Date: Fri, 17 Dec 93 04:30:06 PST
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Subject: Info-Hams Digest V93 #1472
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Info-Hams Digest Fri, 17 Dec 93 Volume 93 : Issue 1472
Today's Topics:
SWR tweeking: Details, details... (2 msgs)
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Date: 16 Dec 93 23:53:58 GMT
From: ogicse!hp-cv!hp-pcd!hpcvsnz!tomb@network.ucsd.edu
Subject: SWR tweeking: Details, details...
To: info-hams@ucsd.edu
George S. Reiswig (gereiswi@nmsu.edu) wrote:
: A question for all you techno-whizzes out there: When measuring
: the SWR with (guess what) an SWR meter, is it really crucial to place the
: meter between the feed line and the antenna, or can you put it between the
: transmitter and the feed line? Would the latter yield spurious readings?
An SWR meter will give you an accurate indication of SWR on a line
of the impedance for which the meter is calibrated (designed), wherever
you put it in that line, assuming that the meter itself is accurate.
The SWR on the line will be different at different points along the line
because of loss in the line, and possibly because of internal reflections
in the line (e.g., if the line is damaged). It is important to note
that the meter you use knows nothing about the line its connected to
except the current and voltage at the point where it's connected--that
is, if you hook it to a line that's a different impedance than the
meter is calibrated for, you will get wrong readings.
If the meter is appropriate for the line you are using and you want
to match the antenna to the line, you will get the most accurate
readings by putting the meter at the antenna end. That's because
you measure all the reflected power, not attenuated by a length of
line, and you measure only the forward power that actually reaches
the antenna, not the full xmtr output unattenuated by the line. But
if it's more convenient, you can start your measurements at the
transmitter end. If you get that end to 1:1, then the other end will
be 1:1 also. Double-check if you wish when you are satisfied with that
by putting the meter at the antenna. But of course if it's up 150
feet in the air, this isn't very practical...
If you know the line loss, you can predict the SWR at the opposite
end knowing the SWR at one end. If your transmitter doesn't complain
too much, you can measure the line loss by opening (or shorting) one
end and measuring the SWR at the other end, and use the same formula
rearranged, since you know the SWR at the open (or shorted) end is
infinity:1. It doesn't take a whole lot of line loss to make the
SWR quite a bit "better".
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Date: 16 Dec 93 23:31:50 GMT
From: ogicse!emory!kd4nc!ke4zv!gary@network.ucsd.edu
Subject: SWR tweeking: Details, details...
To: info-hams@ucsd.edu
In article <2eq8cuINNpgl@dns1.NMSU.Edu> gereiswi@nmsu.edu (George S. Reiswig) writes:
>
> A question for all you techno-whizzes out there: When measuring
>the SWR with (guess what) an SWR meter, is it really crucial to place the
>meter between the feed line and the antenna, or can you put it between the
>transmitter and the feed line? Would the latter yield spurious readings?
> Thanks for the help, and Merry Christmas!
Well that depends on why you're doing the SWR measurement, and how well
you know the characteristics of your line. If you know your particular
line characteristics, you can make the measurement *anywhere* that's
convienent and use the Smith Chart, or the formulas in the Antenna Book
to determine what complex impedance appears at any point in the system.
If you don't know the characteristics of the line, then you can't determine
the "real" SWR on the line anyway, but by measuring at the antenna, with
suitable precautions to avoid near field effects, you can determine the
mismatch magnitude at the antenna, but not it's precise complex nature or
absolute sign without further measurements. For most uses of the "will my
transmitter like this?" nature, measuring at the transmitter end will be
all that's necessary.
Gary
--
Gary Coffman KE4ZV | I kill you, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | You kill me, | uunet!rsiatl!ke4zv!gary
534 Shannon Way | We're the Manson Family | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244 | -sorry Barney |
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End of Info-Hams Digest V93 #1472
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