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:
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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