I made series of CoCos from a coax of unknown VF. The only thing I knew that it is RG6 with FPE insulation, and the VF lies somewhere in the range 0.8 to 0.85. I made CoCos using different VF in this range i.e. from 110 mm to 117 mm, and one of these (114 mm) performed better than others (but still worst than Cantenna & Spider). From 114 mm figure, I inferred that my unbranded coax’s VF is 0.83.
By the way, I have substantially improved the 114 mm coco by inserting a 1.5 pf Capacitor into the feed line at 95 mm from feed point. This shows that SWR of 114 mm Coco was very high.
Today’s simulation shows that 95% of calculated length gives better SWR. I am going to make one more CoCo of element length 95% of 114 mm (=108 mm) to see how it performs compared to the 114 mm one.
Over on Plane Finder forum you posted
(3) The correct length of ¼ wave whip : The 1090 Mhz wave in air is 275 mm. Hence ¼ wavelength is 68.75 mm which is rounded to 69 mm. When RF current travels in a wire, the length of wave is slightly less (1% to 5%), and depends on diameter of wire use. The figure of 65.4 mm originates from ARRL book used by hams, and mainly pertains to lower freqencies of 440 mhz & below (uhf, vhf, hf).
At the ADS-B frequency, does the copper wire affect the propagation of radio waves? Does 4NEC2 take this into consideration?
(1) Yes, at all frequencies, propagation in wire is slightly slower than the air, hence wavelength is shorter, but the amount of reduction depends on frequency & dia of wire.
(2) Yes NEC does take into consideration this effect, but too much quantitatively. I remember when I simulated for a dipole, it gave minimum SWR at 63mm instead of 69mm limb length. When I practically tested, I found that 67 mm for un-insulated wire, and 66 mm for insulated wire gave me the best result, but the difference in performance between 69mm & shorter length, though noticeable, was small.
Got some new information about 5% reduction in length of a dipole as below:
(1) Practical wire elements are not the same length as the free space value.
This is due to capacitive end loading (‘End Effect’) which makes an antenna element appear to be slightly longer than it actually is. So a half-wave element needs to be slightly shorter (usually about 5%) than a half wavelength in free space.
(2) The end effect changes the complex impedance of the antenna/feedline system so that the resonant frequency occurs at a shorter physical length than the one half wavelength predicted by theory without end effect. The end effect is effectively modeled by an additional lumped capacitance.
[Edit] Does the 95% hold true for lower frequencies?
The 95% figure for CoCo is something new I found only today, and I dont know if it is frequency dependent. I am also not sure if it is something real or a simulation error, unless a prototype is made & put to trial.