The line to an antenna has Standing wave ratio.
The actual voltage will vary in the forward and reflected wave in a sine wave
manner along the line, when you have more than 1:1 SWR, but the RATIO is
constant everywhere in the line, other than slight ohmic loss in the wire
conductors. For all practical purposes, for 100 feet of run, you put in
power at one end, and measure the forward power there, and you will have the
same forward power, if you could measure it, at the antenna. But SWR bridges
want to be in a constant impedance line, and the antenna is probably not 50
ohms, and the tuner or antenna matching unit is tuning out the reactance of
the line and antenna to allow maximum forward power. The use of the SWR meter
in the 50 ohm line AHEAD of the tuner toward the rig, is to show when we have
a good match by showing the minimum reflected power. Eventually, after many
reflections, all the reflected power reaches the antenna. Reflected power
does not cross thru the matching device to your final. Those are some of the
misconceptions Maxwell covers in question and answer style. His book was made
from his several pieces he wrote for QST in the 70's or 80's.
Thus the same info is in back issues of QST.
One can spend a whole weekend reading about SWR!
:-)
73, Stuart K5KVH
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|