[Repeater-Builder] Re: Antenna Pattern Question
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 wbe...@centurytel.net 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
Re: [Repeater-Builder] Re: Antenna Pattern Question
Sorry Dan, The Comprod UHF Corner Reflector is the 440-70 or 442-70 URL: http://www.comprodcom.com/en/antennas/base/pdf/440-70.pdf. There are several different models for UHF that offer different gain, pattern shape (beam width) and front to back ratios. You should be able to find a model that fits your application. Same rules apply about getting the other antenna nearby oriented behind yours. Bill - Original Message - From: Dan Hancock To: repeater builders Sent: Saturday, November 21, 2009 8:47 AM Subject: [Repeater-Builder] Re: Antenna Pattern Question 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 wbe...@centurytel.net 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
[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
Re: [Repeater-Builder] Re: Antenna Pattern Question
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
[Repeater-Builder] Re: Antenna Pattern Question
I'm basically looking to null an area of about 45 degrees behind the antenna. Kinda like I want omni coverage except for this 45 degree area. I don't think beans will cut it. I'll check Comprod's web site. Thanks and 73, Dan N8DJP --- In Repeater-Builder@yahoogroups.com, Jeff DePolo j...@... wrote: Comprod makes dipole arrays with a screen reflector, at least for VHF, not sure about UHF. I'm looking in the catalog and it shows a backside null close to 20 dB down from the main lobe. The screen reflector should be nominally about 1/4 wave behind the radiator for maximum forward gain. It could be modeled fairly easily. How wide of a forward beamwidth do you want? Would a couple of corner reflectors be a better way to go? --- Jeff -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Dan Hancock Sent: Thursday, November 19, 2009 11:43 AM To: repeater builders 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 No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.425 / Virus Database: 270.14.61/2498 - Release Date: 11/19/09 07:51:00
