On 29 Jul 2007 22:25:04 GMT, Ed
wrote:

I only wanted the data to help "prod" the
people in charge of the need to change the antenna system. SWR is a
figure that they recognize.

Hi Ed,

After having heard the third chorus of:
I am not communicating effectively with you!
it is time to analyze why.

First and foremost, the correspondents here are focused on technical
solutions. You are seeking a political answer. The two do not
generally reside on the same page.

Technical correspondents here would reasonably expect that an
association composed of amateur radio operators would appreciate a
technical analysis to solve a technical problem. Owen and others have
provided that, and several have been specific to exactly the numbers
you have asked for. By and large, those numbers don't seem compelling
if your audience fails to appreciate the limitations of the internal
tuner's abilities.

SWR is not in and of itself loss. It does not always represent a quid
pro quo for efficiency. Your reliance on these numbers to sway a
group's rather myopic view of the problem gives all the appearance of
a magic beans solution suited for yokels.

Returning to the work we do here, there remain a number of missing
details to help us provide solutions, not political answers.

I've noted a number of correspondents have presumed this metal roof of
yours stretching out in an infinite plain. Clearly no such roof
exists. However, I have seen no further amplification of the details
by you. At a minimum, and as a gesture of good will towards those you
ask favors of, it would be useful to know the size of that roof, if it
is flat (or reasonably so); how high it is, and how high the antenna
is above earth (I presume you have only described how high the antenna
is above the roof).

These parameters have a bearing on the results of the EZNEC
computation you ask for. The metal roof has no particular effect in
terms of loss, and hence doesn't particularly effect SWR if resonance
is not perturbed. However, the proximity of earth does tend to
broaden bandwidth and to lower resistance of dipoles (shunt resistance
doing the job in both situations). This does impact EZNEC
computations you ask for.

Insofar as actually remedying the problem, that too has a technical
solution easily achieved courtesy of your metal roof. You simply
convert the dipole into an Inverted F. Consult:
http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm

Its discussion is couched in terms of UHF, but HF is merely a matter
of scaling. Also, you don't strictly need the wide, bent radiator as
single wires will do the same job to a narrower bandwidth (or you can
build a skeleton version as illustrated).

The Inverted F is suited to a long low horizontal section. This is
something that already exists and is probably driven by the lack of
higher supports. The metal roof conforms to the expected metal ground
conditions (UHF would make this simple to supply, you already have it
handy). The amount of wire is less than you already have, so no new
burden in that regard. The need for NVIS operation is clearly
supported. The design match obviates the transmission line losses
driven through the roof by existing mismatches.

73's
Richard Clark, KB7QHC