> I modelled a two-element 5-band quad with each element fed with 1/4
> wavelength 75 ohm coax to a switchbox. An EZNEC model showed a bad
> affect on performance when the unused 1/4 wavelength lines were
> shorted at the switchbox. When the unused lines were left open at the
> switchbox, there was little interaction with the unused elements.
Shorting a 1/4 wl line at the far end presents a very high impedance at
the antenna and, opening the quad loop. An open at the far end presents a
very low impedance at the antenna junction end, essemtially closing the
loop.
Extensive modeling studies on 5-band 2-element quads shows that the loops
need to be closed when not in use for minimal interaction. Opening the
driven element loops when not in use presents extensive interaction. As
well, for a 5-band quad, or even a 3 band or just a 10-20 meter quad,
separate feeds are needed to prevent serious pattern distortion. The 1/4
wl match line (or even just any line) from the driven element loop to the
remote switch is a good way to effect the closure electrically while
leaving the loop "physically" open. The study, incidentally, will appear
in some distant future issue of Communications Quarterly. If I owned a
GEM or Lightning Bolt Quad, I would seriously think about rewiring it for
separate feed. If I opted for a Cubex, I would choose the separate feed
option over the transformer (which apparently is designed to effect some
isolation). If I opted for the Antenna Mart-Bill Wall quad, I'd follow
instructions. If I built my own, . . ., well, you get the idea.
Incidentally, with a 5-band quad, loop closure is essential for the 10-12
combination, as an inspection of loop sizes will show. No common feed on
that combination.
And NEC-2 will model quads very well if all wires are the same size (for
example, #14 AWG). A good starting point for segmentation is 7 for 10 per
side, raised by 2 per side as you move down the bands (15/side for 20),
which will keep you within the 500 segment limit for programs like EZNEC
and keep you inside upper and lower segment limits of the calculating
engine. Concentric quads (spider hub idea) will show a fairly constant
impedance for all the interior quads, although the end ones (10 and 20)
will be off and may require a bit of special attention to spacing to bring
them into "perfect" alignment with the interior loops.
-73-
LB, W4RNL
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