[Antennas] Hy-gain AV-640 issues

Mon Apr 25 13:23:41 EDT 2011

Anthony R. Gold wrote:
> Andy, I was tracking you until the last paragraph where you lost me.
>> How does the way in which someone uses a transmission line affect the
> truth of what you wrote in the prior paragraphs? Maybe you could give an
> example to illustrate your point.

(I had written: "That is all assuming you are using the coax at its
characteristic impedance, of course.")
Let's say you have a 50 ohm system, meaning, that's what your rig
wants to see, and that's what your antenna looks like. But you got
your hands on some 125 ohm coax.
Under certain conditions, you can use that coax in your 50 ohm system
without much problem. If the length is very short compared to a
wavelength, then it won't matter because the discontinuity is so
short. If it is cut to one half wavelength long or a whole multiple,
then in principle (ignoring loss), the input impedance equals the load
impedance ... so with a 50 ohm antenna on the far end, it looks like
~50 ohms at the driven end, and you'll see a low SWR there, as far as
your 50 ohm equipment is concerned.
But now let's consider loss. If that 125 ohm coax has moderate loss,
then it's going to tend to transform the actual impedance (V/I)
towards something closer to 125 ohms, the more loss there is on the
coax. In the limit as all of your input signal gets dissipated by
coax loss, the input port to the coax approaches Zo = 125 ohms, no
matter what load you put on the far end.
So in that specific case, the effect of coax loss is to increase the
SWR from 1:1 to 2.5:1. But that's only because you used 125 ohm coax.
 You got the length right to supposedly make the coax reflect its load
impedance at the near end; but the loss messed it up.
If you had used 50 ohm lossy coax, coax loss should only reduce the
SWR. By using non-50 ohm coax, loss can increase the SWR.
Be careful to note here, when I say "SWR", I mean what you measure in
a 50 ohm system with a 50 ohm SWR meter, treating that 125 ohm coax
like a black box. I do not mean the actual SWR on the 125 ohm coax,
which is different.
And I should also note that using an N X half-wavelength piece of 125
ohm coax may not be the greatest idea, because the actual SWR on that
coax increases its loss too, beyond what its specs say (which is why
people say not to use coax with a non-resonant antenna).
Regards,
Andy