Hi Bill,
I think you may be misunderstanding a quad antenna:
A quad element is not a bent dipole or a shortened element in any way. It
is a full-wave loop, an antenna that by itself exhibits gain over dipoles in
directions perpendicular to the loop. The calculation for its length (or
circumference, since it's a loop) is 1005/f(mhz), vs. a dipole, which is
468/f(MHz). So clearly, there is more than twice the amount of element. So
an element centred on 14.2 MHz would have 70 feet, nine inches of wire
(forming a box approximately 17.5 feet on each side), as opposed to a
dipole, which is 33 feet long.
If you put a reflector behind that (or a director in front), what you're doing
is
putting another full-wave loop (plus 5 per cent length for a reflector, minus
5 per cent for a director) to parasitically alter the pattern of the driven
element just as a yagi element steers the signal from a dipole antenna.
Also, if you build the quad using a spider hub (so the spreaders extend
out from a central point at angles instead of from the end of the boom
perpendicular to the boom) you can place each element on a multiband
quad at an ideal spacing to maximize gain. (If you Google Gem Quad, you
can probably find some good pix to illustrate the spider hub idea.)
It is generally accepted that a 2-element quad will perform comparably to
a 3-element yagi.
There are antennas on the market that look like quads (the Butternut
HF5B is one), but those aren't actually quads. The wires on those
antennas are actually used to resonate otherwise short elements on
various frequencies. So in essence, they are loaded dipole antennas.
Full-size quads are actually formidable antennas, not just because of their
construction difficulties, but also because they work. A quad element is
generally quieter than a dipole because both feedpoint terminals, at DC,
are at the same potential.
Is it possible you've seen an antenna like the HF5B and mistook it for a
quad?
73, kelly
ve4xt
> From: Bill Fuqua <wlfuqu00@uky.edu>
> Date: 2005/11/28 Mon PM 02:03:07 CST
> To: towertalk@contesting.com
> Subject: [TowerTalk] ALTERNATIVE TRIBANDER
>
> 1. A quad is a compromise in a since because it is the same a 2
stacked
> shortened yagis with a separation of only 1/4 wavelength.
> It can be viewed as two 1/4 wavelength yagi driven or parasitic
> elements with capacitive loading from the 1/8 length ends turned up or
> turned down, which also serve as the feed lines to the upper half of the
quad.
> 2. By what magic does a quad separate RF atmospheric noise from RF
signal?
> The idea that a quad is quite seems a bit strange to me.
>
> 73
> Bill wa4lav
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless Weather Stations", and lot's more. Call Toll Free,
1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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