Loop Antenna Construction Details

I’ve received a few questions about how I constructed my loop antenna. Unfortunately, I didn’t have the foresight to take pictures during construction, and will have to make do with a written description and some general pictures.

The loop antenna was made from some scrap hardwood (possibly birch or maple) that I had lying around. The pieces for the spokes are 31" long and 1/2” square cross section.

To hold the wire for the tuned part of the loop, I drilled 1/8" holes spaced 1/4" apart, and inserted pegs made from 1/8" dowel. The holes were drilled at an angle so that the pegs tilt out towards the end of the spoke to help hold the wire on.

The pickup loop is mounted on the back side. I used 1/8 x 2" dowels on the back side lined up between the two centre turns of the tuned loop. These pegs are mounted straight, and each one has a rubber grommet mounted on it, and then the pickup loop runs through the grommet groove. This allows adjustment of the distance between the pickup loop and the tuned loop, by sliding the grommets back and forth.

The three main frame pieces were dadoed together. The dados on each piece are about 4" apart. This construction method makes for a very sturdy frame. I started with the pieces a bit longer than the final length so that I didn't have to worry about getting the dados exactly centred, and trimmed the ends later once all the pieces were cut. I did the dados on my table saw with the mitre guide set to 60 degrees. I ran the first two pieces through the saw, one at a time, to make the first join, and then temporarily fit them together. I temporarily hot glued a couple of scrap slats across the ends to hold them together at exactly 60 degrees, and then ran the assembly through the saw again to cut both dados for the 3rd spoke at the same time. Then, I set the 3rd spoke piece into the grooves that I'd just cut, and marked it so I could tell where to cut it. Then, I ran that last piece through the saw. I knocked the temporary struts off, and assembled the frame with glue and clamps.

To attach the frame to the stand, I glued and doweled two more short pieces, spaced about 1/2" apart, across the triangular centre section. They have a 3/16" hole drilled through the side to line up with a 3/16 hole through the main vertical strut. There is a 3/16" dowel that runs through them (friction fit) to hold the frame onto the vertical strut. This allows the loop to be tilted up and down, in addition to being rotated on its vertical axis.

The base is made from two unfinished wood plaques (one 3x5" and one 8.5x12") from a hobby/craft store. I used two together, to get enough thickness so that I could drill a deep enough hole for the main strut to stand up straight without wobbling. The vertical strut is square cross-section, but the bottom part where it fits into the base was turned down round using a jig that I made for my router.

I finished the wood parts with a couple of coats of tung oil, after they were all assembled.

The tuned part of the loop is wound with #24 enamelled wire. There are 10-1/6 turns giving the inductance of 152 µH. There are enough pegs installed to wind 13 turns if necessary. After winding, I applied some clear coil dope on the pegs to hold the turns in place. The pickup loop is PVC insulated #22 solid hookup wire.

Operational Observations

I’ve used this loop antenna with both my two tube superhet (for which is was originally designed) and my one tube double reflexed superhet. In use, I’ve found that the position of the pickup loop slightly affects the sharpness of tuning of the tuned part of the antenna. Moving the pickup loop closer to the tuned loop, broadens the tuning very slightly. I haven’t noticed any significant difference in signal strength with different positions of the pickup loop. I don’t believe there is enough range of adjustment to make a very noticeable difference. For future reference, it would be a good idea to allow for an adjustment of more than two inches.

Another observation is that the directionality is not exactly as expected. I had expected a figure-8 pattern typical of loop antennas. However, while getting the expected nulls on the faces of the loop and a maximum on the one end, I got an unexpected null on the opposite end as shown in this diagram. My only explanation is that that it may be due to the pickup loop being grounded on one side.