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PICList Thread
'[OT] SWR'
1998\03\27@112938 by Harold Hallikainen

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       I prefer to look at SWR as the ratio of the magnitude of the load
impedance to the magnitude of the characteristic impedance of the
transmission line, or the reciprocal of this ratio, whichever gives a
number of 1 or greater..  This corresponds to the radius of a circle on a
Smith chart.  An ideal 1/4 wave vertical over a ground plane has an
impedance of something like 32+j0 giving an SWR of 1.56.  Although
transmission line losses increase somewhat with SWR, i don't believe this
is significant.  More significant, in my view, is the mistuning of the
output network of the transmitter by its being loaded with a load
impedance that is different from that for which it was designed.  The
load impedance seen by the transmitter is the antenna driving point
impedance "rotated" around the Smith chart by the transmission line
length (with the radius dropping somewhat due to line losses).  If we
have a 50 ohm line and a 50 ohm load, we "rotate" around a circle of
radius zero as we travel from the antenna (the load) back to the
generator (the transmitter).  Thus, the transmitter still sees 50 ohms.
If instead we terminate the line in our 32+j0 load, we can go 1/4
wavelength down the line and the impedance gets "inverted".  Instead of
being 50/1.56, it's 50*1.56 or 78 ohms (note that we still get the same
SWR).  This impedance then loads the output network.  It is transformed
by the output network to an input impedance that loads the output device.
Any load impedance other than that for which the network was optimized
will result in a nonoptimum load on the device (typically giving less
output power and possibly adversely affecting efficiency, possibly
increasing dissipation to where the device can be damaged).
       Besides antenna driving point impedance, there is the radiation
pattern to consider.  A vertical antenna over a ground plane will have an
omnidirectional horizontal pattern, but will have a vertical radiation
pattern that depends upon the height of the radiator.  Each increment of
the radiator can be thought of as another very short antenna with a
current flowing through it.  The phase and magnitude of that current
varies depending upon where the point is, and the length of the radiator.
The resulting fields are added up to get the overall vertical pattern.
       There's a fair amount of data on vertical radiators (admittedly
the data is at about 1 MHz) in the NAB Engineering Handbook...  (While
you're there... see my chapter on transmitter control systems :)

Harold



On Thu, 26 Mar 1998 13:26:31 -0000 Eric H <spam_OUTefh77TakeThisOuTspamHEXLINK.COM> writes:
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