Circular Polarization for 2 Meters

I’ve been interested in antennas for years and enjoy making them. Haven’t bought one in eons. After being interested in amateur radio since I was a kid, I finally got my basic licence in March, then wrote the advanced exam 3 weeks later, so I have to make quite a few antennas to get on the air.

The 2-meter antenna I built for my car is just a piece of aluminum fencing wire cut to size and stuck in a wood block screwed in under my trunk lid. The resonant frequency is a little high, probably due to the trunk lid effect. I’m going to try threading the end and screwing on a tuning nut. Otherwise, I’ll just cut a longer piece. So far, I’m amazed at how well it’s getting out, despite the 2:1 match. It even made it to London on SSB on 144.2 MHz with 3:1 match.

The real project is my home base antenna for 2 meters. I wanted something omnidirectional, with gain in both vertical and horizontal planes. I decided to try and adapt a common broadcasting antenna to 144 MHz, something I’ve never heard of being done before. Plans and measurements could not be located.

The characteristics are supposed to be as follows:

  • Circular polarization
  • DC grounded for lightning protection (so there’s no gamma capacitor involved for tuning)
  • grounded elements reduce static type interference
  • 50 ohm impedance
  • Low SWR typically 1.3:1 over wide frequency range, predicted to be 10 MHz
  • Gain is -3 db for 1 bay, Unity for 2 bays, 3.2db for 4 bays, 6.3 db for 8 bays, 9.6 db for 12 bays omnidirectional in both horizontal and vertical polarization     (12 or more bays might be feasible if scaled for UHF bands)
  • Can handle full legal power
  • Avoids penalty of 25db common with cross-polarized signals, and array increases system gain
  • Reduces flutter and picket-fencing of mobile signals (caused by cross-polarized reflections)
  • Common in FM broadcasting, often called a “rototiller,” turnstyle or (patented) Penetrator
  • A single bay could be ideal for satellite work
  • 6 bays would make an outstanding repeater antenna

The antenna consists of a pair of crossed-folded dipoles facing each other a half-wavelength apart, and with each dipole shunt-fed about a 1/8th wave from the end with a common feed from the centre. They can be stacked for increased gain and power, at least .7 wavelength apart, and fed with properly phased coax much as any colinear array. Notice the gain values are half those for a stacked colinear array, because power is divided between both vertical and horizontal polarizations.

I built one unit without any plans, using the standard formulas, referring to telephoto photographs of broadcast towers, and gleaning what I could from antenna manufacturer’s brochures. Its performance is in the ballpark but not quite as expected. The VSWR is 1.5:1 at 148 MHz, but 3:1 at 144 MHz. This would indicate that I cut it short. Altering the feed point (called a wishbone) over a wide range made minimal difference in VSWR, which I did not expect. When I lengthened all 4 ends, the VSWR went up instead of down! So I shortened it and it came back down, but was not as good as the way I first cut it. It works quite well though – at 6 feet off the ground I was picking up a Detroit repeater, and I can hit our local repeaters full quieting with 100mW.

I’m certain there is a complicated interaction between the elements which makes it behave counter-intuitively. I haven’t tried changing the coax length yet, but the final version has a phased harness with critical lengths. I heard they are sometimes tuned using ferrite beads placed around the coax inner conductor just before it exits the pipe for fine tuning, and perhaps a capacitor shunted to ground. I have the dipoles crossed at 90 degrees but this might be too much. It would be nice to find some plans and measurements for one of these things. Maybe it’s a trade secret? At least the club has an analyzer I can put on it to see what’s going on.

Well, I’m having fun anyway. Once I get this figured out for one bay, I will make 5 more, phase the cable, and try to mount the unit. Next project? Maybe a 12-bay version for UHF on the same pole, or 16-bays for 902MHz. In fact, there’s no reason why I can’t shunt feed HF into the entire grounded pole either since versions of this antenna operate on hot AM towers.