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Wayne · January 10th 12, 02:37 AM posted to rec.radio.amateur.antenna

"Owen Duffy" wrote in message
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"Wayne" wrote in
:

"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
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On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
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Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

snip
***********

Update. Current configuration is a 14.6 ft element fed with 40 ft RG-8.

Wayne
W5GIE

So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen
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I think that losses were discussed on a previous iteration of the thread,
and I have no quarrel with the loss numbers.

Many years ago, I might have been able to somehow deduce cable losses from a
Smith chart, but nowadays I just use the published curves.

The current configuration (14.6 ft vertical) is promising for 14-28 MHz
operation, as three of the bands have SWRs of less than about 10:1, and the
other two are below about 20:1. The antenna is not checked out on all of
those bands, but it does an acceptable job on 12 and 10 meters where the SWR
is highest. That statement should not be construed as meaning that it is
the final configuration.