Re: NEC-LIST: Propagation Patterns

>Hi list members,
>
>I'm an EMC chap by trade ( don't hold that against me! ). I encounter
>many varied emissions patterns when I'm testing objects. The source in
>many cases is quite easy top locate from the radiation pattern,
>however, there are many cases where it's not.
>
>Could anyone tell me if anyone has documented non-intentional antenna
>radiation patterns.
>
>Derek Walton
>Owner: L F research EMC Design and Test Facility

Derek,

The environment around us is full of re-radiating structures.

During the past couple of decades we (in Canada as well as myself)
have studied the effects of high rise buildings and power transmission
lines on the directional patterns of MF broadcast antennas; the effect
of ship structure on the pattern of HF antennas (principally the
Canadian Patrol frigate). We (at CRC and supported research at
Concordia University) are also engaged in studies of near and far
fields about hand-held radios --- but this is very complicated
subject, being studied by many around the world, since the body, head
and hand of the user are a part of the system affecting the radiation
pattern (largely an absorption problem rather than a re-radiation
problem). And, putting on my amateur-in-radio hat, I have several
papers on the effect of support structures (towers) on the pattern of
HF antennas (principally delta loops and dipoles (balanced,
unbalanced, off-centre fed)); and HF (published) and VHF (unpublished)
antennas on automobiles. And, I have also looked at HF antennas on
sail-boats --- but this study is very preliminary. My interests are
concerned with the performance of antennas in their operating
environment.

There is also a problem, not well documented, if current is induced to
flow on the outside surface of the shield of the coaxial transmission
line feeding the antenna. This effect can totally upset the effective
radiation pattern, a factor that is often overlooked by those
measuring the radiation patterns of VHF/UHF antennas --- totally
overlooked in the case of HF antennas, since radiation patterns are
seldom measured.

I could provide some references to my own work, and to work of others
that I am aware of --- *but* unless you have an interest in the
subject as a whole --- I suggest you look at each situation as it
arises --- since the effects are very environment sensitive.

Take a look at the structures surrounding the antenna, and if they
look like they might be resonant, create a wire grid model that can be
analyzed by NEC.

I speak here about resonant structures. I have not looked at the
problem of shielding, which can be a problem at VHF/UHF.

Jack

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Wed Mar 03 1999 - 21:19:30 EST