I have been using quads an and off for about 25 years. I currently have
a six bander: four elements on 10-20 and seven elements on six meters
on a 24 foot boom. The HF bands are fed via a remote Ameriton switch so
that there are separate feeds to each driven element. The six meter
quad also has a separate feed. With a four element quad, the feed
impedance is closer to 50 ohms so that 1/4 wave coax matching
transformers are not necessary. However, the coax links between each of
the remote switch terminals and its corresponding driven element should
still be 1/4 wavelength long. The switch end of the coax links for the
inactive bands are left ungrounded, which effectively (electrically)
closes the inactive driven element loops. Frankly, I am not entirely
clear on the theory here, but there is definitely less interband
interaction and the behavior of the quad is less sensitive to small
changes in relative loop separations, minor dimensional errors, etc. (I
have done it the other way - random length coax links and grounded
inactive terminals - and, believe me, the differences in behavior are
major.) The use of 1/4 wave links has been confirmed and recommended by
K2OB, W4RNL and EI6BA, all well known quad aficionados. I strongly
recommend separate feeds for a multiband quad.
As far as mechanical strength is concerned, the latest fiberglass
spreaders are much sturdier than what used to be available in the past.
I get my spreaders from Lightning Bolt. I hear Max-Gain also supplies
high quality spreaders. It is my understanding that the Gem quad was
designed for Canadian winters. It is trussed so that if there is a wet
snow or ice build-up, the spreaders are prevented from bending and
breaking. Although my quad is not trussed, it has been up for 2-1/2
years now without any problems. I live on a hilltop and the quad is
constantly buffeted by high winds without any ill effects. The biggest
problem is wet snow and ice coupled with high winds. When I know that
kind of WX is coming, I lower the quad as close to the house rooftop as
possible and pray that heat escaping from the house will minimize any
heavy build-up. So far, so good. I have 200 lb. test fishing line that
I plan to use to truss the quad, but I'm a hopeless procrastinator and
will probably wait until after the next ice storm.
Generally speaking, while the quad is less sturdy and more vulnerable to
the elements than a Yagi, it's just not that much of a problem in
practice. Admittedly, it's an ugly looking, 3D object up there, but a
four element, five band quad on a single support is at least the
equivalent of five three element, monoband Yagis. That's tough to beat.
73, joe
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