"I don't think on half wave is the real problem point"
Agreed. I looked at patterns for my 40 foot base-matched vertical:
http://www.n3ox.net/projects/lowbandvert
I found the same thing, for a 40 footer, 20m is OK, 17m has high
lobes. The cutoff point is somewhere between 5/8 and 3/4 wavelength,
if I remember. I didn't really care, I was going to go horizontal at
20m and up.
The lack of low angle response is fairly obvious on RX comparisons
here. I use a 20m delta loop on 20m and 17m with a matching network
at the antenna, and comparing the (unmatched, networks bypassed)
vertical to the delta loop shows a much bigger difference between the
delta loop and vertical on 17m than the delta loop and vertical on 20.
A little bit of that might be attributed to the slightly higher gain
of the 20m delta loop on 17.
Personally, I wouldn't go shorter. The 40 foot high antenna has
relatively little loss on 80m vs. a quarter wavelength antenna,
probably not more than a dB or 1.5 with a low-loss matching coil. It
will probably put out a weak signal on 160 with a good coil (-6dBi
modeled over the radials I've got). The lower bands are where
vertical-ness is really going to shine, at least in my experience. I
had a 17m ground plane once and it was a miserable antenna compared to
a low full-wave loop.
I'd look into other antennas for the higher bands... if you really
only can go vertical, though, and want to homebrew, there's probably a
decent combination of a handful of parallel wires that would make the
antenna work well on all the bands when base matched.
The SteppIR does get around the problem very elegantly; unfortunately
not in my price range right now, and I don't really need 17m and up in
my vertical.
73,
Dan
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