Wednesday, May 15, 2024

MFJ-1896: 6 Meter Moxon

It may seem surprising that I have an MFJ antenna. The story behind it is interesting. While chatting with a friend last year I mentioned that I would like to have a second 6 meter antenna. It would be used to check for DX in directions other than the one the main antenna is being used for. Turning antennas to find openings is tedious and during those minutes I can miss stations from all directions.

He mentioned that he had a small 6 meter yagi that he bought at a flea market but would never use because he had no room for it. When he brought it over I was surprised to find that the carton was unopened; it was a new antenna. It turns out that the fellow that sold it to him for a few dollars had the same problem: no room for it. So I found myself with a free and brand new MFJ-1896 6 meter Moxon.

It languished for a year. The 6 meter season was drawing to a close at the time and I had more urgent projects. I felt a little guilty with the carton standing there over the winter since a gift ought to be enjoyed. This month, with the 6 meter season started, I finally made the effort. It's a small project that I could tackle despite the family issues that are occupying so much of my time at present. Assembly and tuning took just 2 hours spread over a couple of days.

I opened the carton and found most of the parts loosely packed. The hardware was bagged and a there was a bit of bubble wrap around the tubes. There were two assembly guides, a large one inside and a smaller version stuffed in a fold of the carton. I don't know why. I didn't bother to compare them in detail, though they appeared to have the same content.

The instructions were skimpy, as noted by others in the small number of eHam reviews. All the listed parts were located, including a few spare lock washers. The heads of the #10 hex bolts were smaller than standard size. I think all the hardware is stainless. A little time spent outdoors will soon confirm that.

As noted in the reviews, finish quality was lacking. A couple of the corner pieces were not bent to right angles, tube slots had hazardous burrs and the coax pigtail wasn't wide enough to fit onto the driven element studs.

I filed the burrs, bent the corner tubes and slit the coax a bit to fit the studs. A bolt hole on one side of the driven element was askew, which made it difficult to push the #10 bolt through it. 

Let's talk about the aluminum tubes. The extraordinary light weight of the antenna offers a hint. They are thin, very thin at 0.03". The same goes for the 1.5" diameter boom. This is half the wall thickness of the usual aerospace 0.58" wall used for telescoping yagi elements in better antennas. Even that isn't the entire story.

For proper telescoping the step is 1/16" rather than the usual ⅛". The main tubes are ⅜" diameter and that of the corner pieces is 7/16". This by itself isn't worrisome for a fairly small VHF antenna. The problem lies with the aluminum alloy.

This is not high tensile strength aluminum alloy. It crushes all too easily. There is almost no resistance to the pressure from screwing on an ordinary #10 nut. No matter how careful you are, the tube will crush before the hardware is properly torqued. Eventually the tube will further yield under wind load and weaken the mechanical and electrical bonds.

The situation is less dire for the joints with the elbows and element tip insulators. The distributed pressure of hose clamps avoids the risk of crushing the tubes. The driven element in particular needs support inside the tube to reliably secure the low tensile strength aluminum tubes. I did not attempt to "fix" the antenna's design woes.

I put my concerns aside and completed assembly of the antenna. I followed the dimensions printed in the manual. The critical element coupling inherent to the Moxon's operation requires more care in measurements than for a conventional yagi.

The picture shows how I modified the feed point to reach the studs and used silicone caulk to seal the unprotected open end of the RG58. RG58 of any length at VHF is a bad idea but I had no compelling reason to upgrade it. It's only a few feet and I doubt that I'll be tempted to put a kilowatt into it. I can replace the supplied coax if I'm even so inclined.

The other end of the short coax has crimp UHF female connector. I taped the coax to the boom and attached a (tested) length of RG213 to allow the analyzer to be connected from a distance where ground and a human body would have no significant impact on antenna tuning. The absolute minimum height should be ¼λ (1.5 m) and more is better. Happily that's easy to manage for a VHF antenna.

This was my testing setup. The antenna is approximately ½λ above ground (3 meters), which is more than sufficient to stabilize the feed point impedance. The SWR should remain unaltered at greater heights, assuming there are no interactions to contend with. It is easy to tune a VHF yagi this way in comparison to tuning HF yagis. I crouched down and moved around during testing to confirm that my body didn't have an effect on the measured impedance.

The initial SWR measurement was out of bounds, with the minimum SWR found at 49.5 MHz. It is almost exactly 1, which was promising. One of the critical dimensions to get a 50 Ω match on a Moxon is the distance between the element tips and the quality of the insulator connecting them. 

I was careful to get that distance exactly right during assembly (3-¾"). The plastic rods supplied with the antenna are of unknown material but at first blush appear to be adequate.

I remeasured the element dimensions and found a few small errors. Mistakes are easy because the rounded corner complicates the measurement from the outside of elements to the boom centre. After correcting those dimensions the minimum SWR rose to 49.7 MHz. That's still far too low.

The elements were shortened to raise the frequency of minimum SWR. The elements must be adjusted in concert to maintain the frequency relationship between the driven and reflector elements. I therefore chose to slide the inside end of the elbows into the element tubes to shorten the elements. I did it in ¼" steps until I achieved the SWR curve I wanted.

The minimum SWR is now slightly above 1. There is no advantage in fussing with the antenna to make it exactly 1. In any 2-element yagi, which includes Moxon rectangles, the frequency for maximum gain is below that of minimum SWR. My chosen SWR curve is a compromise that should provide good performance from the low end of the band (CW and SSB) up through the digital windows above 50.3 MHz.

Mission accomplished, I leaned the antenna against a wall of the garage until I am ready to raise it. I hope to do so in the coming weeks and use it during most of this year's sporadic E season. I will probably feed it with LMR400, since that is cheap and convenient, and the run will be less than 50' (15 m). There is no compelling reason to fuss with Heliax to reduce loss over this short distance.

Is this antenna worth paying the retail price? Obviously I paid nothing for it but it is a valid question for almost everyone else. A 6 meter Moxon is an easy and inexpensive antenna to construct from raw materials. But that would require an investment of time to create a mechanical and electrical design and to find, purchase, machine and assemble the components. The driven element insulator might make an interesting 3D printing project. 

Not many hams would want to do this, preferring to invest their limited time on other projects or on operating. MFJ has made a business of meeting the needs of hams on a budget for many decades. I wonder what if anything will take their place now that the business is shutting down.

All I can do is shrug. If an antenna like this inspires more hams to enjoy what the magic band has to offer, it serves a purpose. Don't expect it to last many years in fierce winds and winter weather. Hopefully by the time that happens the owner will have become a 6 meter enthusiast and will be ready to make a larger investment in their next 6 meter antenna.

I have a place for this antenna, at a modest height on a tower currently unused. I had intended that small tower to put up the radio/antenna for my wireless internet service. Since that had to be higher for a reliable connection from my isolated QTH it has only been used for climbing onto the roof of the house. I'll have to dig up a small TV antenna rotator to turn it. If I get a couple of years out of the antenna it'll be worth the effort.

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