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Build a 17 Meter Reduced Size Coaxial Moxon
A good use for the MFJ-259B.
If you have tried the Moxon Generator
program to design a Moxon Rectangle, then you are familiar with the
Moxon antenna. For those of you who are not familiar with the Moxon,
it resembles two letter "U"s with the open ends almost touching but
is squared off at the bends. One director and one reflector, each
with tails pointing at each other. This antenna can achieve a
respectable gain of about 3 to 4dbd and good front to back ratio and
is usually made from wire or small diameter tubing. It is usually
designed and operated on the higher HF bands due to the physical
sizes of HF antennas on the lower bands. A standard Moxon
rectangle designed for 17 meters would be about 19 3/4 feet wide.
Using these plans provided by N0KHQ, the same Moxon retangle is
reduced to about 11 feet wide. Man that's a big
Unfortunately the MoxGen program was written only for normal
wire sizes. In order for the program to generate values for
different types of coax cable, the author would have to investigate
different coax types. It could be done, but would require a lot of
work. Here is a trick I have learned about
using coax for antenna elements utilizing an antenna
If you have an MFJ 259B or better, plug a coax tee
into the top, terminate one side of the tee with a 50 ohm resistor,
cut a piece of RG-58/U VF .66 using the standard formula 234/freqmhz
for one side of a dipole. You will find after following the
instructions below that it is too long and will have to be
example we will use 18.050). Connect the center conducter to the
other side of the tee and the shield to the ground side of the tee,
leave the other end of the coax open. You will notice that when the
MFJ is adjusted to resonance (50 ohm,X=0 and1:1) it will be
somewhere around 17.6 or 17.8. Divide that number by the desired
frequency (18.050) you will probably end up with something like .90,
take the length of the coax and multiply it by .90, then cut
off the valued displayed, check for resonance again....repeat as
required until your coax reaches resonance at 18.050. Note, I always
leave mine a little long to make connections to dogbone
insulators. I have built a 75m coaxial inverted "V" using the
same method. Works great!
formula for using coax instead of wire for shorter
984 x .66 = 650/18.050 =
36' / 4 = 9' x 1.2 = 10.8'. Now start testing with the MFJ 259B or
the old trial and error method with an SWR meter.
The Coaxial Modified Moxon Rectangle
explanation by N0KHQ/St. Louis
MFJ 259B or better, connect a coaxial tee to the analyzer, on one
side of the tee install a 50 ohm termination resistor, the
other side of the tee will be connected to the coax under
We will cut a 1/4 wave length piece of coax
to resonate at 18.050. When I say resonate, I'm saying that when we
are done trimming the coax, the following will be displayed on the
MfJ 259 = 49 to 52 ohms, 1:1/0 swr and X=0 or real close to
Cut a piece of RG-58/U PE
VF.66 10' long. At one end of the coax trim back a couple of inches
of the PVC jacket, separate the shield from the center
conductor, this end will feed into one side of the coaxial (center
to center and shield to ground) on the MFJ. At the other end of the
coax, remove a couple of inches of the PVC jacket slide the shield
back, strip 1" of the PE from the center conductor, short the shield
to the center conductor.
Turn the tune knob on the MFJ to
18.050, you will notice that the piece of coax is not resonate (too
long). Turn the tune knob slowly down in frequency until the display
reads 49/52ohm, 1:1 SWR and X=0, you should end up somewhere around
Use this formula to determine how much of the 10'
piece of coax you will have to trim off.
This procedure is
covered on page 20 of your Mfj 259b operator's
manual. In this example:
16.200/18.050 = .897. Take 10' x .897 = 8.970' this is the length
that your 10' piece of coax should be trimmed to resonate at 18.050.
Check the frequency again with the MFJ, repeat trimming as
necessary. When you have resonated your piece of coax, only
the center conductor of the driven elements (remove shield) will be
connected at the feed point, the other end will remain
How about that, an electrical 1/4 wave length
with over a 30 percent reduction in size verses using AWG. You
will need 2 of these for the driven element. The reflector element
will be .05 percent longer (lower in frequency), X will = 0 at
17.150 (both ends of coax are open under test). When installing the
reflector element short the center to the shield on both ends. The
element spacing between the driven elements and the reflector
element can remain at 7'. When fabricated and
tuned at 9 feet (almost 1/4 wavelength at 17m) the coaxial
antenna produced the following readings: (using the Mfj 259b), (some
trimming or lengthening of the driven elements may be
|R = 59
||R = 52
||R = 51|
|X = 6
||X = 0
||X = 6
power handling capabilities of this antenna is approximately 500
this is due to the limitations of RG -58/U PE
In my opinion, the mark of a good
antenna system is how well it receives signals.
this antenna at this QTH was done over a period of a few weeks. The
receive of the coaxial antenna was compared to the receive of my All
Band Sterba Curtain up at 50'. Because the Coaxial Antenna is
only up at 9 feet, the take-off angle is pretty high, as a
result the receive is a couple of "S" units down from the
The Coaxial Antenna
(weight 7 lbs.) should be raised to an operating height of 34
feet to 40 feet for optimal performance. A take-off angle
of about 18 deg. To 15 deg. can be expected.
Click here for a large
Its a lot of fun, there is no HF antenna that cannot
be built with a 30 to 40 percent reduction in size.
very, very light weight antenna. With this kind of reduction in
weight......now we can start
them............mmmmm......mm. What a DX antenna that
would be.....I can see the gray line already.
Talk to you
Thanks for keeping the Sterba on your site. Hams are building those things lke crazy and
blown away at the performance!
N0KHQ St. Louis