TECHNICAL FEATURE JULY 2010. RADCOMRON STONE, GW3YDX . E-MAIL: RON@VINECOM.CO.UK

The GW3YDX Super MoxonAdding extra directivity to the Moxon Rectanglefor 6m, 4, and 2m

PHOTO 1: Completed 4m GW3YDX Super Moxon.

MOXONFAN. The author isa keenenthusiastof the Moxon Rectangle antenna, inventedby the late Les Moxon, G6XN.ln its basic

form it's easy to build and is a great performer.The reduced wingspar.1 of the rectangle alsoappeals to the QTH-restricted UK radioamateur and it is a surprise that the antennahas not become more popular.

Spending much of the day in theworkshop, I wished to keep an eye (and ear)on the 6m band one June. Something isoften happening on six, so I wanted a simpleantenna to monitor the band. Although myneighbours are quite understanding of thestructures that go up from time to time tobe tested, this would be a semi-permanentantenna and thus something was requiredthat was not too visually obtrusive. A Moxonrectangle was built for 6m and put on a shortmast outside the workshop door. An FT-847(a trusty test radio for antenna projects) wasinstalled for monitoring the band.

Much interesting DXwas worked,

including a memorable evening in lateJune where more than 80 stations in the

USA were contacted, extending as far aswest Texas. This was just with lOOW froma poor VHF QTH, and there seemed nodifference in results compared with the(alleged) kilowatt-and-Iong-Yagi G stationson the band.

BASIC MOXON DESIGN. Making the Moxonrectangle was not difficult. There are someuseful design formulae available that arequite accu rate. The best one so fa r fou nd is

Moxgen (Figure 1), which allows input ofwiregauge and frequency and that results in adimensional guide. Moxgen is downloadableas free software from [1] and, most usefully,allows creation of a model in either NEC or

EZNEC formats for further experimentation.

Continuingto usethe Moxonrectangle,thefairly broadfrontal lobeof the antenna(seeFigure2) of 80° between3dB powerpointswassometimesnottoogoodin QRMwhen beamingto Europe.

IMPROVEMENT.Something better wasneeded,but how couldthis beachievedwithouta greaterwingspan,yetwithaworthwhile increaseingainfor nota hugeincreasein boomlength?Lyingin bed onenight and thinking about radio antennas(asonedoes)the ideacame to meof addingdirectorsto the Moxon,but in theformofanotherrectangle,ietwodirectorswiththeends bentbackandjoinedwith an insulator.Severaleveningswerethenspentmodellingthe antenna usinga combinationof 4NEC2and EZNEC+.Foran increasein boomlengthto just under2m (justoverdoublethe originalboomlength)and no increaseinwingspan,another3dB gainwasachievedfromthemodelling,with a 26.5dB frontto backratioand aVSWRof lessthan 1.5:1 between50.0 and 50.3MHz whenthe modelwasoptimisedfor 50.1MHz.

Figures3 and 4 illustratethe modellingresultsobtained.Notethedecrease in -3dBpowerpointsto 60° comparedwith80°forthe original.Thisnarrowingiswheregaincomesfrom. Frontto backratioofaround26dB isvery respectableandthe patternhasa niceclean'light bulb'shapeto it,withnominorlobes. Althoughmodellingindicatedthat a 380 feed impedancewas ideal,

;:: Moxon Rectangle Generator @

Frequency~ MHz \Noesizeji'2:7" ~

DrivenElement

I

'~~I1

11

A

Reflector

I ~e I ! .!ienereteModell:i -Fonnal-. : r. EZNEC

r NEC

-RetUb Unk---"'jrFeetr Inchesr Met",.r. :M5,"""'~

Emt

Closem__m ~_M m

-------

FIGURE 1: Calculating Moxon Rectangle parameterswith Moxgen.

A r 2150.1mm81 283.6mm

cl 101.5mm

DI 413.6mm

E j 798,7mm

I RADCOM. JULY2010

PHOTO 2: The 2m version of the Super Moxon

is just 30" x 25".

practical models have been fed with 500

cable and VSWR plots that follow the model

graphs have been obtained with just slight

adjustment of the driven cell element lengths.

VSWR bandwidth is probably a little narrower

than comparable Yagi designs, but at below

1.5: 1 for the most-used part of 6m is very

acceptable. The author has quite extensive

knowledge of Yagi designs and so far (this

tempts fate of course) has seen no 6m antenna

design with comparable performance on

such a short boom. A regular Yagi with the

same gain would need a boom length of

nearly 50% more and a turning radius nearly

double of this design. One can truly say that

this design packs more dB into its size than

anything else so far realised.

BUILDING THE PROTOTYPE. The next

stage was physical implementation, which

was done with aluminium alloy for the

elements and fibreglass rod for the splitdriven element and the element-end

insulators. This is a lightweight antenna,

easy for one person to manage, and only

a 1" square boom was required.Construction of the antenna was

principally with-W' aluminium alloy, withthe bent corners in 3/8" material. 3/8"

fibreglass rod was used for the elementinsulators both at the driven element centres

and at the element ends. The 6m antenna

has been tested with 1000W from an

ACOM 1000 without complaint. (Theinsulators on the directors on the 2m

version are only 12mm long but at the

50W power level no arcing or instability

was noted. Unfortunately no greater power

was available on 2m for testing,)

Table 1 gives constructional sizes for

6m, 4m and 2m versions, the dimensions

relating to Figure 5. The front part of the

driver element is of course split at the centre

and 500 feed line connected there througha balun.

Although the final dimensions are as set

out in the table, it is useful to slit the ends ofthe main element tubes and install stainless

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