Don't forget the "magic" 50 ohm point for a single band dipole is not
the natural impedance of the dipole. The dipole over a certain
conductivity of earth at approzimately a quarter wave height, will show
72 ohms approximately. Now, if it rains this changes, it there is
drought this changes, etc. Thus, you never can depend on having 72
ohms, or 50 ohms at some other height, all the time. A mismatch of 72
to 50 is only about 1.5:1 SWR, which is of minimal concern. All of the
reflected power at SWR 1.5 will eventually radiate from the dipole less
cable losses for each trip up and back and up to the antenna.
Using a particular impedance cable does not change anything at the
antenna.
It is a combination of matching the rig impedance, and using a tuner in
between to remove SWR at the rig end, that creates an antenna system.
You will still have 72 ohms at the antenna center, and 50 ohms on the
end of the tuned cable, but this short distance reflection at the end of
the coax is of almost unmeasured loss from the coax end to its
attachment to the elements.
Even lower loss than using tuned coax would be to use higher than coax
impedance balanced feeders. Check the losses per 100 feet for 450 ohm
nominal window line and that for any half inch coax, in the catalogs.
Higher impedance lines mean you have lower current and less I squared R
losses in the line.
-Stuart Rohre
K5KVH
Jim WA9YSD wrote:
Lowest loss matching an antenna has to be the antenna that does not
need a metal composite balun or a tuner and use only Type "N" connectors
through out.
With that said, and saw some one say use single band antennas.
Use a COAX CHOKE at the feed point.
You want a 50 ohm center impedance at the feed point and use 50 ohm coax.
At this is telling me that an inverted vee would be the optimal antenna to use.
If you have a 100 foot tower and a yagi at the top and all what I just had said
was can be ignored.
Keep The Faith, Jim K9TF/WA9YSD
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