I have an AV640 vertical from Hy-Gain, which is about 18' off the ground at
the feed point. Overall I have been satisfied with its performance, which
has generally met or exceeded (apparently realistic) expectations.
My chief complaint comes with respect to QRO. When running a KW or more
into it (fed with LMR400, about 125'), my SWR will initially stay flat. I
am a CW operator and after two or three ragchew exchanges the SWR will begin
to increase a bit, and then suddenly go high (8:1 or better). It seems that
this happens more frequently if it is damp/has been raining.
Can you help me understand what is likely going on? Is this likely a
symptom of balun core saturation which is somehow related to heat, rather
than simply the power/magnetic flux?
Or is it more likely this has something to do with the RF choke from the
vertical element used to give a discharge path for static electricity? That
choke is 470uh, .3 amp. Is there any reason not to replace it with
something of greater current handling capability, and is the inductance
critical (would 500uh work just as well?).
Thanks for your replies.
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