Bill, thanks for the tip, however the repeater is a UHF. I don't see anything on Comprod's site showing a UHF with a reflector screen. I guess I will just have to improvise! I was looking to see if any other members had done such a thing before so I didn't have to re-invent the wheel so to speak.
73, Dan N8DJP Re: Antenna Pattern Question Posted by: "William Becks" [email protected] wgbecks Date: Fri Nov 20, 2009 7:18 pm ((PST)) 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
