In the weeks before CQWW SSB I installed my first 80m 4SQ. I am mostly a
CW guy and want the antenna to work well at the bottom of the band.
The vertical elements are Titanex V80S verticals which I bought having an
80m 4SQ in mind some 15 years ago. It takes me a while to get around to
things at times! The V80S is set for mid EU band i.e. 3650kHz and without
adding extra alloy or cutting some off there is no facility for
adjustment. I started out with the idea I would rig the 4SQ for 3650 and
see how effective it turned out to be at 3500 and 3800. The sides of the
square are 20.54m. The four 1/4 wave phasing lines have been cut for 3650
and the array is fed via a home built hybrid coupler.
4SQ experts will likely have already spotted my mistake. I overlooked the
inevitable rise in resonant frequency of the vertical elements due to
mutual coupling. Anyway in some respects this was kind of lucky or would
have been had we not had serious electrical storms last weekend.
First tests were very promising but with best results at 3800 not 3650kHz.
F/B was a good 20dB and F/S not much short of that. Power dumped to the
50R load was around 2% but rising to around 15% at 3500 where both F/B and
F/S were significantly less impressive at around 10-12dB.
With CQWW SSB visible now only in the rear view mirror I have turned my
attention to shifting the array down band. I reasoned I would need to pull
the individual vertical resonance down by around 250kHZ to 3400kHz for
optimum operation in the CW band. Taking down each vertical then adding
further alloy would be a far from easy option so I have instead added a
small amount of base loading. In fact 6 turns of wire around the bottom
30mm diameter insulator on each vertical did the job. The effect of the
adjustment is as follows:
Power dumped
3500 = 5.0%
3580 = 3.5% (minimum)
3800 = 11.5%
The above looks acceptable but....
F/B and F/S are around 10dB across the band so far as I have been able to
determine. So to my question..........
What step do I best take to improve the F/B and F/S in the CW part of the
band. My options appear limited to adjusting the L&C in the hybrid and
adding length to the 1/4 wave phasing lines. My gut feel tells me to hit
the hybrid first. Does this make sense or is there something else I am
overlooking?
Advice from the experienced will be much appreciated.
Thanks.
73 Bob, 5B4AGN
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