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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