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

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_OUTefh77TakeThisOuTHEXLINK.COM> writes:

{Quote hidden}

>{Original Message removed}

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