Hi Jim & All, Thanks for the info & responses.
I took down the 224 & found that the N connector on the end wasn't quite up to snuff. I am going to put a ham antenna on it - think it's a ringo2 or some such thing, just to see how an antenna with good swr will work in the desense arena. Then I'll put up a different 224 & see how it goes. I did see the posting about how to lower the frequency of a 224. Not sure I want to go that route, but might in the end! I built a small matching unit that I'll try out... basically a tuner that will go between the duplexer & feedline. One of the guys talked about using a smith chart & putting some stubs at the feed line which would change the matching. gee, lots of things to experiment with. That's the <fun> of it no? :-o Thanks again, & I'll post my progress. Tim W5FN --- In Repeater-Builder@yahoogroups.com, Jim Brown <w5...@...> wrote: > > One of the local repeater operators used an antenna at the top of a 100 ft > tower that got bent over during last winter storms. He put up a temporary > antenna at the tower base and is experiencing some really bad desense with > the low antenna. > > He is using a GE Mastr II base station repeater and had reasonable operation > with little desense on the antenna 100 ft above the equipment. The antenna > only 15 ft or so above the equipment now and has the bad desense problem. It > would appear that the antenna is flooding the equipment with more RF than the > shielding can handle. > > BTW, take a look at some of the previous posts on modifying a DB-224 by > adding a 2 inch extension to each end of each dipole to bring it down into > the ham band. The SWR does not go completely to 1:1, but does hit a minimum > in the middle of the 2 meter ham band. No change to the harness was required > to move the antenna frequency. > > 73 - Jim W5ZIT > > --- On Tue, 9/1/09, tahrens301 <tahr...@...> wrote: > > From: tahrens301 <tahr...@...> > Subject: [Repeater-Builder] Antenna SWR = Desense? > To: Repeater-Builder@yahoogroups.com > Date: Tuesday, September 1, 2009, 2:03 PM > > > > > > > > > > > > > Hi folks, > > > > Just a bit of an update... got the 6 cavity Telewave > > duplexer tweaked up - looks like it pretty much hit > > the specs in the data sheet. > > > > With a dummy load at the 'antenna' port, I used an > > "iso-tee" to inject a signal at both the receiver > > input, and between the antenna port & the dummy > > load. With a weak signal, both places showed me that > > there was no desense. Very weak signal would hold in > > the repeater. > > > > However, putting the system on the antenna (a 150-160 mhz > > DB-224 100' horizontally & 10' vertically separated) > > through a metal building fed with 7/8 heliax, there > > seems to be no end to the desense! > > > > The wattmeter shows 30 watts forward & 3 watts reflected > > at the antenna port, if my math serves, it's less than 2:1. > > > > Can the less than 1:1 match be the culprit? > > > > Thanks, > > > > Tim W5FN > > __ >
