ARLA/CLUSTER: 2m Mixed Mode Helix

[Carlos Fonseca - CT1GFQ]

2m Mixed Mode Helix


I finally got some time to construct a 2m version of the Mixed Mode Helix. The intention is to try this out on our microwave  WEB SDR.

The helix is about 1m long, 300mm in diameter with 300mm spacing between turns. The exact length of helix has to be trimmed for resonance (zero reactance), and the gamma match is then adjusted for lowest SWR.









I built this version using 5mm copper brake pipe and 22mm plastic water pipe with 90 degree bends ends and a Tee.

The pipe joints still need to be glued, and the whole assembly waterproofed and painted.

The antenna has a fairly high Q and moderately narrow bandwidth, so any flexing of the helix can change the impedance match quite dramatically.

The 5mm brake pipe I used is really a bit too soft for this size of helix, so I had to add an additional bracing section to keep it in shape.









The turns of copper pipe are held in place by a single turn of thin copper wire wrapped around the pipe and soldered close to the plastic pipe. The antenna is quite high Q so requires some fine tuning once built. I made the last turns slightly longer than required and then cut them to resonance. Painting the antenna will lower the resonant frequency slightly. Very fine tuning can be achieved by bending the gamma match wire.

Two or three suitable ferrite beads also need to be placed on the coax inside the boom arm near the feed point, in order to choke off common mode currents and create a balanced feed.

The support boom was made with an inner of 22mm water pipe and an outer of 25mm plastic electrical conduit. A plastic conduit straight coupler can be persuaded to fit over the 22mm Tee by warming it with a heat gun.

The whole assembly is not waterproof, so the coax connection to the feed point needs to be protected with something like epoxy resin, hot melt glue or silicone sealant. A small hole also needs to be drilled in the bottom of each length of plastic tube, to allow any water that does collect inside is able to drain away.

Although I made the first antenna with 5mm copper car brake pipe, I suggest that 8mm copper central heating pipe may be more suitable for the 2m version and provide a slightly wider operational bandwidth.

I also had to use a short length of rope from the top corner of the antenna to the support mast, in order to help support the weight of the antenna and prevent the 1.5m long boom from sagging. A length of aluminium tubing placed over the boom arm may help improve the stability in high winds.















Modelling produced some good plots, which seem to be representative of the measured performance.

When constructed as shown, the radiation pattern is predominantly Right Hand Circularly polarised at low angles of elevation.







Pattern is Omni-directional with very slight asymmetry due to the gamma match and interaction with the mast and feed coax.





This antenna is now in use on the Southampton University Wireless Society uWave WEB SDR







73's de Carlos CT1GFQ
SKCC#466C www.skccgroup.com
REP#1406 www.rep.pt
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