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
----- Original Message -----
From: Dan Hancock
To: repeater builders
Sent: Thursday, November 19, 2009 10:42 AM
Subject: [Repeater-Builder] Antenna Pattern Question
I have a repeater that I need to have as tight a cardiod pattern as
possible. I have looked at the dipole antennas such as the DB-411 and they
don't really shut down the back door quite enough.
Does anyone have any antenna modeling software that would show the
result of adding an 18" wide screen to the back side of the mast on the DB-411?
Does anyone have any experience in home-brewing a modification like this?
Dan Hancock N8DJP
