To: [email protected]
From: [EMAIL PROTECTED]
Date: Sun, 8 Jun 2008 20:08:38 -0400
Subject: RE: [Repeater-Builder] Re: antenna suggestions for 440mhz
> even if the harness is balanced it's only effective at the
> design freq so a freq change makes a phase shift and alters
> the tilt / transmission angle ?
>
Don't think of the feed system in terms of wavelength multiples - I'm
guessing that's where your train of thought is jumping the tracks.<< I am
thinking of it as a resonant resistor at the design >
Consider a two-element array. We'll ignore matching issues here to keep
things simple.>> although they do have a major effect and I don't believe its
possible to do so> ?At the antenna feedpoint is a tee, and from the tee there is
four feet of coax to each element. At some frequency (say, 150 MHz), the
total electrical length from feed to element is some fraction of a
wavelength; since I didn't specify the velocity factor of the coax I'm just
going to pull a number out of thin air here, so let's say that our five feet
of coax is 1.0 wavelength at 150 MHz.
Now let's add 10% to the frequency, making it 165 MHz. The cable length
from the tee to each element is now 1.1 wavelengths at the new frequency.
BUT, the *difference* in phase between the elements is still zero!
<<I think incorrectly , I am not going to pursue the maths as I am rusty ( did
the ma a while back) but the feed impedance changes a freq shift to effect
radion angle certainly ?>It's the difference in phase that causes the beamtilt,
and since the elements are
still in phase, there is none.
It's no different than keeping the frequency the same but adding
identical-length extensions to each of the two feeder cables.<<it makes sense
but I cant agree ?>
Did that make sense? to a segree ,<< rationally if the freq is altered a retune
will allow the aerial to work when retuned so the argument only effects an
unaltered stick ?
, I must admit I have not messed about much with low band stick of late as my
primary ocupation is much further up the band with telephone and satellite gear>
--- Jeff WN3A
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