John, designed this wonderful vertical reversible 20
meter moxon: Here's his story.
Reversible Vertical Moxon for 20M
decided to try a vertical moxon rectangle at my new
QTH which has limited rear garden space. The rear garden
runs roughly NW to SE, so a reversible moxon gives useful
central mast consists of two 9 metre fibreglass fishing
poles, tie-wrapped together for extra rigidity, and
fixed into the ground via two PVC waste pipe sections
set into the lawn. For the cross-pieces I originally
used pairs of 8 foot varnished garden canes. However
I recently discovered at a local £1.20 shop, some 2.5
Metre fibreglass fishing rods at half price, so I bought
six of these at 60p each, to replace the canes.
whole assembly is quite light in weight. I have it guyed
with polypropylene cord, and there is very little movement
even in strong winds.
used 3mm bare copper flexiweave for the elements. The
left hand element is cut to act as a driven element.
It is end-fed via a LC tuned circuit mounted in a plastic
box at the base of the antenna. The right hand
element is cut as a director. On the right hand end
of the central cross-piece I have mounted a plastic
pill box which houses a small dual pole C/O reed relay.
Applying 12V to the relay will insert an extra length
of wire (a stub) which converts the director into a
reflector, thus reversing the direction of fire. When
de-energised, the stub is shorted out. This stub is
just a loop of insulated wire attached to the centre
crosspiece. Its length is adjusted for best F/B. I found
a total length of 120cm about right. The 12V is fed
via a length of thin microphone type coax, which runs
along and down the director side guy rope. This way,
it has negligible effect on the polar diagram or tuning.
LC circuit consists of about 19 turns of 16 gauge enamelled
or cotton covered copper wire wound on a one inch dia
teflon rod. The inductance is about 2.5uH which resonates
on 14.2Mhz with around 50pF of parallel capacitance.
Wound on top of the “cold” end is the secondary winding
of 3 to 4 turns of insulated stranded wire.
tune the antenna, connect a VSWR meter to the output
of the LC circuit, briefly apply a low power carrier
(5W) and adjust the capacitor for minimum SWR. A sharp
null will be found. If the minimum is not low enough,
then add or subtract one turn from the secondary winding.
have been getting excellent reports with this antenna.
The F/B is on average 4 points, and often much
a lovely feeling to be able to instantly reverse the
direction by flicking a switch in the shack. One of
my first CQ calls received a reply from CO6LC in Santa
Clara, Cuba. He gave me 58 with my 100W, so I knew it
was working. My QTH is in a valley, 50 feet ASL with
rising ground to the west and gently falling ground
to the SE and the sea half a mile away.
had a go at modeling the antenna using the demo version
of eznec. Of course, if I could plant this antenna next
to seawater in a portable situation, I would get a take-off
angle of around 8 degrees and a forward gain of about
7dBd. Now, my back garden in Wales in the winter is
permanently waterlogged, so I modeled it using very
wet farmland type ground, with the bottom of the antenna
at its actual height above ground of 80cm. Any lower
than this has a negative effect on the F/B. The result
compares well with practice. I found the optimum F/B
was obtained with a reflector stub length of 75cm. In
practice, I ended up using about 60cm (120cm total wire
think the only other way I could get similar low take-off
angle would be a two or three element beam at least
40 feet AGL, which in my location would be impossible
and a lot more ugly. This spring I may try the alternative
approach of having both sides cut as driven elements
with a relay on each side to add a reflector stub to
each side in turn. This will also need a relay in the
plastic box to switch the feed. I may also try using
insulated wire for the elements. Since they will be
5 percent shorter due to velocity factor, I will have
a bit more room to raise the bottom a bit higher, however,
I am loath to disturb this antenna since it is working
thanks to SM0DTK
and others whose web-pages helped to inspire me to build
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