Titan) See if you can "chase" it with your amp  LOAD control. If so
that's it. Remedy: key it for while and tune it there. So when you transmit
you would "drift" into better setting. Otherwise mark two extremes and do it
manually as you go.

Otherwise could be the case of capacitor heating up somewhere in the antenna
matching section (ceramic door knob caps would do that - use air caps or
ceramics with good temp coefficient.) or sometimes ferrite chokes/cores can
overheat and drift in value causing change of inductance.
GL  Yuri K3BU, VE3BMV

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