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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. - PowerPoint PPT Presentation
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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)
1
2
3
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/
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