[TowerTalk] Fan/Parallel Dipoles

[Steve, W3AHL]

Parallel multi-band dipoles require some attention to detail during construction and tuning, but they certainly are a good solution for many sites.  I have hosted several antenna building parties for our local club where we have successfully completed almost 30 antennas, in a wide variety of band configurations and wire types.  Four bands are the most I recommend, but we have built several five bands antennas that tuned fine.  Many were built by new hams that hadn’t gotten on HF yet, due to the lack of an antenna.  After the antenna is tuned, another mini-party is usually scheduled to help get it up in the air at owners site.

All will cover the entire band (for 40-10M) at less than about 2.5:1 VSWR, except on 80M, assuming a reasonable height (40-70+ ft.).   We try to tune them at the height and vertex angle for the final site, but sometimes a little tweaking is needed at the final installation site, especially on 15 and 10M when mounted as an inverted V.  Tuning is done with an AIM-470C, allowing the owner to have a record of what the tuning was for future reference.  I usually include an EZNEC plot for azimuth and elevation patterns and sometimes an HFTA example of how their site affects the pattern in key directions. 

Tuning is fairly straightforward if the antenna is built correctly and measured accurately.  Start with the longest elements first and work your way down.  Tuning the shorter elements will not affect the longer ones.  One problem to be aware of is the interaction between 40 and 15M elements, especially if 40M is tuned a little high to favor the upper part of the band.  It can make tuning 15M very frustrating, until 40M is tuned just a little lower.  Yes, you can operate 15M using the 40M element at its third harmonic if you use a tuner and don’t mind the nulls in the pattern.

They make a good club project, since it really helps to have someone on the team that has built and tuned one before!  The most critical part is getting the wires dressed correctly where they fan out from the feed point.  The hardest part is getting people to follow the directions carefully and measuring the wire accurately so the two sides stay balanced as they are tuned.  It is hard to build one by cutting the wires to length from a table and expecting it to centered in the desired band.   I find that 6” spacing between elements work well.  A four-band 75/40/20/10 antenna can be built for less than $40, plus feed line and supporting ropes, pulleys, etc.  It takes 4-8 hours to build and tune (with 2-3 people helping for some parts of the project) depending on experience level and how much time everyone spends chatting (antenna parties tend to be social events!).

While just about any piece of wire can be made to work to many operators’ satisfaction, dipoles always seem to perform more consistently, produce less RFI in the shack and can run the legal limit without a tuner in most cases, at least for me.

Steve, W3AHL