How to make a Turnstile Antenna for Satellite Communications

Here are the building and construction plans for the Turnstile antenna that I use to communicate in space on the 2 meter amateur radio band.


A Turnstile antenna with a display underneath it makes a good antenna for local communications because it creates a circularly polarized signal pattern and also has a wide, high angle pattern. Because of this characteristic, there is no need to rotate the antenna.


My design goals were that it had to be cheap (of course!) and made from readily available materials. In looking at other styles of gate antennas, one thing that has always bothered me is that they use a coax (unbalanced feedline) as well as a vertical feed antenna (well balanced). According to antenna literature, this situation tends to cause the coax to shine, and upset the overall radiation pattern of the antenna.


The Antenna

What I decided to do was to use "double dipoles" instead of normal. After that feed the gate antenna with a 1/2 wavelength 4:1 coaxial balun. This type of balun also addresses the "ratio-to-disorder" issue that is often encountered as well.



The diagram below shows how to make a gate antenna. Please note, this is not an exception.

 

2 meter gateway antenna for satellites

Construction of a gate reflection antenna with 2 1/2 wavelength vertical dipoles displaced 90 degrees from different angles (such as a large X). Then feed another dipole 90 degrees out of the second phase. Another problem with Turnstile Reflector antennas is that the structure of raising the reflector part can be difficult.


Unfortunately (some may disagree) I decided to build my own conversion antenna in my attic. This solves another problem in that I also don't have to worry about setting up the antenna.


For the paired dipoles I used 300 ohm television twinlead. What I had was a reduced loss "foam" type. This particular double lead has a ratio of 0.78.


You will also notice in the above diagram that the size of the dipole is not what you would expect for two meters. This is the length I tied when I finished tuning for low SWR. Apparently it is the ratio of the number of twinleads in the resonance of the double dipole. As they say, "Your mileage may vary" at this distance. I would also like to mention that in the picture above the feedpoint of the paired dipoles is in the center of the paired dipole. I created a picture this way to make it clear.

The Reflector

In order to obtain the radiation pattern in the upper directions of the aerial communication antenna, it needs a reflector below it. For a wide pattern antenna the literature recommends 3/8 wavelength (30 inches) between the display and the gate. The product I picked for display is a typical home window display you can pick up at a hardware store.


Make sure it is a metal screen as there is a non-metal type of window screen they offer as well. I bought enough to cover 8 square feet of wall space in my attic. The hardware store couldn't give me a big deal on each of these, so I wrapped the display material with a foot on the joint. From the center of the screen, I measured 30 cm (30/3 wavelength). This is where it lies between, or goes beyond the object of paired dipoles.

The Phasing Harness

This is not done hard at all. It is nothing more than a piece of 300 ohm twinlead that is electrically 1/4 wavelength in length. In my case, with a different measurement of 0.78 the length is 15.75 inches.

The Feedline

I made a 4:1 coaxial balun to match the feed to the antenna.

2 meter balun for turnstile antenna

Use high quality, low loss coax if you have a long way to run your feedline. In mine, I only needed 15 feet of coax so I used RG-15/U coax. This is rarely mentioned, but with this short feedline there is less than 8 db loss. The dimensions of the loophole depend on the velocity factor of the coax used. Connect the coaxial balun to the feedpoint of the rotating antenna, as shown in the figure above.

The results

I am very pleased with the performance of the antenna. Because I didn't want the extra money for the AZ/EL rotor, I really felt it was worth buying the Mirage preamplifier. Even without a preamplifier, the MIR spacecraft, as well as the ISS is completely silent in my receiver when it is related to 20 deg. or great in heaven. By including the preamplifier, the full scale on the S-meter is about 5-10 deg. about the view.