Kinda what I thought... rg-11 is an 84% VF. No clue for sure what
the original vapor block was. I was going to try one at 38", but
thought I should ask around. Counting the VF, the original 36"
doesn't compute to a half wave, which would be 30.43" at the
original frequency, but maybe that isn't what they were shooting
for..
Thanks
dave
--- In Repeater-Builder@yahoogroups.com, Mel Farrer
<[EMAIL PROTECTED]> wrote:
> Let's say you don't know the velocity factor, but are going to use
the same coax. The easy solution is the ratio of the frequencies.
1.0544 or 5.44% longer. 36" at 155 and 37.95" at 147.
>
> na6df <[EMAIL PROTECTED]> wrote:
>
> I'm rebuilding a db-252 corner reflector for use on 2 meters. The
> original (broken) dipole radiator was fed with a 36 inch piece of
> the "vapor block" db feedline. I think this stuff is 70 or 75
ohms,
> judging by the center conductor diameter. It's pretty much
identical
> to RG-11 (75 ohm) cable. The question is, how do they calculate
the
> length of the matching section? 36 inches is a physical half wave
> (not electrical length) at the original center frequency of 155
mhz.
> I guess I could just replace it with the same length, but I was
> thinking that it needed to be a tad longer for use at 147 mhz.
> Whadda ya'll think? I'm really just guessing at the 70 ohm
> impedance, but a straight dipole *is* 75 ohms, isn't it?
>
> Thanks in advance...
>
> na6df.. dave
>
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