Tuesday, December 29, 2009

Quadrifilar Helix Antenna for 2m


I've built the QFH antenna recently published in QST. The result is very omni-directional, as advertized; but I found the construction rather difficult. Here are some notes on the process.

First, use the article as linked above and note the change in dimensions of the shorter loop. The dimensions published in QST are too short. Secondly, you should note that the dimensions given in that article are for the loops as laid out in the manner provided. They are not linear lengths, which are not provided in the article but should be several inches longer due to the curves of the loops.

You can see in the picture provided here that I had to splice in some smaller-diameter lengths of tubing because I had made everything too short the first time around. Because everything was shorter on the first go-around, I ended up cheating on the horizontal run of the loops at the top of the antenna. Moreover, I had to re-shape the whole antenna several times.

The good news is that 3/8" flexible copper tubing works just fine in this application. I was concerned that it would sever when crimped to form the 90 degree bends, but it didn't. I wouldn't want to bend them many times, but one single bend does not introduce a dangerous level of flexibility.

I found that bowing out the loops had a great effect on the tuning of the loop. A greater bow tuned the antenna to a lower frequency.

One further note: the diagram in the article shows the loop connected to a pcb board as a means of electrically connecting the feed cable and the antenna elements. As far as I can tell, this board must be on top of the elements, not underneath as shown in the diagram.

If I were to build this again, I would use a direct soldering technique on the elements and feedline. Or I would have the main tube split with a coupling for final assembly. I'd have a short length of tubing at the top so that I could get a nut on the underside of the tubing to hold the pcb to the elements by means of a bolt. I used self-tapping screws and found they were happy to strip from time to time. I would also use a large-diameter former for initially curving the elements. I wasn't very good at eyeballing the helix.

Finally, note that if you follow the design in the article above the result will be RHCP. This should mean that the downlink for VO-52, which is LHCP, is attenuated considerably. My experience suggested that it is, but that the signal is so strong in general that it makes up for this. 

3 comments:

  1. What is the length of the loop that you ended up with? I am about to try and build thie antenna.

    Bill AE5BK
    ae5bk@yahoo.com

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  2. Hi, Bill, I wish I could tell you, but the antenna is now mounted. I would recommend laying it out as in the diagram, in the pdf of the article and measure the linear length.

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  3. It works fine... Noobs.

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