[Antennas] Hy-gain AV-640 issues

Mon Apr 25 15:03:51 EDT 2011

On 2011年4月25日 13:23:41 -0400, Andy <ingraham.ma.ultranet at rcn.com>
wrote:
> 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).

Thanks for your various comments on transmission lines, but they did not
answer my question. You deleted the paragraphs I was commenting on so let
me recap: you wrote that VSWR reduces with increased line losses, which
agrees with the arithmetic, but then you added that this relationship was
only true if one used the line in some particular manners. I did not
understand that last part. Can you give an example of a situation where
one's manner of use of the line will change the relationship between
increased losses and reduced VSWR?
Tony