The Sigma IV explained in simple terms.
Checking the statistics of my FMBroadcastAntenna.com | HIGH GAIN FM BROADCAST ANTENNA web site allowed me to see traffic resulting from links in this forum. That prompted me to explore the link and I've enjoyed reading the posts here. In a way I felt obligated to share what I've learned in the past three decades working with this "Sigma IV" style antenna. This is not a plug for my FM scaled version of the antenna since CBers have no use for that product. However, the same principles of how the antenna works and what improves it's performance still apply on 11 meters.
First, the antenna is definitely not a J-Pole. This name originates from Sirio who called it a coaxial J-Pole. There is no 1/4 wave element driven with RF to inductively couple to the main radiator. Therefore the name does not fit. There is also much confusion as to if the bottom 1/4 wave portion of the antenna radiates, or not. Using a simple field strength meter will confirm almost nothing is being radiated in the area of the cone. The more important question should be "how does the cone section improve performance"? The cone makes this antenna superior in two ways that only this design has been able to accomplish.
The cone "forces" RF up the main radiator with very little lost in the first 1/4 wave. The longer wavelength main radiator causes much more RF voltage to build across it's length then could ever be possible by simply end feeding a half wave. Higher RF voltage on a resonant radiator creates a stronger electromagnetic field. The "cone effect" as I call it also provides much needed focusing of the radiation pattern in the .82 wave design. Without it, the antenna would have the disastrous calculated results of a 45 degree radiation angle! Thankfully the real world results are closer to 12 degrees above the horizon. Much more useful for land based communications even with low antenna height.
Back in the days before getting into FM broadcast, I found this radiation angle to give a profound advantage in ability to work DX on HF. This doesn't happen too often at VHF but I can tell you for lack of better terms, it sure kicked butt over any other single element antenna. I now refer to the new generations of the Sigma IV style antenna as a .82 wave coaxial antenna due to the coaxial cone structure at the base. I've tried every vertical antenna possible with on the tower tests and no single element vertical puts more signal on the horizon then this antenna. The only possible exception is the Big Mac. This antenna is not a single element and they are so scarce I could not find one to test.
It is important to understand that the Sigma IV gets it's gain by lowering the angle of radiation on the horizon. This is not like the effect of a beam that concentrates most signal in one direction. The result with the Sigma IV is that the improvement in signal is typically not noticed locally. The advantage only becomes obvious in testing over 20 or 30 miles. It is the shape and electrical length of the coaxial base that is entirely responsible for lowering the angle of radiation. The Sirio is not designed for maximum gain in this area. It has been slightly compromised to reduce the mechanical size of the loop.
As noted in Avanti's original patent maximum gain on the horizon occurs with a cone angle between 25 and 30 degrees with respect to the main radiator. Since the coaxial base is electrically equal to a 1/4 wavelength, we must shorten the stock four support rods and increase the diameter of the loop to reach the 25 degree angle. Yes, four support rods work better then the original three that Avanti used. Adding more then 4 will not help much. Sirio was on track when they increased the wavelength from 3/4 wave but they past the maximum gain by going to 7/8 wave.
The Sigma IV design provides more gain when the main radiator is inductively reactive. This is why it requires the capacitance of the gamma match to cancel out the inductive reactance. Some here have suggested the gamma match has inefficiencies that can cause a loss of 6 db. Totally 100% false! If you give up the gain added by going beyond 3/4 wave you can directly shunt feed the 3/4 wave without the gamma. The radiator can even be isolated from ground and directly fed. The result in either case is less gain. Therefore any loss introduced by the gamma is more then compensated for. On FM broadcast, on the tower testing has proven that .82 wave gives the most gain. Just slightly shorter then the 7/8 wave.
With respect to modeling this antenna with NEC, this has proven to be a complete waste of time. I've had at least a half dozen "RF engineers" over the years tell me that NEC shows the primary lobe at 45 degrees above the horizon. This couldn't be anymore inaccurate! If that were the case my "money back guarantee" would have put me out of business years ago. The FM version has replaced countless 5/8 wave Comets and in every case provided a larger coverage area. Didn't matter if the antenna was 30 feet AGL or 200 feet.
The biggest mechanical failure with the Vector 4000 results from screwing together the telescopic whip. Use a saw to slit the top of each section so that a hose clamp can tightly lock the sections together. Then install the bottom screw only in each section so they can't slide down. This will stop the top sections from breaking off. The RF components on the stock antenna leave a lot to be desired if you run power. The SO-239 is specifically designed with little space between the center conductor and ground ring so that it will arc under power. The stock gamma is no better.
Remove the original SO-239 and drill out the mounting hole so that it will accept a single hole Teflon insulated UHF connector. Solder an eye connector to the center pin so that you can bolt a Maco 5 KW match to the connector. Some have made comments about making the antenna handle 25 KW. You bet it can be done if you can weld a DIN connector to the original bracket and use a vacuum variable cap. I know of one person that is dumping the full output of a 4CX15,000A into one as I described.
When using this design on HF the Sigma definitely benefits from the use of a coaxial balun in the feed line. This can be as simple as a few loops of coax just below the feedpoint. When testing the antenna against the Interceptor 25K we clearly noticed an improvement in gain by no less then 1 db. When the loop is expanded to the 25 degree angle this goes up to almost 2 db over the 5/8 wave. The drawback was that an increase of unwanted RF interference was noted due to radiation taking place on the coax line. While I can't confirm that adding the balun increased signal any, it certainly reduced RF interference to the levels of the Interceptor.
As a side note, I've been working with Stefania at Sirio to improve the Vector 4000 for CB and 10 meter use. I've provided her with both my mechanical and electrical improvements and she has passed this along to her research and development team to see if they can economically implement my suggestions. Unfortunately the anechoic room at Sirio has been designed to model antennas operating at UHF or higher. According to Stefania it was installed at their facility to design 800 Mhz. and Wi-Fi antennas.
I hope this information has been helpful to those searching for the best omni directional antenna. While the Sigma IV design is often misunderstood or not even heard of, I can assure you beyond any doubt you cannot purchase a more effective ground plane. You can spend more money on an Interceptor but you can't buy more signal unless you install a beam. To the few that have claimed the Vector does not outperform the 5/8 wave, your tests absolutely contained other variables that effected your results in an adverse way.
