Moxon antenna

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70 cm moxon made from 6 gauge wire

The Moxon antenna (invented by Les Moxon (SK 2004), G6XN) is sort of a two element yagi with the elements folded and spaced to eliminate the need for an impedance match. The moxon can be further simplified by also eliminating the yagi's boom support.

A carefully optimized and constructed moxon can easily exceed the gain and bandwidth of a sloppily designed and constructed yagi. Also, the moxon is smaller than the typical yagi and much smaller than the typical J-pole antenna.

This design is based on L. B. Cebik, W4RNL's Moxon model (see calculator at the end of the linked page). Note that wire diameter is critical. If you use something different, recalculate the values.

We are also working out the details of a copper pipe version of this. Hard part to find for both antenans is the spacer between the elements.

The club recently built a 6m version of the moxon out of wire. Eventually we will post details of that, but several of the parts were manufactured (makerbot, machine shop). Contact me if you are interested in this, and it'll motivate me into working on writing up the plans anyway.

Contents

[edit] Radiation pattern and gain

  • Gain is typically 7-14 dBi, depending on accuracy of construction
  • Although this is a directional high gain antenna, it has a very large apature (about 100 degrees) with a high front to back ratio.
  • This antenna may perform much better than a poorly tuned three element yagi, but the yagi is still a better direction finding antenna, as the Moxon's forward lobe is nearly flat across the entire apature.
  • If pointed directly up, this can make a decent sattelite antenna. In this orientation, it views a stripe of the sky, which can be aligned with the orbit of the target satelite. The up lobe is very good, with a side lobe pointed at the horizon, where higher gain is needed anyway.

[edit] Construction

Please note!! These construction details (as well as the dimensions below) are preliminary. Details are still being worked out, this is being used as a scratch pad as we go.

The designs here are for flimsy light weight moxons designed to be carried around or used inside. With some care, this can be hung from the ceiling on a piece of string using a thumbtack or a paperclip. The ideas below are for a 6 gauge wire moxon (70cm or 2m) although the picture on the right shows a 1/2" copper pipe moxon.

A more durable (or larger) moxon might use pipe instead, or PVC as a support for 12 gauge wire, as depicted on Cebik's page above. 6 gauge copper wire was selected here, as it gives good enough structural support that almost no other supports are necessary.

2m pipe and 70cm wire moxon

Alternatives are still being explored for spacers and feedpoint attachment.

  • spacers alternatives (dimension C)
    • long tube
      • fill with epoxy or wedge with heat shrink to secure
      • wooden skewer to hold distance inside tube
    • plastic block
      • drill holes in ends
      • heat wires to melt to correct positions
  • feed attachment and support alternatives
    • pannel mount BNC; solder to base directly on one side
    • (70cm) feed at right angle to plane of antenna to minimize distortion?
    • (70cm) try facing feed towards reflector
    • lexan plate to support wires and coax connector
      • bend wires through holes in lexan and melt in place
    • (70cm) lexan plate across feedpoint and reflector
      • feed coax through and secure near middle of reflector
    • sandwich wires between two sheets of laxan, melt to fit?
    • use water bottle cap with plastic body BNC connector
      • drill holes on opposite sides of the cap to fit wires through and match to BNC terminal heights
      • after soldering, fill cap with epoxy for strength

[edit] Dimensions

Moxpage-1.gif

Dimensions are for 6 gauge wire or 1/2" copper pipe. Note that when measuring the 70cm elements, include the wire diameter(!) on corners in the element length, and measure to at least the nearest half mm. Dimensions for the 1/2" pipe are already adjusted for pipe diameter.

freq units A B C D E A/2+B 2D+A A/2
146 mhz
6 gauge
in 29 3 7/8 1 3/8 5 9/16 10 3/4 18 3/8 40 1/4
cm 73.8 9.78 3.44 14.2 27.4 46.68 35.75
442 mhz
6 gauge
NOT ADJUSTED
in 9 17/32 1 19/128 9/16 1 55/64 3 73/128 5 117/128 13 1/4
mm 242 29.1 14.3 47.2 90.6 150.1 336.4
146mhz
1/2" pipe
inside dims
in 28 1/4 3 1/4 1 11/16 5 3/8 10 1/4 14 1/8
cm 72.0 8.15 4.34 13.7 26.14 36.0

[edit] 1/2 pipe calculations

2m (folded) pipe and 70cm wire moxon
  • sample without elbows: 4.47cm
  • sample with one elbow: 6.35cm
  • sample with two elbows: 8.01cm
  • 1/2" pipe (outside) diameter: 0.625 in

Dims in () are fittings.

Elbow center to pipe
0.875
coupler edge to edge
1.0625
coupler edge to second pipe
0.5

(dimensions in inches)

freq A B C D E
center to center 28.8 3.37 1.79 5.63 10.79
outside 29.42 4.00 1.79 6.26 11.42
pipe lengths (3x2-fold) 8.87 + 10 + 8.87 + (1.0625x2) + (1.25x2) 2.43 + (0.875+0.5) 2.75 4.69 + (0.5+0.875) (same)

pipe lengths

# length (in)
copper pipe
A 4 8.87 8 7/8
Ar 1 10 10
Ad 2 10/2 - gap
B 2 2.43 2 7/16
D 2 4.68 4 11/16
PVC pipe
C 2 2.75 2 3/4
Ad 1 gap + 1

[edit] Hints

  • Spacing between elements (dimension C) is very critical.
  • Try to make the corners as square as possible. A rounded corner is less effective.
  • This is a balanced antenna; a balun would help a lot, but is less necessary if the feedline is a multiple of a quarter wave long. (Remember velocity factor!)
  • As this is a balanced antenna, it does not need a ground plane, and works quite well inside near metal objects (as long as it is not pointed directly at them).

[edit] measured components

all units in inches + 1/128 inches (unless noted)

pipe
HD SKU
part#
outer diameter 5/8
coupler
NIBco
HD SKU 039923001696
part# PP-600
outer diameter 90
inner diameter 80
NIB 4
length 143
elbow
HD SKU 039923001849
part# PP-607
insertion depth 68
added length 65

[edit] folding equations

2m moxon mesurments

Using 146 mhz as center frequency with 1/2" copper pipe:

A=28 51/64"
B=3 47/128"
C=1 101/128"
D=5 81/128"
# var equation value
2 b B-57/64" 2 61/128"
2 c C+35/32" 2 113/128"
2 d D-57/64" 4 95/128"
4 x (A-31/16")/3 8 61/64"
1 y x+5/4" 10 13/64"
1 f h+35/32" 1 11/32"
2 g (y-f-1/16")/2 4 51/128"
h feed point gap 1/4"


[edit] External links

[edit] 6m moxon

6m moxon on top of a 28 foot rotating mast
printed feedpoint and air balun
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