[Antennas] SWR Isn't Enough

2002年4月28日 08:47:14 -0600

I use a 44 foot doublet fed at the center with ladder line on 10 M through
40 M here. The length of the ladder line is about 20 ft long. From there I
use a coil of coax for a balun and a short length of coax to the auto tuner
of my TS-850. I have some different lengths of coax I can insert to help the
match on some bands. More on that later. It is a good antenna.
I find feeding the antenna quite manageable. Although there are no "magic"
lengths for center fed doublets and their feed lines, some lengths work
better than others. The above antenna was described in a QST article from
the early 80s and is reprinted in the "Wire Antenna Classics Volume 2" from
the ARRL. The author crudely modeled this antenna a shorted one sixth wave
length transmission line (the antenna) connected to an open one sixth
transmission line (the antenn)a. This gives a fairly low primarily resistive
feed point at the end of the transmission line at unusual multiples of the
one sixth wave frequency, 1,2,4,5,7,8, ... and so forth. Like I said, this
is a crude model but it is helped by the fact that the equations describing
the transmission lines are slowly varying logarithmic.
For 30 M, the antenna is a half wave, so I add enough feed line to bring the
total feed line length to a half wavelength and the feed point resistance of
the dipole is seen at t the feed line feed point. I add other feed line
lengths on 17 M and 12 M to give a low feed point resistance.
Cecil Moore, W5DXP explains in detail how to use different transmission
lines to tune the antenna system on his web page
http://www.qsl.net/w5dxp/notuner.htm
It should tell you all you need to know.
Good luck with your antenna project. Let us know how it all turns out. - Dr.
Megacycle KK6MC/5
-- 
James R. Duffey KK6MC/5
Cedar Crest, NM DM65