[cma-l] AM v FM

[Two Lochs Radio]

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? 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).

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!

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.

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. 

Alex
  ----- Original Message ----- 
  From: Ian Hickling 
  To: The Community Media Association Discussion List 
  Sent: Sunday, December 20, 2015 6:14 PM
  Subject: [cma-l] AM v FM


  That's the trouble.
  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.
  All my practical experience tells me that only by measuring the real thing can you be sure.


  Ian Hickling

  Partner
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