“The other horizontally polarized omni!”
by Dave Clingerman – W6OAL
Those of you that know me know that my avocation on horizontally polarized omni-directional antennas is the Wheel antenna; however, I have been known to use a “Turnstile” on occasion when the requirement for a more robust and less lobed, omni pattern is not needed or required.
Recently our 1.35M (222.055 MHz) beacon went down so it was brought out from the “interior” of Colorado and placed on the OAL bench for analysis and repair. To our amazement we found no problem as it keyed right off with the initiation of 12 V and a shorted key line. Anyway, I did a good going over of it and peaked it up whether it needed it or not. It appears to be very healthy with a good 30 watts of output on a Bird wattmeter.
It does take a bit of planning to get a beacon back into service at our South Park site, a trip that consumes the better part of a whole day. So I thought I’d run it here at the DM79ql location for a while (with the “W6OAL” ID’er) until we could get it back to our DM79ch location. I don’t have the altitude of the beacon site which is at 9500 feet but have no problem being heard up and down the Front Range on any band up to 10 GHz from my place here at 6500 feet 10 miles east of Parker in Elbert County. Over a couple of week period I have received reports ranging from an S-3 in Fort Collins to 40 over 9 in Bailey.
To return to the subject, the antenna used was a Turnstile as I didn’t have a 1.35M Wheel here and Turnstiles aren’t all that hard to whip up in an afternoon. As it has performed so well in beacon service here I thought that with the recent resurgence of activity on the 1.35M (222.100 MHz) band that a few new comers to the band as well as the “old timers” might like to construct one prior to buying or building a beam for the band. Besides, an “omni” saves you from working a rotator to death on activity night if for instance one lives in the Metro area and wants to work and hear both Northern CO and south of the Denver Metro Area.
I believe first I should describe just what a Turnstile Antenna is, for those new to the hobby or have not had much involvement with the various types of antennas used on the VHF/UHF bands. The Turnstile is basically a pair of orthogonally positioned dipoles sharing a common axis, crossed dipoles if you will. Since the pair of dipoles is physically 90 degrees out of phase with one another they need to be fed 90 degrees out of phase with one another via a quarter wavelength section of coaxial line. The phasing section is made of 75 ohm line in order for the pair of dipoles to be matched to a 50 ohm transmission line.
Since this type of antenna or array of dipoles will need to be broken in the center I elected to use flat stock elements rather than round stock. My intention was to mount the Turnstile on a push up pole, so I built the pair of dipoles on a hub of sorts. The hub I chose was a 3″ length of 1.5″ PVC pipe. The elements were made of flat stock (0.095″ thick X 0.50″ wide). I used 0.095″ thick material because it was available and of sufficient strength for the length of the elements. In this case, for resonance at 222.055 MHz the length required was 10.75″. I made a 90 degree bend at one end of the elements of 0.625″. If it is desired to make elements any where in the 2M, 1.35M or 70cm bands the following equation can be used for element length.
L (in inches) = 2248.3 / F (in MHz)
Then add 0.625 inch to it as that will be the part that is bent 90 degrees and drilled for mounting to the length of PVC (hub). Drill 4 #6 (0.140 inch diameter drill bit) holes 0.250 inch below the lip of one end of the PVC hub so that when the elements are mounted an X will be formed by the elements. The screws (#6) used to attach the elements to the hub will need to be long enough to hold a second nut to attach the 50 ohm transmission line solder terminal and, in the case of the first dipole hooked up, it will be holding a pair of solder lugs.
Preparation of the phasing section follows; the phasing line will be a quarter wavelength section of 75 ohm (RG-59) coax. The length of the phasing section is determined by the following equation:
L (in inches) = 2952.5 X VP / F (in MHz)
The VP is the velocity factor of the coax. In the case of RG-59 that uses a dielectric of polyethylene, the VP is 0.66.
Example: The phasing line for a Turnstile to be used on a frequency of 222.100 MHz using RG-59 would be:
L (in inches) = 2952.5 X 0.66 / 222.100
L (in inches) = 1948.65 / 222.100
L (in inches) = 8.7738 (8.8 inches is fine)
Remove approximately 0.5 inch of the outer insulation from both ends. Comb out the braid and twist it into a single wire. Solder a #6 solder terminal to the twisted braid wire shield taking care not to melt the dielectric of the center conductor. Remove approximately 0.25 inch of the center conductor dielectric and solder a #6 solder terminal to it. Do this to both ends of the phasing section.
Attach one end of the phasing section to the dipole screw set on which the 50 ohm transmission line was attached previously, braid to braid and center conductor to center conductor. The other end of the phasing section is to be attached to the other set of dipole screws. Order does not matter in this instance. Order would matter if you were to use the Turnstile as a feed for a dish with a reflector behind the Turnstile by 0.25 wavelength. Then, depending on which way the braid and center conductor were connected to the second dipole, this would determine if the circular polarization of the array would be left hand circular polarization (LHCP) or right hand circular polarization (RHCP). But I digress!
This completes the construction of the Turnstile. Mount it on a pole and give it a whirl – so to speak. If it has been properly constructed the VSWR should not exceed 2:1 and be more on the order of 1.3:1 at the design frequency and not over 2:1 over several +/- MHz of bandwidth.