I'm not far from Tim and have been emailing with him offline. Yesterday he disconnected the rx from the duplexer, terminated the rx port of the duplexer and injected weak signal directly into the receiver. He did not remove any original cables between the rx and duplexer, so he simply divided the rx path. When he ran another test he had no desense. This test should help prove / eliminate radiated antenna RF from 100ft away as the cause of his desense. He's describing pretty significant desense.
It all works well ( without desense) into an end of feedline terminated dummy. I'm kinda questioning the quantar circulator somebody mentioned. He's planning the antenna site install soon. Don Kirchner W5DK -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Eric Lemmon Sent: Thursday, September 03, 2009 8:01 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Re: Antenna SWR = Desense? Tim, A high-gain antenna that is only 100 feet away horizontally is far too close. It is "bathing" your repeater equipment with RF, and very few machines can tolerate being in such a high RF field. Vertical versus horizontal separation is very roughly in a 1:45 ratio, that is, the isolation of 100 feet of vertical separation is roughly equivalent to about 4,500 feet of horizontal separation. In other words, your 100 feet of horizontal separation is no better than if you put a mag-mount whip right on the top of the repeater cabinet. You would likely have less desense if you mounted your DB224 antenna on the roof of your equipment shed, directly above the repeater, so that the repeater cabinet was in the "shadow" beneath the antenna. You might also consider filtering the DC power leads to prevent RF ingress through that path. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of tahrens301 Sent: Wednesday, September 02, 2009 9:38 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Antenna SWR = Desense? Hi Eric, Hmmm, don't think I said above the station, but if I implied it, no, it's horizontal. I can look out the window & see it! Nate - I only tried the 'ham quality' antenna because I knew it would be a better match than the DB224. It was easy to change, standing on a 6' ladder! Just wanted to see if a poor swr would induce the desense. There are no other communications systems within miles of my location, so who knows. Perhaps the metal building is the problem. A side question, dealing with separation. Obviously, when you are using a split site, vertical separation makes you a lot more $ than horizontal does. But, in this type of situation, where you are a single antenna with a duplexer, what real difference does vertical or horizontal separation from the station make? If I'm horizontal, & could turn the whole system on it's side (including the antenna system), then it would be vertical. The straws that I'm grasping are getting smaller!! Thanks to all! Tim W5FN ------------------------------------ Yahoo! Groups Links
