Unfortunately this is another spin off topic from the other recent feedline & antenna topics (or arguments
)
After much discussion I decided to try some experiments today and I discovered a very interesting thing. I connected an old 1:1 current balun that came from a ready made G5RV antenna made by
Radio Works that I purchased sometime back in the early 1990’s to my center-fed Zepp. The antenna is 130' with 90' of 390 ohm 14-gauge ladder line. The 390-ohms is now debatable. The velocity factor of the 390 ohm line in question is suppose to be really close to 450 ohm line, but I could never find an exact number. If you research on the net you will see VF numbers for both 450 ohm and 300 ohm line ranging anywhere from .98 to .80 and there is no consistency. Most of what I found said a VF of .90 should be about right.
Here is what's interesting; I connected the 50 ohm input side of the balun to my MFJ-259 HF/VHF SWR analyzer and it shows my antenna is resonate right at 4.7Mc instead of 3.6Mc. I've recently been using a Viking II with an old Ameritron "T" tuner with a built in balun and it won’t tune on 75 meters, but it tunes fine on the higher bands (well sort of on 40 meters). I think that’s partly due to the limitations of the "T" tuner because my HB linked coupled tuner tunes the Zepp just fine on all bands including 160 meters.
I found several sections of my old #14 ladder line and I kept adding 10’ at a time. With every 10 feet it dropped the Zepp’s resonance down lower, but here is the deal; I had to add about 50’ more to get the resonance down 3.6Mc. To make a long story short the feedline must be slightly longer than the antenna itself to be self resonate at 3.6Mc. The Zepp is 130’, but the ladder line must be around 140’ and that is what doesn’t make any sense.
The next step was to determine if the balun might be bad. I know for a FACT each 1/4 wave leg of the Zepp is exactly 65’. I measured both legs twice before putting it up and I know the ladder line’s length is also correct. Fortunately I had another 1:1 current balun, which is nothing more than a 1’ piece of 50 ohm coax with ferrites slipped over the coax’s PVC jacket. Needless to say the results were EXACTLY the same as the other balun.
The only other thing I can think of is the velocity factor must be near or better than perfect, but I find that impossible to believe. The VF would need to be near 1 or even better and like that’s going to happen. However, the VF must be really good like .98, but something else must also be in play. Does anyone have any ideas?
I’m curious about this because it might possibly explain some of the other recent discussions on issues relating to antenna matching via feedline, etc. An example of this would be for the feedline below using a G5RV antenna.
L= (492 x VF)/f
f = 14.286
VF = .99 = 34.1’
VF = .90 = 31.0’
VF = .81 = 27.0’
