I have seen the Sigma 4 type antenna work quite well, I used one for a few years.
I have heard about the interference problems also, and this may be from the vertical element is not insulated from the mast, and uses a Gamma Match.
My complaint comes from the tuning of the Sigma 4 type antennas. I installed mine and had a difficult time adjusting the antennas SWR. As the antenna was seting on the retracted push-up telescoping Mast, it had a low SWR, but once it was raised into the air the SWR went to something like 3 to 1.
After messing with it a few days I settled for the antenna being raised almost to the full length of the push up mast, then lowered a few feet to allow tha SWR to acceptable low. This was indicating that the mast and metal guy wires was also part of the radiating element.
(A unfriendly call to Avanti did not get a answer. But I came up with my own. You will not see a antenna like this design in a military, commerical, or amateur ares. After the time spent on the roof and fighting the SWR as cpmpared with the antennas heigth, I felt a little cheated.)
The original Avanti manufactured Sigma 4 held up to some pretty high winds for a few years, but I had to take it down, and drill the antennas telescoping aluminum tubing and install stainless steel 10-24 screws and locking nuts to keep the vertical radiator aluminum tubing sections from coming apart. The foreign made versions of the Sigma 4 use sheet metal screws, a bad idea to save money by the manufacture.
If I was going to install a Sigma 4 Antenna I would use the good UV resistance rope (None Metal) sold by AES or HRO, as guy wires. And get ready to measure the antenna SWR as it is raised up, if your your using a telescoping Mast. As of course the higher the better.
The Sigma 4 type antenna uses the hoop as a radiation restrictor, to get the antenna to operate as a 5/8 wavelength antenna, and not a 3/4 wavelength antenna. This is done so that a very expensive insulator (and supporting hardware) does not have to be purchased by the manufacture.
Some foreign venison's of the Sigma 4 Antenna use a butter type aluminum tubing, that is very soft. After a strong wind it looks as if its still is in the wind, being constantly bent! I think that copy came from France.
Using the LMR400 coax is a good call.
Jay in the Mojave www.a1antennas.com
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