Okay Cort - this one is straightforward. That coax forms both a 4:1 impedance shift and the balun function. The shorting bars are part way out the DE, just at the point to provide a 200 ohm balanced match, hence the 4:1 balun. For that setup, the coax needs to be an exact 1/2 wavelength. It is not highly sensitive to length, but having it off by more than a few percent can definitely shift the minimum SWR up 100 KHz. I can calculate the different lengths for you, but to get started, the velocity factor of:
RG58 = 0.83
RG58A (foam) = 0.78
RG58A (solid) = 0.66
RG8 = 0.66 to 0.88 depending on manufacturer and solid vs. foam
RG213 = 0.66

So, you really need to know what coax you are using to get the Vf correct. Better still is to cut the cable with an antenna analyzer to be exactly 1/2 wavelength. ALso, you could cutit long and trim it in.

As an aside, if you use foam I would not wrap the coax into a tight bundle. Over time the center conductor will migrate and short out. Solid won't. Foam is fine, but can't be coiled into a tight radius.

73,
Hal
N4GG

On 9/9/2013 6:19 PM, Courtney Judd wrote:

ok thanks all that sent answers. Let me go over this again. Originally it came with rg58 single length piece of coax 80 1/4 in total length with 78 in of shield total. each end of the coax is attached to the driven elements.... one on each side... and the shields go to the boom. A beta match tube with shorting bars also attaches to the driven element on each side. according to the manual to use with high power just subsitute rg213 same dimension as the 58 and it should be ready. I used a piece of RG8 with solid dielectric cut the same. I have a hard time believing that this could shift the resonant frequency by 1.5 mhz. the manual doesn't say anything about it being 4:1 balun it's just a matching line, i would guess. Also I have interaction with the 7 el 10 ft above it; when the two are aligned in the same direction, the swr on the 7 el also jumps to 2:1... go figure! Cort
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