[Antennas] VSWR on Log Periodic.

2002年4月27日 18:20:54 -0600

Jesper,
A circle on the Smith Chart signifies constant SWR, although you may not know
the exact impedance, any of the impedances on the circle represent the same
SWR. In the ideal lossless case you simply rotate around the circle of constant
SWR.
In the case of a lossy line the Circle turns into an inward spiral. However, if
you did a spiral from all the possible starting points for the initial SWR (say
1.6:1) and rotated around the smith chart for the length of the coax, all the
possible end points would form another circle of lesser SWR (say 1.4:1). In
summary the starting point could be any point on the 1.6:1 SWR circle and the
ending point could be any point on the 1.4:1 circle and you don't need to know
the actual impedances of either. It is also true that if you have actual
knowledge of the impedance at either end you can calcuate/trace to determine the
impedance at the other end. ...but the point remains that know SWR at either
end and the length and loss of the coax is adequate to determine the SWR at the
other end.
73
chris, n4vi
Jesper W Jespersen wrote:
> I have heard this statement mentioned before, that you could calculate the
> SWR at the antenna from knowing the SWR at the end of the feedline and the
> feedline propperties, but I have to disagree.
>> If you knew the impedance at the end of the feedline and the feedline
> propperties you would be able to calculate the impedance of the antenna. But
> that is another matter entirely.
>> Picture a smith chart, a given SWR say 1.6 is a circle arround Zo. Any given
> feedline will move a point on this circle to another point in the smith
> chart, forming another figure. All the points on this distorted circle are
> possible candidates for the ipedanse of the antenna and will give wildly
> different SWR's.
>> What makes you guy's think that there is a way to calculate 1:1 from SWR at
> the feedline to the SWR at the antenna ?
>> Do you silently considder the load to be purely ressistive ?
>> Greetings from Denmark.
> Jesper Wolf Jespersen
> OZ8ACE
>> ----- Original Message -----
> From: "Harvey&Bessie" <[email protected]>
> To: "A10382" <[email protected]>
> Cc: "Antenna reflector list" <[email protected]>
> Sent: Thursday, April 25, 2002 8:30 PM
> Subject: Re: [Antennas] VSWR on Log Periodic.
>> > If the line loss is known, it a simple matter to compute the actual SWR
> > (SWR at the antenna) from the reading at the transmitter. As someone has
> > pointed out, Maxwell shows how to do this in his book. Why make a big
> > deal of it? It is simply too inconvenient to measure it at the antenna,
> > for the value one gets from that reading. Of course, in cases of high
> > line loss, one sees a very favorable SWR "downstairs" even when there is
> > a poor match "upstairs."
> > Harvey/W4TG
> >
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