Hello TopBanders!
In reply to my own posting a fortnight ago, I'd like to thank each and
everyone offering advice and insights.
It seems that each of the options (loading coils, lineair loading and traps)
to make a dual band vertical has its own fan club.
Given the fact that there seems to be no free lunch and in that respect that
you can't have a cake and eat it too, I settled for a coaxial trap. Either
solution has losses, but a trap does not need switching like a coil for dual
band operation would. And no relay means no control cable, which in turn
saves a small interface to switch the relay outside from within the shack.
That said I made a coaxial trap with thicker coax (H100) to cope with the
QRO, and wrapped it tightly around a PVC form with 125mm diameter. I did not
even get out my antenna analyzer. This option simply is too heavy to suspend
on the L wire. I tried but the sag is too big and the support to keep the
end of the L wire up in the air would suffer in the end.
There had been a little flame burning in the back of my head. So I unleashed
the carburetor and injected some fuel into the antenna brain. There is
another solution. A second antenna sharing the feed point and radial system.
Worth a shot. Luckily I have a big pile of scrap bits and pieces and
leftovers from previous projects. Here's what I came up with. I took some
pieces of 16mm PVC electrical conduit about 30-40cm long. I drilled holes
through those and used them as spreaders. That created two parallel wires
sharing a common antenna feed point. They go up the telescopic tower (with
pulley and rope) and up the tower they split ways. The 160m L goes to the
left and the 80m L goes to the right. For testing purposes the tower is only
2/3 up (15m). For contesting both tower sections will be cranked up (21m) so
the 80m vertical will be full size and the 160m L wire will be approximately
50/50 vertical/horizontal.
Sure enough the antenna analyzer showed two nice dip (160+80)s. In the shack
I found that the MFJ auto tuner at the antenna's feedpoint accepted the
impedances and took 900W on both bands. That's the good news. Now the better
news: with some pruning I think I can get both bands to work WITHOUT the
MFJ-998. That would be super. However I now have a working concept. A simple
concept. No coils, no traps to burn, no relays to arc.
I had a ball last year on 160m with a totally random length system 'fooled'
into 50 ohms by the autotuner. I worked into USA almost daily and worked a
lot of new ones outside EU. I hope to do even better this year with the
'nearly resonant' antenna. It may be far from the textbook figures for
efficiency and take off angles but I think this is the best I can do given
my location, budget and size of my garden.
Now if I could only squeeze in something for better RX. A K9AY loop fits but
would be crammed and jammed between all the other antennas, which is not a
good thing.
73 de Franki ON5ZO = OQ5M
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