2 Meter Turnstile Antenna For Amateur Satellite Communication

For the folded dipoles I used 300 ohm TV twinlead. What I had on hand was low loss “foam” type. This particular twin lead has a velocity factor of 0.78.

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You will also notice in the above drawing that the lengths of 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 construction details.

2 Meter Coax Balun

2 Meter Coax Balun