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<DIV><FONT size=2>You can never be 100% sure, only achieve a
sensible degree of confidence, but that should be enough, and more to the
point that applies to FM as well, surely? <FONT size=2>I guess the most
significant issue is that for more predictability you do need more ground
space and a clearer antenna environment for AM than you would for FM, which does
of course come back to your central point that AM is harder for town-centre
based stations (apart from those with handy mill chimneys
etc).</FONT></FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Of course, with the much shorter wavelengths involved, it's
easier to get an FM installation to approach a modelled version without undue
interference from nearby buildings, ground conditions etc, but even so,
mounting a Band II antenna on a typical commercial tower replete in
random metalwork, feeders and other antennas, can result in all sorts of
variations that are near impossible to model accurately (or at least aren't
worth the effort involved). When we tried modelling one of our antennas to
include the support work and all the main tower legs and diagonal spars the
resulting pattern looked like a horribly spikey fuzzball!</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>I also spent a long time once carefully modelling
the entire metalwork of a typical 350mm triangular lattice tower that was
to have dipoles mounted parallel, only to find out that in some similar cases
the BBC modelled the entire lattice work and three legs as a simple cylinder on
the primary axis and ignored the difference.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>But if you can use a T or L arrangement to help get the
vertical radiator and capacitance hat well clear of the ground and
low-level objects, maybe 20 metres at MF, and can use or create a decent earth,
then there's no reason the modelling shouldn't give you similar confidence
as you would have with a couple of dipoles mounted within 1-2m of a a lattice
tower. Extra height is also a win-win because as well as making the
pattern more predictable, it will improve antenna efficiency and also keep the
ground wave to a low angle. A short, low-level monopole is going to shoot off
rather more power skyward than is ideal for a local station. Of course,
measuring afterwards is a good check for anything unanticipated, but it
shouldn't be wildly out. </FONT><FONT size=2></FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Alex</FONT></DIV>
<BLOCKQUOTE
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=transplanfm@hotmail.com href="mailto:transplanfm@hotmail.com">Ian
Hickling</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=cma-l@mailman.commedia.org.uk
href="mailto:cma-l@mailman.commedia.org.uk">The Community Media Association
Discussion List</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Sunday, December 20, 2015 6:14
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [cma-l] AM v FM</DIV>
<DIV><BR></DIV>
<DIV dir=ltr>That's the trouble.
<DIV>Common sense says that modelling is only as good as the information you
can give the computer, and how well the software can use it.</DIV>
<DIV>All my practical experience tells me that only by measuring the real
thing can you be sure.<BR><BR>
<DIV
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size=3><B>Ian Hickling</B><BR></FONT></DIV>
<DIV><FONT
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