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 Windows (Freeware) It will automatically generate a model in EZNEC format.

0, 0, 10’ 2 1 3 4 6 Wire # 5 Denotes Feedpoint

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 antenna]). – this is the gain over an Isotropic antenna (one out in free space – don’t exist [theoretical • Some programs will give it in dBd over a ½ wave dipole. – gain • 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)

Outside Ring = 6.27 dBi Design Freq. Azimuth Plot – different frequencies Beam Width Angle where signal is down 3 dB ~ 125 degrees

Outer Ring = 8.65 dBI Front-to-Back ~ 18 dB - 16 dB - 20 dB Notice the outer circles for the Azimuth & Elevation do not match. The reason for this is that the Azimuth plot is the signal at 20 º - max. gain @ 12 º (can be changed in the program). Elevation Plot Elevation Angle – Main Lobe angle ~ 12 deg.

Center of Chart 50 ohms resistive SWR 2:1 Circle Smith Chart – shows antenna impedance at different frequencies SWR 3:1 Circle

EZNEC 5 SWR Plot (not exactly the same model)

Effects on Antennas

• The ground effects antennas. If you are a 1 λ 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.

D C B A Reflector Driven Element

D C B A Direction of Radiation Reflector Driven Element

Qty.

4 36” 2 1 4 8’ Bill of Materials

Description

6 X 32 5/8” Brass Screws ½” PVC Sch. 40 Pipe ½” PVC Tees ½” PVC Cross ½” PVC Plugs #8 AWG Bare Copper Wire RG-8X or RG-58 Web Site http://www.moxonantennaproject.com/