[Antennas] VSWR on Log Periodic.

2002年4月28日 16:47:00 -0600

Jesper,
Ok, I understand your point of view and I will have to put on my thinking cap.
It seems to me the results are predictable if you know the impedance at the
antenna, but if you only know the SWR I'm not sure you can determine the final
answer.
Of course from a practical point of view, since there aren't transmitters with
Zo of 600 ohms you use a tuner to alter whatever impedance is presented by the
transmission line to the tuner to the Zo of the transmitter.
BTW, in the past I have used the exact 1/2 wavelength concept you mentioned so
that I could do antenna switching and have the balun at ground level.
73
chris, n4vi
Jesper W Jespersen wrote:
> Hello Chris.
>> Thank you for the explanation, it makes good sence when the Zo of the
> feedline is the Zo your transmitter wants.
>> But often we are feeding our antennas through feeder systems that dont have
> the Zo of the transmitter, or not all the way.
>> We use open wire feeders since they have much less loss, particularly with
> large SWR's on the feedline.
>> Lets take a simple example to illustrate.
>> Here say a dipole at resonance giving a 72 Ohm resistive impedance meets a
> 600 ohm feed system giving a VSWR of 8.3.
>> There is practically no loss in a open feeder so we can ignore that, but the
> impedance is transformed through the length of the feed.
>> At one point we attach the feed line to a 1:1 transformer and want to match
> the impedance to the transmitter with Zo of 50 Ohm.
>> If we have a length of multiples of half wavelengths we will see the 72 ohms
> at the end of the feedline and will now have a VSWR of 1.44 (50 Ohms).
>> If we chose an uneven multiple of 1/4 wavelengths, the impedance will also
> be resistive and of a magnitude of 3750 ohm, this gives a VSWR of 75 (50
> ohms).
>> Other wire lengths give other results, but I claim that you cannot predict
> the WSVR at the antenna from the VSWR at the end of the feed line because of
> the different Zo's.
>> This also illustrates why you have to take care what your feeder length is
> or you may ruin the performance of a perfectly fine antenna :-)
>> I apollogize if my message offends some in this newsgroup.
> I never meant it sarcastically, just wanted an explanation.
>> English is not my mother tongue, so maybe I cross the line between giving a
> thorough explanation, and ridiculing my audience.
> For that I appologize.
>> Greetings from Denmark
>> Jesper Wolf Jespersen
>> OZ8ACE
>> ----- Original Message -----
> From: "Chris Adams" <[email protected]>
> To: "Jesper W Jespersen" <[email protected]>; <[email protected]>
> Sent: Sunday, April 28, 2002 2:20 AM
> Subject: Re: [Antennas] VSWR on Log Periodic.
>> > 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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