Bill,
The antenna will be a building-top installation. The only thing near field is a
couple of VHF fiberglas omni sticks that are over 6' away. Interaction with
anything else will be insignificant.
Thanks
Dan N8DJP
Re: Antenna Pattern Question
Posted by: "William Becks" wbe...@centurytel.net wgbecks
Date: Thu Nov 19, 2009 4:13 pm ((PST))
Dan,
Before
you make any decisions about selecting any specific antenna for your
application, you need to know or specify top or side mounting. Then
you'll need to consider any and all metallic objects present in the
near environment of the antenna because they constitute reflective or
parasitic sources that can have a profound impact on the actual far
field radiation pattern obtained.
I have done a lot of NEC
modeling in order to make a more informed scientific estimations of how
these factors modify the final radiation pattern before attempting to
choose any specific antenna for a given application vs. placement and
orientation about the tower. NEC modeling is only and good as the
modelers ability to accurately construct a model that truly depict the
real world electrical environment of the antenna. It's doubtful that
you would ever get an antenna manufacturer to model, or to guarantee a
particular pattern outside of those field patterns derived from their
antenna test range due to the complexity of modeling and of offering
such service.
Cellular and other similar providers largely
employ directional panel arrays are virtually free of any significant
radiation in the direction of the tower, supporting structure, or other
antennas in the near environment. Therefore, their engineers don't
need to consider the unwanted effects of parasitic radiation sources
external to the array. Unfortunately, those of us relegated to VHF
and UHF systems end up illuminating a rather large area of the tower
resulting in a complex number of parasitic radiation sources that
change the resultant pattern that otherwise might radiate per the text
books if located in free space.
Bill, WA8WG
