Dan, The VHF fiberglass Omni's within 6 feet of your building-top antenna can cause pattern distortion just as is the case with side mounted tower installations. However, I recall from your original posting that your application requirement is to produce a deep null +/- 45 degrees with as much gain as possible elsewhere around the compass. The best candidate for this application would be a corner reflector array such as the Comprod 470-70 (Assuming VHF) that develops 30 dB Front to Back with a main horizontal beam width of 67 degrees. Check URL: http://www.comprodcom.com/en/antennas/base/pdf/471-70.pdf
If the 67 degree horizontal beam width is too narrow for your intended coverage area and you can live a little less front to back ratio, then a better choice would be the Comprod F-3713 that has a flat reflector screen mounted behind the folded dipole radiator elements. Check URL: http://www.comprodcom.com/en/ecatalogs/BaseStation2005-Full.pdf You should expect to obtain nearly the same published pattern shape and gain with either of these antennas for your building-top installation provided that you are able to mount your antenna such that the fiberglass Omni's are behind the reflector and not out in front of the main beam of the array. This gives you the advantage of a large reduction in radiation (excitation) toward the Omni's that significantly reduce overall parasitic radiation from these sources with little or no net change from published pattern shape and gain. A secondary benefit is an increase in isolation from the other VHF systems that may prevent or reduce the possibility of receiver desense or transmitter IM among the three systems sharing the rooftop. Good luck with your project! Bill, WA8WG ----- Original Message ----- From: Dan Hancock To: repeater builders Sent: Friday, November 20, 2009 8:15 PM Subject: [Repeater-Builder] Re: Antenna Pattern Question 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
