An easy to build, 6m halo antenna. By Adrian Jarrett-K6KY
The Conejo Valley Amateur Radio Club was looking for project ideas for the members. Early in 2017 the Radio Society of Great Britain published a 2-meter band halo design, which looked easy and fun to build. This was chosen as one of the club’s projects. However, the Conejo Valley is surrounded by hills, and so long haul 2m may be a bit problematical. That, and after having a great time in summer 2017 on 6m sporadic E, first with JT65 in the June ARRL VHF contest, and then with FT8, a 6m halo seemed like a worthy project. The gestation period was long, but after building and testing two prototypes, a design was developed that was easy to make and seemed to have good performance. It drew on the RSGB published 2m design, and on various designs on the internet. No green fielding here, as we say in England!
The element is ¼ inch copper refrigeration tube, cut to 110 inches long. Another 14 inches of ¼ inch copper tube is needed for the gamma tube. The 110 inches length is excessive, but it is a lot easier to cut-to-tune, rather than extend-to-tune. Typically, 1 to 1 ½ inches is removed from each end, making the finished length 107 to 108 inches. 108 inches was used for a subsequent build.
The tube was straightened, as the it came rolled in a carton, cut to length, and the center was marked. I did try 3/8 inch tube, that was a little more sturdy but heavier, so I decided that ¼ inch was more manageable. Straightening the tube made it much easier to measure the length and center than if it was in a loop.
The tube was then formed into a circle using a nail board. I am sure there is a posh word for such a board, but I cannot think of it. The diameter of the nail circle was 36 inches. Once in a circle and cleaned at the center, a SO259 bulkhead connector flange held in place with two twists of 12 gauge solid, bare, copper wire and was soldered to the exact center of the loop, with the connector inside the loop, facing downwards. This is best performed with a large soldering iron, typically 250 Watts. It is possible with 250 Watt soldering gun but they dwell a little longer to wet the joint and, as we found out, the heat transferred can distort the connector dielectric and make mating the connector difficult or impossible.
Once the connector is in place, the gamma wire is soldered to the connector center pin solder bucket. Both ends of the gamma wire, and the connector joint, were covered with heat shrink tube. The former to make sure the wire did not contact the gamma tube. The gamma wire was a 14-inch long piece of RG8X dielectric and center conductor. The jacket and braid were removed.
Next we made the mark 1, double croc, gamma test clip. No shop should be without one. Two 20mm crocodile clips were soldered back to back. This makes a nice movable variable shorting bar for tuning the gamma match.
The next part is where things start to take shape. The element is passed through the appropriate hole in the pre-drilled boom, until the connector approaches the boom. See drilling dimensions below. The free end of the element passes through its hole. The gamma tube is positioned in the gamma hole and the gamma wire threaded into the tube. The test clip is attached between the loop and the gamma rod.
Next is the tuning. I held my loop upright to tune, in the clear. An antenna analyzer is useful here; I have not tried to tune the loop without an analyzer so I do not know if that approach is possible. Assembled as above, the antenna should show a dip in VSWR at around 48 or 49 MHz. Carefully trim both ends of the element, I suggest by ½ inch at a time, to pull the dip up towards 50.2 MHz. When you get close, as I said I would expect you to remove 1-1/2 inches of tube from both ends of the element, move on to tuning the gamma match. If you do trim the loop too short (done that!) solder some 12 gauge plain copper wire onto each end and try again. Not so pretty, but effective.
The gamma match is tuned by leaving the gamma tube in place and rotating the element, to pull the gamma wire in or out of the gamma tube. The position of the test clip can also be adjusted. Adjust until a 1.5:1 or less VSWR, preferably with reactance = 0, is achieved at about 50.200 MHz. Once the sweet spot for the test clip is found, mark the position and solder a piece of 12-gauge plain copper wire in the place of the clip.
The typical dimensions are shown in the photo on the right.
Finally, the gap at the end of the halo is supported by a length of ¼ I.D. polythene tube, and another wire.
Parts can be held in place with a small amount of hot glue.
Tools are sharp and can be dangerous. You undertake this project at your own and your equipment’s risk. No guarantee of performance.
Thank you to Kat and Norm for patching me up when I found out just how sharp box cutters can be, and thank you to all the participants for enjoyable activities. And an extra thank you to Norm-AB6ET and Stu-KK6VYS for the photos.
Build #1, from lower left, clockwise, AB6ET, K6POI, W2EGB, K6KY, WB6PYK, K6VQN, K6HHW, N6PK, AI6MW
Build #1, from lower left, clockwise, K6POI, W2EGB, WB6PYK, K6VQN, KA6HHW
Build #2, from right, WA6RIE, W6VO, KQ6K; thanks for an enjoyable Memorial Day