Channel Jumper wrote:

"Sal M. O'Nella[_4_ Wrote:
;820890"]I try to take a new antenna to Field Day every year. This year
I decided on
the Moxon, AKA the Moxon Rectangle. I signed up for 10m Phone, figuring
I
could build that antenna and it would be small enough to be
transportable.
I began reading about the Moxon design a few years ago but this was my
first
time to construct one.

I drive a van. For a long-past antenna takedown party for the widow of
a
Silent Key, I made a wooden frame to add length to my roof rack, about 8

feet, total. The Moxon antenna is 12 1/2 feet long, so my frame should

work.

My Moxon structure is PVC pipe and the elements are #14 wire. The
dimensions fall out of a program called MoxGen and they will come out as
an
EZNEC .ez file. Nice. (I bought a licensed download of Roy Lewallen's

EZNEC for the occasion.)

I noticed a couple of things I didn't expect. One, setting the EZNEC
height
of the horizontally-polarized Moxon anywhere from 0.2 to 1.0 wavelengths

above ground seemed to affect only the radiation pattern in elevation
view -- including take-off angle, upward-pointing lobes. The resonant
freq
didn't seem to change much , which surprised me. I've found a common
dipole
to be quite sensitive to height vs. resonance. Does the addition of a
parasitic element, in this case a reflector, make the
height-above-ground
less influential?

Two, MoxGen seemed to produce a model that was too big . . . and EZNEC

seemed to agree. However, when I built the antenna, it came in around
27.600 MHz, not the 28.4 I asked for. My elements were all cut and
installed to tolerances of a millimeter or two, although I don't know
what I
should have had for bend radii -- I just shaped the bends around my
finger
so the measured bend point was halfway though the bend. After I checked
it
with the analyzer (AA-54) I trimmed the elements and I have a nice low
SWR
where I want it.

The Moxon is supposed to offer a few dB gain over a dipole. Its selling

point is a high F/B ratio, exceeding 15 dB. I can aim to Hawaii, for
example (from San Diego) and all of North America gets two S-units
quieter.
Whether 10m will be open for Field Day remains a question, I may sit
next
to the coffee pot for more time than I spend logging contacts.

Aside: I looked in EZNEC for a quick way to change the height of an
antenna
model and found no obvious signs so I just "arrowed-down" through the
"Wires" table and changed all the Z-axis numbers. With only twelve
entries,
it was not bad -- this time. Is there a hidden trick to quickly alter
the
height-above-ground an EZNEC model? Yes, I could email Roy but he has
other
things to do.

"Sal"
(KD6VKW)

I would venture to say that the reason why you had a problem was due to
the fact that you used plastic pipe and a piece of wire as opposed to
using aluminum tube.

That would depend on whether the pipe was only used as a support as
opposed to the wire run through the pipe.

The relative permittivity or dielectric constant of the copper as
opposed to aluminum tubing probably caused a mismatch.

Unless the conductor is very small, it would be difficult to measure
any difference between copper and aluminum and in any case the antenna
was modeled with EZNEC where one can select the material.

If the wavelength is too long - you simply cut it down to match the
frequency desired.

All a Moxon is - is a folded dipole antenna.

That's like saying a 7 element yagi is just stacked dipoles.

Its design causes it to have a little directivity, which causes it to
pretend to have some gain.

A proper Moxon has about 3.3 DBi forward gain and a F/B ration approaching
30 Db.

snip remaining babble



--
Jim Pennino