Thursday, October 05, 2017

Takedown Magnetic Loop Antenna

 Over the past couple of years, it seemed like every time I opened a ham radio publication, or found myself working the bands, someone was either talking about a magnetic loop antenna that he or she had just acquired, or was building, or.....well you get it, there seems to be a lot of interest in MLA antennas. So with that, I thought that maybe it was time that I built an MLA to see what all the hype was about.

It was a given that the MLA that I would build would have to be capable of being quickly assembled or dismantled, as I knew that a TD-MLA would be the perfect antenna for my Manpack that includes the Yaesu FT-897D running on internal batteries.
Because the FT-897D running on internal batteries is limited to 20 watts out, and after doing research as to what was available for a variable capacitor in the marketplace that would suite my needs, I found a split stator style air variable capacitor rated 12.5 pF - 432 pF per section, and rated for 25 watts out and ordered it up. This variable capacitor has a geared 6-1 front end making for very fine adjustments when tuning my take-down MLA built for the bands of 10 through 40 meters.

Because the primary loop had to break down for storage, I put some thought into how I could build it from aluminum, and be able to dismantle the loop for storage in the storage bag included with the manpack. I constructed the primary loop using thin-wall hi-tensile 3/4" OD aluminum tubing, and once I had the 36" loop formed, I then divided the loop off into 12" segments. With a tubing cutter, I separated all the segments from the loop, ending up with nine 12" segments, 4 matching segments on each side, and one unique segment for the top of the loop. I then inserted a 2" thin-walled aluminum insert into one end of each loop segment as joiners for assembling the loop. These inserts are held in each loop segment by tig-welding them in place. Each segment then was modified for keying to one another, allowing for quick assembly and disassembly with just one's hands, no tools necessary.

The feed loop where the SO-239 connector resides for the transceiver to connect up to, is constructed from 1/8" x 3/4" flat bar-stock 6061 aluminum, and this also was by design so that when assembled the mast and the feed loop along with the control housing become a ridged assembly to which the main loop mounts to and holds its form.
It should be mentioned that loop balance is important, as any electrical imbalance will result in common mode currents flowing on the braid of the coax that will mess up the radiation pattern of the MLA. I wound a common mode choke and placed it at the feed point of the small loop to prevent this from potentially happening.

 Of course it goes without saying that there were a lot of one off parts built in my workshop, like the component you see that resides at the very top of the feed loop that was formed from Delrin, and has a precision sized slot cut into it at the top, that the primary loop drops in to upon assembly of the TD-MLA.
It also was by design that all the antenna components measure 12" or less when disassembled, so that they would readily fit in the storage bag that resides on the frame of my manpack.

The mast components were formed from non-conductive composite materials, with the mast joiners made from white Delrin round stock. The enclosure that the air variable capacitor mounts in is made from a heavy duty non conductive composite material. Also mounted in this enclosure are the two SO-239 panel mount connectors that the primary loop fastens to upon assembly of the TD-MLA.

I have to say that upon finishing the building of this TD-MLA and once I had it connected up to the transceiver and on the air, I was very impressed with how quickly I was able to tune in a station, and how well the MLA was hearing and receiving stations on the band (20 meters).
Initially it was as simple as choosing a frequency, then turning the tuning knob of the variable capacitor until the receive signal was at its strongest on the S meter of the transceiver. With a few watts output, it was simple to quickly tweak the variable capacitor knob while watching the SWR meter on the display of the FT897D, adjusting it to its lowest reading. I should add that I used my antenna analyzer to make the initial adjustments required to the MLA, but for the most part, the antenna was perfect from the start.

My first contact was with a station in Oklahoma who was talking to another station in upstate New York, and after asking for a break I was immediately acknowledged. After establishing where I was located and considering I was being heard off of the side of his beam, I received a good report. 

I found it interesting that stations I was hearing from across the US, I could maximize the receive signal by rotating the mast of the MLA and watching the S meter on the transceiver, as the antenna is vertically polarized and has fairly sharp nulls along the axis of the loop, with the radiation pattern peaks lying in the plane of the loop. Yet it made no difference on DX stations that I was receiving from countries in Europe when I rotated the MLA, as they were equally strong regardless as to what direction I pointed the loop. I understand based on the research I've done on magnetic loop antennas, that this is a characteristic of MLA's.
Although I haven't spent a lot of time running tests with the MLA, at this point I must say that I'm impressed with the performance of my newly constructed Takedown Magnetic Loop Antenna, and it will be my favorite manpack antenna for the foreseeable future.

Note....all photos expand

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