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2 Meter Turnstile Antenna
For Amateur Satellite Communication
Here are construction plans of a Turnstile antenna that I use for space communication on the 2 meter amateur radio band. Specifically for 145.80 mHz. A Turnstile antenna with a reflector underneath it makes a good antenna for space communications because it produces a circularly polarized signal pattern and also has a broad, high angle pattern. Due to these characteristics, there is no need to rotate the antenna.
My design goals were that it had to be cheap (of course!) and made from easily available materials. In looking at other turnstile antenna designs, one thing that has always bothered me is that they use coax (un-balanced feedline) and directly feed the antenna (balanced load). According to the antenna books, this situation tends to cause the coax to radiate, and upset the overall radiation pattern of the antenna.
The Antenna
What I decided to do is to use "folded dipoles" instead of traditional ones. Then feed the turnstile antenna with a 1/2 wavelength 4:1 coaxial balun. This type of balun also takes care of the "balance-to-unbalance" problem usually encountered as well.
The drawing below shows how to make a turnstile antenna. Please note, this is not to scale.
For the folded dipoles I used 300 ohm TV twinlead. What I had on hand was low loss "foam"
type. This particular twinlead has a velocity factor of 0.78. You will also notice in
the above drawing that the lengths ot the dipole aren't what you would expect for 2 meters.
This is the length I ended up when I was finished adjusting for minimum SWR. Apparently
the velocity factor of the twinlead figures into the resonance of the folded dipole. As
they say, "Your mileage may vary" on this length. I would also like to point out that in
the drawing above the feedpoint of the folded dipoles is actually in the center of the
folded dipole. I made the drawing this way for clairity.
The Reflector
In order to get the radiation pattern in the upward direction for space communications
the turnstile antenna needs a reflector underneath it. For a broad pattern
the antenna books recommend 3/8 wavelength (30 inches) between the reflector and the
turnstile. The material I chose for the reflector is ordinary window screen you can
pick up at a hardware store.
Make sure it is metal screen as there is a non-metal type of window screen they
sell as well. I purchased enough to lay out an 8 foot square on the rafters of my attic.
The hardware store couldn't give me one big piece for all of this, so I overlapped
pieces of screen by about a foot on the seam. From the center of the reflector, I measured
up 30 inches (3/8 wavelength). This is where the center, or the crossing point of the
folded dipoles are located.
The Phasing Harness
This is not complicated at all. It is nothing more that a piece of 300 ohm twinlead that
is an electrical 1/4 wavelength in length. In my case, with a velocity factor of 0.78 the
length is 15.75 inches.
The Feedline
I constructed a 4:1 coaxial balun to match the feedline to the antenna. In the drawing
below are the contruction details. Use a high quality, low loss coax if you have a long
way to run your feedline. In my case, I only needed 15 feet of coax so I used RG-8/U coax.
This is not normally recommended, but with the feedline this short there is less than
1 db loss. The dimensions for the loop are dependent on the
velocity factor of the coax used. Connect the coaxial balun to the feedpoint of the
turnstile antenna, as shown in the drawing above.
The Results
I am very pleased with the performance of this antenna. Because I did not need the
added expense of an AZ/EL rotor, I felt justified in purchasing a Mirage preamplifier.
Even without the preamplifier, the MIR spacecraft, and ISS are full quieting in my receiver
when they are about 20 deg. or higher in the sky. By adding the preamplifier, they
are full scale on the S-meter at about 5-10 deg. above the horizon.
Vel. Factor 0.66
Vel. Factor 0.78
26.73 in.
31.59. in.