After looking at Shockwaves posted model, finding the thread, reading through the last 10 pages, etc. I've come to a couple conclusions. I was one of, if not the first person to model the antenna, and have been running through my models for the last couple weeks, as I had an I10K come down, and had to go back to my Sigma...... I have an ORIGINAL Sigma IV. The only difference is mine has an N connector at the base. Otherwise, stock, all the way to fitting in the stock 'stops' for length.
Yes, the antenna 'basket' or cone does confine radiation.
Yes, the TOP section is a HALF WAVE. The model shockwave posted, as well as the model(s) I've done up and checked out confirm this. A half wave radiator radiates evenly, from maxima to minima.... and then back again to voltage maxima at the feedpoint (read this as HIGH impedance, REAL high impedance).
After having some debate in the A1 antenna forum about it, I set out to check it out even more. Well, turns out an enterprising amateur did some test range experimentation and found that on THE HALF WAVE MONOPOLE (or ground plane) you only need .05 wavelength radials to approximate THE SAME EFFECT AS FULL LENGTH 1/4 WAVE (per side) RADIALS! Check the radius of the hoop on the Sigma.
This EFFECTS the ground radial or ground planes that we see on the quarter wave, five eighths, and (erroneously) on the half wave ground planes. Modeling confirms this: It ALSO confirms that if you try to shorten a qtr wave, 5/8 or anything OTHER than a half wave, it WON'T WORK!!!! The pattern goes to hell in a handbasket.
THEREFORE!!!! The model that shockwave posted, the modeling I've seen and performed myself, and the theory of slotted antennas and others points to this: The BOTTOM section (the basket) works on the principle of a transmission line, and it transforms the impedance seen at the halfwave feedpoint (which is at the basket) down to a MUCH lower feedpoint impedance which is also slightly inductive.
Proof in the pudding you say???
OK, here ya go!
Get some GOOD ferrite beads and put them JUST below the basket attachment point to the 'radials'. This will prevent radiation, JUST LIKE A BALUN OR CHOKE ON A COAXIAL FEEDLINE!!!. Do NOT try this with full power, you'll crack'em good! The INSIDE of the basket will STILL contain the radiation pattern (and the 4 radial version does better than the original siggie design), BUT you will get LITTLE 'feedline radiation' from the OUTSIDE of the basket, or as Shockwave had put it, the colinear effect....
The antenna BASICALLY performs just like the marketing BS from the Antron.... It's a half wave over a quarter wave... The ODD thing that makes this NOT apparent and nobody takes into account one thing (in the basket): VELOCITY FACTOR. Because the basket flares out and is a transmission line, the internal conductor will HAVE to be shortened to maintain ELECTRIC length.
The "sigma" design found a novel approach of getting rid of the BIGGEST problem in the antenna industry: Matching a low impedance to a high impedance. The half wave has > 1K ohm of input resistance, coupled with reactance. The 5/8 'ground plane' is close to 50 ohms, and a bit of capacitive reactance, easily tuned out with a coil or other inductor. In talking with Jay, the BIGGEST problem he sees with the half wave as a radiator for us on 11 meters is the transformation ratio from 1Kohm to 50 ohms. I concur, having had to design quite a few matching networks.... The bigger the ratio, the more loss is inherent.
Lastly, in looking at the antenna this way, it shows why I see the effects I do... The 5/8 does NOT gain me CRAP locally.... I lost. In DX, I gained. The half wave antenna has a bigger lobe contained at the lower portion of radiation... I'm 6K feet above EVERYONE.
Anywho, open to any discussion or thoughts.
--Toll_Free
