At 10:57 AM 5/6/03 -0500, Tree wrote:
>I would be concerned about 8 element quads.
>
>I love quads. My first real antenna for 20-10 was a 2 element Cubex quad.
>It worked great. I have used many wire quads on 40 meters and they also
>work great.
>
>However, I had a friend with a 6 element quad and it never really worked
>right (it was on a 60 foot boom for 20 meters).
>
>After I saw some modeling for them, I stated to see why. The two element
>quad is a great antenna, but as you add more elements, it becomes less
>clear that you are really getting much advantage.
>
>Maybe someone smarter than me can explain why.
I'm not sure that I'm smarter than Tree, but here are some reasonably
well-informed observations.
I've modeled a lot of quads using good modeling software (NEC/Wires by
K6STI). Many of the published quad designs don't look very good because
the element lengths were guesses and not verified that well. It always
amazed my that in the old brown ARRL Antenna Book, one 20M quad would have
a 70' driven element and 72' reflector, while another would have a 71'
driven element and 73' reflector.
One big variable which I think traps a lot of quad builders is bare versus
insulated wire. I never thought there would be much affect to this, until
my good friend Jeff/N8CC told me of an experiment he ran. He and some
buddies built a 15M dipole out of insulated, solid Romex house wire, hauled
it up in the air and carefully measured the SWR curve. They then let it
down, and peeled off the insulation with a knife. When they hauled it back
up and again carefully measured the SWR curve which had moved 2% in
frequency! Same conductor, same coax, same end insulators, same height -
remove the insulation and the resonance moves 2%. So if you're going to
copy someone's quad design, make sure you know whether its designed for
insulated or bare wire.
IMHO, the definitive HF quad article was written by Terry/N6CW, Hardy/W7KAR
(don't know if he has a new call) and (I think) Glen/K6NA. It was in QST
in either 1976 or 1977. They used a commercial antenna range with scale
model antennas (up in the VHF range). Their goal was a 20/15/10 triband
quad, so they scale modeled and tested quads with three sets of
loops. Their final result worked very well.
My modeling of quads seemed to indicate diminishing returns as the boom got
longer with more elements. However, it was always possible to make a good
design, although the old adage "quads have 2 dB more gain than a yagi of
the same boom length" certainly was not evident.
I once sat with N6CW at a hamvention talking about his original quad
work. We came to the conclusion that the quad made a really good multiband
antenna, because of the loop geometry allowing the different bands to be
concentric on the same boom.
73,
Dave/K8CC
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