ARLA/CLUSTER: 70cm mixed mode helix

[Carlos Fonseca - CT1GFQ]

70cm mixed mode helix
This is an experimental omni-directional antenna for 70cm which has both horizontal and vertical gain, which is now in use with the Southampton University Wireless Society WEB SDR 

Please note that the radiation pattern is optimised to peak toward the horizon, with a gradual reduction in gain towards the overhead sky. This should give good results for Satellite operation and High Altitude Balloon tracking, whilst still providing maximum gain towards the horizon for terrestrial operation using both horizontal and vertical polarisation. This makes it ideal for use on bands such as 2m & 70cm, where both polarisations are commonly used.

As a starting point I had been experimenting with single turn 'twisted halo' design, and decided to try stacking them to see if I could achieve more gain. Modelling suggested that a stretched 3 turn helix with a helix circumference of approx 1/2 wave length and an overall length of 1/2 wave at 70cm, and fed with a gamma match at the centre would offer reasonable gain, an omni-directional pattern and mixed polarisation. Adding more turns changed the propagation mode towards that of an axial mode helix. Less turns reduced the gain in both planes of polarisation, and caused an imbalance between the horizontal and vertical gains.

Since originally designing this antenna, I subsequently found a paper entitled 'The Helical Antenna' written by  J. Kraus, W8JK (SK) which was published in the Proceedings of the IRE Volume: 37 Issue 3 - 1949 Page(s): 263 – 272.

In this article a distinction is made between the Normal (Omni) mode of radiation when the helix circumference is smaller than about 1 wavelength, and the Axial (Beam) mode of radiation when the helix circumference is about 1 wavelength.

In the normal mode, depending on the helix geometry, the radiation may, in theory, be elliptically, plane, or circularly polarised.

For circular polarisation to occur the ratio of π x Diameter of Helix has to be equal to the Square root of   2 x the Spacing between turns x wavelength. So any helix which has a suitable Diameter to turn Spacing ratio will produce a circularly polarised radiation pattern at right angles to the length of the helix. However this formula starts to break down when length of the helix starts to exceed that of a single ½ wave dipole.

Any deviation from this ratio would result in elliptical or linear (only horizontal or vertical) polarisation.

In the case of my antenna is about the same length as a dipole and the Diameter to Spacing ratios as defined in the formula is fairly close 0.377 ≠ 0.387 – So only 65 years after Kraus published, I manged to hit upon the same ratio !





Next a plot showing the modelled gain in both planes, when mounted 10m above ground and fed with coax.







First attempt at a practical design - approx 300mm long 100mm between turns





Fixed the antenna to a short length of plastic tube in the workshop, and checked with a hand held fluorescent tube to see if the voltage distribution looked even between turns. Got the lamp to strike and then wound the power down to about 5 watts so that I could see the areas of the tube that remained lit up more clearly. Observed four distinctive bright spots of similar luminance (although it's not that clear in the photo) at each of the high voltage points.





Decided to check the difference in field strength between Horizontal and Vertical polarisation, by connecting the antenna to the VNA output and a small dipole to the VNA input. Held the dipole about 2 wavelengths away and rotated it between horizontal and vertical polarisation. Although this isn't the ideal environment for antenna measurements, the received signal levels on each polarisation are remarkably similar. Looks like about 8MHz bandwidth between 3dB points, not too bad for elements made from 2mm diameter wire. Larger diameter tubing would give a much wider operational bandwidth.





Final version built from 22mm plastic water pipe and 5mm copper car brake pipe.





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.








Graph showing the measured input impedance and SWR.





So far so good,everything seems to work as predicted. 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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