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14MHz Moxon Rectangle

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Dave Marshall

Sun Nov 25 2007, 04:34AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

My current project is revamping of my ham radio station to better facilitate my run at DXCC and WAS before I leave this house in Georgia and move back to Pennsylvania in 2009. After that move, I'll have to start over from my new location (them's the rules unfortunately).

While my shack has gotten significant reworking in layout and equipment, my most involved projects have been a number of antennas I've recently constructed. Others will be posted in the projects board as they're completed, but this one has been tested with great success.

A Moxon rectangle is essentially a folded 2 element yagi, a driven dipole element and a reflector. Electrically the two are very similar, but there's a twist.

The peak current in a typical half wave dipole occurs in the first 1/8 wave length of each element. This fact allows for a dipole to be folded, bent, and/or mutilated after this 1/8 wave point without substantial change in performance or impedance. The Moxon takes advantage of this, bending the ends of the elements towards eachother, forming a rectangle (Diagram Here).

The distance between the two elements is also somewhat smaller than a typical yagi. This provides an overall footprint thats 25-30% smaller than a yagi for the same frequency.

The Moxon is often thought to be a 'compromise' antenna in that it has reduced performance over a yagi for the sake of reduced size and complexity. In fact, Moxon beams typically have a forward gain less than 1dB down from a yagi, and actually have a 4-7dB advantage over a yagi in Front/Back ratio due to the closer spacing of the two elements.

This reduced spacing isn't really practical in a standard yagi as this would increase feed point impedance and throw off the SWR. However, the feed point impedance of a folded dipole is lower than the standard half wave dipole of a yagi, negating some of this shift in feed point impedance. They also exhibit a pair of extremely deep (up to -30dB) nulls between the main and reverse radiation lobes. The Front/Back ratio and deep nulls make Moxons exceptional antennas in high noise environments or on crowded bands, using the side and rear attenuation to remove loud QRM or neighborhood noise makers.

I decided on a Moxon because I was looking for a light weight antenna (tubular mast support only, no 3 post lattice towers), I needed a somewhat compact turning radius (nearby trees that couldn't be removed), and its simplicity and affordability were very attractive.

Other antennas come close or beat the Moxon in one or two of those areas, but not all of them. Among the contenders were the SpiderBeam, Butternut 'Bow Tie' Quad, and a few trapped/loaded yagi designs. The SpiderBeam was the closest competitor, but it required a 17' turning radius, which would have required substantial trimming or removal of trees in the yard.

Unfortunately, I didn't get any photos during construction, but I do have a few of it and the tower.

Here you can see the entire antenna. Dimensions are approximately 25.25 feet long by 9.5 feet wide.

The central support structure is 1.25" PVC, with the parallel support arms made of 20 foot long telescoping fiberglass fishing poles (largest and smallest section removed to bring them down to 12.5 feet each). The rope and small length of PVC extending beyond the parallel support arms give tension to the fiberglass poles and prevent torque on the structure from wind loading.

The gaps between the ends of the two elements are bridged by electric fence insulators and UV resistant rope, and the elements are secured to the fiberglass poles in the same way. Moxons are fairly finicky in the tuning department, so having the driven element in direct contact with wet rope would probably be a bad idea.

Here is the entire setup as installed. To minimize weight aloft and wind loading on the mast, the rotator is placed at ground level, and the entire mast and antenna assembly turns. Torque is the primary problem for rotators, as even the light duty models can turn substantial weight (this one can rotate up to 175lbs, but is rated for only 3 square feet of wind load). By turning the entire mast, the guy wires and house bracket handle the wind load.

The antenna sits 30 feet above ground and has a 13 foot turning radius. As constructed it weighs in at just over 9lbs, and cost approximately $65 to build ($40 of this was just for the fiberglass poles). The tower and rotator added an additional $85 to the overall cost. The guy wires are made of milspec UV and weather resistant rope known as 550 Chord. They've got a working load of 550lbs (hence the name). The Moxon is fed directly with 50 ohm RG-8X coax

Overall performance is excellent. It has a perfect 1:1 SWR at 14.150MHz and a 2:1 SWR bandwidth from 13.925MHz-14.375MHz. Overall numbers are pretty close to the published numbers from various articles. Forward gain is 5.5-6dBi, F/B ratio is 22-25dBi, and the nulls provide right around -30dB attenuation. The azimuth beamwidth is generous to say the least.

Takeoff angle is difficult to ascertain, as it can't be easily measured by a nearby station and I can't get up above the antenna with a field strength meter, but I've worked 25 new countries in the week that its been up, including a long-path contact with Tonga in the South Pacific. The substantial amount of DX, as well as the several 150-200 mile Line Of Sight contacts I've made so far lead me to believe that the elevation beam width and takeoff angle is quite sufficient.

The antenna has already withstood two windstorms, one with gusts to 35MPH, so I hold few concerns about its durability. This is an exceedingly capable antenna that just about anyone should be able to build in a weekend. Mine took about 8 hours of work to construct, and another 6 hours preparing the tower and rotor for installation and actually putting it up.

The only issue I had with construction was the tuning process. There are six separate dimensions that have to be adjusted, the length of the two different wire elements (adding and removing wire on each of the four wire ends) as well as adjusting the gap between the wire ends. It would have been possible to tune it using a standard SWR meter and an HF rig, but it would have been painful to say the least. A hand held antenna analyzer made it about a 45 minute job.

A word of caution to anyone who builds one of these antennas. If you're tuning it at a height of <10 feet above ground with the intention of raising it after its tuned, aim for a frequency 100-150KHz below your actual target center frequency. My antenna had a 1:1 SWR at about 13.980MHz when 5 feet above the ground. When raised to 30 feet, that climbed to 14.150MHz.

Dave

Steve Conner

Sun Nov 25 2007, 11:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yay, looks neat! We'll have to try and get a contact again. My HF antenna is pretty hopeless, though. At the moment, it's a 30 foot dipole in the attic.

I should probably go to my mum and dad's house and set up a big magnetic loop, since they actually have a yard to put things in.

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