2M Moxon Antenna

Moxon Antenna

• Moxon antenna is a 2 element reduced size antenna.

• The Elements are bent (reducing the size)• It has similar characteristics as a 3

element yagi. • It has a large beam width and very good

Front-to-Back ratio.

Moxon cont’d

• Use Moxon Generator (freeware) to calculate the size of the antenna.

• Insert the dimensions into a Antenna Modeling program. (Most of the diagrams are from cocoaNEC – MAC program)

• Determine the characteristics of the antenna from the model.

• Can be horizontal or vertical polarization.

Moxon Designer for the Mac computer(Freeware )

Moxon Generator for WindowsIt will automatically generate a model in EZNEC format.

(Freeware)

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2

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4

5

6

Denotes Feedpoint

Wire #

0, 0, 10’

Antenna Modeling

• Most Antenna Modeling programs use NEC 2 (freeware) or NEC 4 (commercial).

• The Antenna Modeling program is the interface to and from the NEC computational engine. It makes it easier to understand.

• You describe the antenna using wires (this could be tubing). This is the most tedious part.

Antenna Modeling cont’d

• You have to designate on which wire the feed point is located.

• You select type of ground, and other things related to the antenna, number of segments.

• The model will generate Azimuth, Elevation, SWR, other curves and information.

• You can then change the model to get the best patterns/SWR etc.

Antenna Modeling cont’d

• There are limitations to the models on certain items (it will not do under ground radials – you have to fudge them).

• If you stay within the limitations of the program, it will model the antenna correctly.

• Use modeling to build better antennas.

• Antenna modeling programs for amateurs can be freeware or cost up to ~ $ 200.

Antenna Modeling cont’d

• The one that is used most by hams is EZNEC. The current version is 5, and costs $ 89. It has several different versions with different features. EZNEC 5 is for an antenna with 500 or less segments.

• EZNEC has a demo version with a limitation of 20 segments (can model a dipole).

Antenna Modeling cont’d

• Each modeling program has advantages & disadvantages.

• EZNEC is the most widely used for amateurs – so getting models from other amateurs is easier.

• Do a Google search for address.

• Freeware Antenna Modeling program – do a search for 4nec2.

Antenna Modeling cont’d

• The gain shown in the model is dBi – this is the gain over an Isotropic antenna (one out in free space – don’t exist [theoretical antenna]).

• Some programs will give it in dBd – gain over a ½ wave dipole.

• To go from dBi to dBd you subtract 2.1 dB.

• 5.6 dBi = ~ 3.5 dBd.

Antenna Modeling cont’d

• Information that is nice to know about a Yagi:– Beam Width– Elevation Angle– Front to Back Ratio– Antenna Pattern– Gain– SWR – Bandwidth (range of frequency < SWR 2:1)

Azimuth Plot – different frequencies

Outside Ring = 6.27 dBi Beam Width

Angle where signal is down 3 dB~ 125 degrees

Design Freq.

Elevation Plot

Outer Ring = 8.65 dBI

Elevation Angle – Main Lobe angle ~ 12 deg.

Notice the outer circles for the Azimuth & Elevation do not match. The reason for this is that the Azimuth plot is the signal at 20º - max. gain @ 12º (can be

changed in the program).

- 20 dB

- 16 dB

Front-to-Back~ 18 dB

Smith Chart – shows antenna impedance at different frequencies

Center of Chart 50 ohms resistive

SWR 2:1 Circle

SWR 3:1 Circle

EZNEC 5 SWR Plot(not exactly the same model)

Effects on Antennas

• The ground effects antennas. If you are a 1λ above ground it almost no effect. (Practically if you are ½ λ – little effect – on 2M ~ 6’). Antenna Models model different types of Grounds (so you can get more realistic results).

• Diameter of the elements.

• Distance from antenna to other objects.

Building Moxon Antennas

• The measurements are the same (assuming the same freq. etc.) – how it is constructed can be totally different.

• For the 2M Moxon, I use ½” PVC pipe & fittings and #8 bare copper wire.

• These materials can be purchased at Home Depot for under $ 15. (Lowes only carries #6 bare copper wire – changes sizes slightly.)

Building Moxons cont’d

• I use PVC pipe because it is cheap (the watch word for amateur radio) and readily available.

• The #8 wire is stiff enough to keep it’s shape (pretty cheap and is available).

• I made a 10M Moxon using # 14 AWG wire. • Some hams build them with aluminum

tubing.

A

D

C

B

Driven Element

Reflector

A

D

C

B

Driven Element

Reflector

Direction of Radiation

Qty. Description

4 6 X 32 5/8” Brass Screws

36” ½” PVC Sch. 40 Pipe

2 ½” PVC Tees

1 ½” PVC Cross

4 ½” PVC Plugs

8’ #8 AWG Bare Copper Wire

RG-8X or RG-58

Bill of Materials

Web Site http://www.moxonantennaproject.com/