[TowerTalk] Comments on Antenna Peformance-SWR Bandwidth vs Gain

[James R. Duffey]

Dave - Your comments were received and thought about;

"However, isn't the opposite true with respect to SWR?  Most SWR curves are
steeper on the low end, so that an antenna that is adjusted for CW has a
lower SWR at the top of the band than the SWR would be at the bottom of the
band for
an antenna that is adjusted for Phone."

This is generally true and illustrates the problems I pointed out in
simultaneously optimizing  gain, F/B, and SWR bandwidth foir a trapped
tribander. It seems to me that of these three items, SWR bandwidth is the
item most open to compromise as it can be compensated, at least to some
extent, in the shack with an antenna tuner.

"Regardless of the gain, f/b, etc., if an antenna has a high SWR then many
rigs and amps become unhappy.  That is why I always adjusted my tribanders
for CW (plus the fact that I don't do much phone), and I was happy with the
resulting SWR curves.  However, your info makes me less happy now if this
method lowers the gain. Maybe I was better off ignorant  :-)"

Well, nobody is better off ignorant, at least in my opinion.

I think, however, that most trapped tribanders, when used as all purpose
phone/CW antennas, would probably be better off, as far as gain goes, when
cut for the phone band, or somewhere in between the phone and CW band. A
suggested compromise for an antenna whose SWR bandwidth is less than the
band in interest would be to trim the antenna so that the SWR at the bottom
edge of the band is 2:1 or whatever the rig/amp in use can handle. This
will place the region in which the gain is falling higher up in the band,
with luck outside of the band, or at least sufficiently high in the band to
not be a significant problem. Good, or at least acceptable, gain will
probably be maintained in the lower part of the band. You pays your money
and takes your choice.

Most modern rigs can handle SWRs up to 3 or more with their automatic
tuners, so I think that trimming the antenna for further up the band might
result in a better dual purpose CW/Phone antenna, while still maintaining
the ability to simply match the antenna. Of course it will probably be
outperformed by a well designed monobander, and often by one with one less
element:^).

Thanks for the input.- Duffey KK6MC/5




James R. Duffey          (505) 764-3143
Principal Scientist      (505) 843-7995 (FAX)
Maxwell Technologies Inc/Albuquerque Division
Suite 300
2501 Yale Blvd SE
Albuquerque, NM 87106



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