Building antennas is a time-honored ham radio tradition. Shortwave antennas tend to be bulky but at VHF frequencies the antenna sizes are pretty manageable. [Fjkaan’s] 2 meter quadrifilar helicoidal antenna is a good example and the structure for it can be created with 3D printing combined with electrical conduit.

Many people, including [G4ILO] use PVC pipe for the structure, and that design inspired [Fjkaan]. Despite being a bit less substantial, the conduit seems to work well and it is easy to cut. The helical design is common for satellite work owing to its circular polarization and omnidirectional pattern.

A quadrifilar helicoidal antenna is really two antennas in one, with a phase difference of 90 degrees between the two. There are several ways this can be accomplished, but in practice, most of these antennas use different loop sizes for the two antennas. One loop will be a bit larger than the frequency of interest, and thus will be inductive. The other loop will be a little smaller, and therefore will exhibit capacitive reactance at the center frequency.

Even though the antennas are both reactive, in parallel, the reactances cancel leaving a nice resistive load to match the radio. The feed at the top, however, needs to balance through some form of balun or choke.

We’ve seen these antennas do great things before. If you need a satellite receiving primer, we saw a good one last year.

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Author Of this post: Al Williams

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