Nice try Jim. Let's get the facts straight though.
You were trying to tell us that the SWR on a transmission line would be
different at one end of the line verses the other end because of the
transformed impedance due to mismatch. Now you are telling us it will be
different because of a different reason.
If you look closely I addressed the line loss situation in my first post
about this.
Jim said:
> > A half wave, more or less in the clear, will radiate just fine.
> > What you measure as vswr at the shack end of the feedline reflects
> > impedance transformation in the transmission line. Seeing 1:1 at
> > the end of 500' of line doesn't tell you a damned thing about the
> > antenna match, or system loss. Nor does 8:1, although that surely
> > raises questions about what's wrong.
Gary said:
> Seeing 1:1 swr at the shack means that the swr at the antenna is also
> 1:1 (provided you don't have excessive feedline loss). Swr will be the
> same anywhere on the line.
Then Jim said:
> Well, yes, Gary, that's true.
>
> But suppose it's 1.2:1? is that because it's 1.2:1 at the antenna, or
> because whatever wierd impedance is presented at the feedpoint is
> transformed to something close to r50 j0?
It seems Jim is still saying that the impedance mismatch transforms the swr
from one end to the other here. Also note that there is no mention of line
loss either.
Gary said:
>If it is 1.2:1 at the shack it is also 1.2:1 at the antenna. The impedance
>may be transformed but the swr will not be.
Please note that I did not think it necessary to repeat again that line loss
could change that as stated in my first post. We were only talking about
impedance transformation causing or not causing swr to be different from one
end of the line to the other.
Now Jim seems to realize he was wrong and goes off on line loss causing a
different swr reading so he can still try to say someone is WRONG! This kind
of thing gets old on here after awhile.
We all know how line loss affects swr readings Jim.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Saturday, September 08, 2007 11:51 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Carolina Windom, etc.
>
> On Fri, 7 Sep 2007 22:13:55 -0500, Gary Schafer wrote:
>
> >If it is 1.2:1 at the shack it is also 1.2:1 at the antenna. The
> impedance
> >may be transformed but the swr will not be.
>
> WRONG! The loss in ANY transmission line will cause the SWR to be reduced
> as you travel along the line AWAY from a mismatched load. The loss can be
> predicted by Smith Chart calculations, by some equations published in the
> ARRL Antenna Book, and by N6BV's TLW program (on the CD in the Antenna
> Book). The amount of reduction in SWR increases with the loss in the line
> -- it will be greater for longer lines and for lossier lines.
>
> As an example, a mismatch of 2:1 at the antenna may be reduced to 1;5:1
> when measured at the transmitter when using RG58, but 1.8:1 when using the
> same length of RG8. An 8:1 mismatch at the antenna might be only 3.5:1 at
> the transmitter!
>
> The best way to undersand why this happens is to realize that both the
> transmitter wavefront and the reflected wave are attenuated by the loss in
> the line. Thus, at the transmitter end, the direct wave is strongest and
> the reflected wave is weakest.
>
> It's quite instructive to do an NEC model, read the values of R +jX at
> various points in the band, plug them into TLW to compute the loss for
> various types of transmission line, and plot the result as a graph. By
> replacing the RG59 feedline for my high 80/40 fan dipoles with RG1l, I
> figure that I've picked up about 1.3 dB of transmit power (averaged over
> the two bands). That's a 1.35 power ratio. No big deal for casual Q's,
> but
> can make the difference if you're right on the edge of the other guy's
> noise.
>
> 73,
>
> Jim Brown K9YC
>
>
>
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