Seductive serendipity / Verleidende serendipiteit

February 26th, 2015

145 MHz / 2m Moxon (pt. 2)

More than one year ago I published my interpretation of a 145 MHz / 2m Moxon
for ARDF / fox hunting purposes. The main advantages of a Moxon over a HB9CV
while foxhunting are its smaller dimensions and elimination of eye poking.
Last, but not least, it contains a direct 50Ω match.

I made this Moxon with junk material. Contrary to most designs, this antenna was
calculated and optimised with 4NEC2 instead of Moxgen.

Over the last year I received some questions concerning its dimensions and
calculated performance.

Below the radiation pattern, optimised for maximum F/B, and the dimensions are depicted
(click on images to enlarge and open in a new tab)

Of course the left figure above is the result from a computer experiment, so the
ridiculously high, almost 60 dB (F/B), notch is purely theoretical.
However, in practice the notch is very deep.

Two notches in the horizontal plane at around + and – 112° are present.
In practice these notches proved to be very handy while hunting as the
generic walking direction can be derived from minimal signal strength
more easily while ‘sweeping’ horizontally with the antenna to find foxes.

The -3 dB opening angle in the elements plane amounts ca. 70°.
Perpendicular to this plane this angle is around 150°.

To finish, below some more pictures from 4NEC2.
(click on images to enlarge and open in a new tab)

Pictures of the Moxon can be found at the end of the original article.

Edit: on April 26th 2016 PA1OKZ made a Moxon according to my design.
Below a picture of his measured return loss. Spot on !

Edit: on December 27th 2016 Wim PA3EJE brought his freshly built Moxon to our radioclub.
He made a 10 mm diameter version which I especially designed for our radioclub (PI4RCG).
Below a picture of Wims beautiful interpretation of building a rock solid and good performing Moxon!
(click on image to enlarge in a new tab)



January 16th, 2014

145 MHz / 2m Moxon

An antenna was needed after building a FOKZBox, a modern 2m fox hunt receiver.
Although HB9CV antennas are popular among ARDF/fox hunt enthusiasts,
more simple/better performing alternatives are available nowadays.

One alternative is the Moxon, described by Les Moxon G6XN in 1993.
elaborates on its effectiveness and ‘unbelievable’ (> 30 dB) F/B ratio.

Moxons (mechanical construction) outperform(s) a HB9CV, and… its width is ~30% less.
Imho the biggest asset of a 145 MHz Moxon is lack of eye poking!

Moxon calculators are online (see Google). However, I used 4NEC2. I found some 5 mm diam
‘scrap’ elements in my shed. One element transformed into the radiating element.
Another element was extended with Al-pipes (after tap wrenching) to serve as reflector.

The radiating element is fed through a common mode choke / 1:1 balun  (~50 cm RG58, coil wound).
The contraption was strengthened using RG213 PE-dielectric.
PE-dielectric may increase (capacitive) ‘Moxon coupling’ somewhat, but.. what the heck ; -)

The result has an excellent VSWR, F/B ratio, and deep notches around + and – 105° azimuth.
Pictures of my 145 MHz Moxon implementation are depicted below (your mileage may vary).

(Click on images to enlarge, open in new tabs)

145 MHz Moxon, top view.

145 MHz Moxon, side view. Notice the hand grip ; -)

Trespa feed point close up. A ~25 uH choke (‘anti static’,

Xl = ~23 kΩ) across the terminals was added later.