---------- Forwarded message ----------
From: "Raoul Coetzee" <raoulc@smmcape.co.za>
To: "Topband Reflector" <topband@contesting.com>
Date: Tue, 15 Apr 2008 10:03:27 +0200
Subject: 2nd Harmonic suppression
I am having a bit pf a problem with a second harmonic when operating
on 160m. This is of course falling in the 80m band segment and causing
mild to severe qrm to my local 80m friends on phone.
My SR is getting later and so this time of the year my 160m operation
overlaps with their local chats.
My radio is a FT1000 MP MKV Field. I believe that the radio is fb (by
on air testing) but that the harmonic suppression of my homebrew
linear may not be so good.. I use classical PI input networks for
matching the two 4-400 tubes to 50 ohm input and to reduce spurious
output. The output coupler is also a PI network. It is difficult for
me to determine the exact energy from the second harmonic but I use a
second receiver to monitor it during loading the amp and currently
tune for minimum second harmonic whilst keeping plate and grid current
within reason. I have also reduced the grid biasing voltage. It is
still a bit high, about -9 volts but the amp is used mostly for cw.
The idle current is about 50mA on keyed, no carrier input. I have
thought of using an 80m ¼ section of RG6 coupled to the feed point of
the 160m vertical as a series resonating circuit to 'absorb' the 80m
harmonic, but will this also radiate some of the 80m energy?
I suppose the best way is to reduce all harmonics before it enters the
tubes, then to make sure the output is clean by using a good band
pass filter. My difficulty is to build a decent low loss high power
band pass or low pass filter for 160m. Your comments are very welcome.
I would be able to borrow a spectrum analyzer for a day or so but at a
later stage only.
Regards,
Raoul, ZS1REC
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