I think you need to multiply the physical wavelength by the velocity
factor... which is about ~.8 - .88 or so depending on the specific
cable...on the longest frequency..
Doug
At 05:19 PM 3/14/2005, you 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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