[NLRS] Anyone using Quads?

[Dr. Gerald N. Johnson, electrical engineer]

Perhaps I've been bad impressed by a couple quads. Back about 1957, my
dad and I built a spider tri band quad, 15, 10, and 6 (didn't have a 6 m
rig though and with channel 2 TV it wasn't necessarily advisable). It
was supposed to have a front to back, but didn't. It was bidirectional
despite the time we spent up on top the telephone pole adjusting the
reflector stub. It did have a lot of front to side. I thought a waste,
my dad liked it. The three element 10m yagi we made by wrapping the
bamboo poles with aluminum foil, I think, worked significantly better.
After I left home, and he broke the big 20 meter beam we'd built
together (4" irrigation tubing for elements), he put up a Hygain HF
triband quad. Whether wind or hail, it needed repairs a lot and the new
rotating mast takes major engineering efforts with levers, cables, and a
sturdy winch to bring it down for those repairs. After one particularly
bad hail storm, he gave up on quads.

I'd think a 6m quad on a rover would suffer from wind while driving as
well as tree limbs. Can't really put the bottom wire AGAINST the truck
roof, so you are looking at maybe 8' + 5' for the top wire... That's
just a foot shy of typical bridge clearances and lower than tree
clearances on many roads, including my driveway.

NEC is easy to use with some of the front ends commercially available,
like EZNEC from W7EL. Though his entry scheme is handy only for antenna
elements that are parallel to the three axes. Round elements and
diagonal elements are a geometric pain. And I'm not to sure that
elements that don't meet in right angles are computed correctly.

I've not noticed the 45 degree response in the loop yagi I have up for
1296, maybe that's because I've not heard any signals on it that were
closer than 50 to 100 miles in range. The square loop puts the vertical
sides in phase at an angle of about 45 degrees, doesn't matter whether
its one element or 50. Its worse in a vertically polarized quad because
the sides are what are fed and coupled best. Even FM users around here
who have tried quads have noticed those big sidelobes. I can believe
that a poorly designed quad will have enough lower forward gain that the
corner lobes seem stronger. Most long quad articles I've seen, have
tended to use a more or less continuous taper. The multi element loop
yagis we use at 902 and above taper in groups. There ought to be a way
to compute them, but the geometry of NEC, I think, prevents good
computation of the circular loop and thus computer optimization.

Even with computer optimization, the gain (and G/T) has to be proven
with actual work on the air after decent results on the test range. And
the handiest test range for many of us is WB0TEM at a CSVHF conference.
Look for VHF quad results there. They never are great.

Perhaps quads and loop yagis could be optimized by creating special
purpose programs, especially for the circular symmetry of the loops. But
good math for the coupling gets daunting. Some of the non-NEC yagi
optimizers depend on an element coupling formula from W8JK's antenna
book that is only correct when the elements are parallel and close to
half wave resonant. That's why those tend to fail to show interaction of
elements so accurately for multi band stacks or interlaced beams. And
the longer the yagi, the more directors that operate further from half
wave resonance. Hence a yagi optimized with one of them needs further
optimization with a NEC package to achieve the claimed gain. But they
can be much faster at computing than any NEC.

Do the stray sidelobes hurt? Yes they do, if the receiver NF is really
good and not affected by the reciprocal mixing of the typical VHF
transceiver. Otherwise if the receiver effective NF is 10 dB from phase
noise and blocking from local FM repeaters, then an antenna NF of 3 dB
from the sidelobes looking at the ground (and the local vertically
polarized repeaters and pagers) isn't a problem.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.