Loosecannon: I do not isolate the antenna from the support pole for two reasons. Lightning protection is great with this antenna since it is DC grounded with no inductor in series to ground to burn out. To make use of this we still have to ground the mast, tower, or antenna base anyhow. Also, just wrapping a coax choke at the base seems to kill the entire problem.
Bob85: I am familiar with the skeleton sleeve antenna and in many ways the Sigma IV is similar. In fact Sirio makes a CX line of antennas that use a 1/4 wave tube as the sleeve rather then a skeleton. At one point Sirio made one that went down to 60 Mhz. and the gain dropped to 1.2 db. While their high band model of the CX is still around 2 db. This shows that angling the base section out to form a 25 degree cone enhances gain at lower frequencies.
The modified skeleton sleeve you mention may be doing this. I haven't seen one so perhaps you can explain the differences. Additionally the sleeve type seems to be directly end feeding the radiator and that forces you to tune it for minimum VSWR rather then the longer length that produces maximum gain. This requires series capacitance to cancel the inductive component.They don't appear to be DC grounded either.
What you noticed in the Big Mac's signal fluctuation is typical with any long collinear. It was great to actually hear this from someone who had the chance to test one. I use to see it all the time in marine VHF installations with sailboats. Customers would insist on the big collinear until the first time they were in rough water and the pitch of the antenna completely chopped up the transmission with every roll.
When experimenting with you hybrid Bob, the first thing you want to do is fabricate a loop that will proved the required 25 degree angle with respect to the radiator. Then you can fine tune the cone for maximum gain by sliding the top four support rods in or out of the lower, larger diameter base rods to find the sweet spot. Once you peak the cone out for maximum gain, there is more gain to be found by re-peaking the main radiator length. Typically you will have to shorten this as the loop is lowered on the radiator to get maximum gain. Take caution to keep the gamma match at a reasonable VSWR after each step for accurate comparison.
CDX-007: Your antenna at 26' 7'' seems a bit short for 27 MHz. Perhaps you tuned the VSWR too much by whip length and not enough with the gamma. If the antenna had the Sigma style base, it had a good low angle radiation pattern. It's not so much the whip length here that controls the angle of radiation. It's the cone base structure. Adjusting the whip length is done to maximize gain in conjunction with the 25 degree angle of the cone from the radiator and loop diameter.
About the only one of these antennas you can get now is going to be the Vector 4000. It's the only one being manufactured now. Order in the USA from H&Y Electronics. Consider the modification I mentioned in my previous post. At least do the ones to the main radiator and the antenna will be good to 1 KW. More then this will require the connector and gamma swap.
Jazzsinger: The main radiator length of the original Sigma IV was 27' 7'' and closer to 3/4 wave while the original Vector 4000 was 29 feet and closer to 7/8 wave. Interestingly it seems the Vector 4000 has be shortened to about .81 wave and they added the birdcage on top. I don't like the cage because it sure does catch birds sitting on the weak whip. Seems they did take some of my advice already on the wavelength.
Keep in mind I did my calculations for maximum gain at 98.0 MHz. and arrived at the .82 wave. This was done with in the field RF testing and not on paper. We can understand the small change of .81 wave at 27 MHz. At VHF the actual change between these wavelengths is small in terms of inches on that band. The change in wavelength at 27 MHz. will be larger physically however, 2 feet seems more then I would expect. Bottom line, peak it for far field gain and worry about the VSWR with the gamma.
