Hello Matt-- Sounds like you may be desensing yourself----What happens to the signal to noise ratio of a received weak signal when you turn off your repeater transmitter? The signal to noise of the output of your repeater receiver should stay the same and if it does all is well as far as desense is concerned. If the noise drops, you have desense and the amount is indicated by the drop in noise. Desense in the neighborhood of 17 db. is a definite possibility. The source of the desense can be inadequate attenuation of the transmitter and receiver frequencies by the appropriate sections of your duplexer or, and in fact, wideband noise produced within your antenna or other 'hardware" in it's vicinity due to poor connections and or corrosion when excited by your transmitter. Remember that your duplexer is wide open to receive frequency noise and that noise of this frequency may well be produced within your antenna as a result of your transmitter excitation. This is one reason that good commercial antennas are usually welded or at least soldered together ----no clamped or bolted connections to invite corroded connections.
Good luck! Scott, N6NXI ----- Original Message ----- From: "Matt" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Sunday, January 02, 2005 2:57 PM Subject: [Repeater-Builder] Rusty Bolt effect and noise floor questions > > Hi All, > > A very Happy New Year to you all on the R-B list! > > I have a couple of questions which I was wondering if anyone else out there > had any experience with. > > I run a 50MHz repeater in Somerset SW England, but it is troubled by two > things...... > > First, I'm using a shared commercial mast and so antenna space is limited. > The repeater uses a single dipole antenna and a WB5WPA type Heliax duplexer. > > I carried out the procedure described by Kevin on the RB web site to asses > the effective sensitivity of the repeater and was amazed at the results! The > effective sensitivity at my site is around -100dBm! So although my Rx is > good for -117dBm for 12dB SINAD the mobiles have a hard time getting in, > although they can hear the repeater's 22W output fine. > > With a weak signal, all sorts of "birdies" can be heard in the background > instead of a nice smooth white noise that you'd expect from a weak signal. > > So my first question is has anyone any experience or thoughts on using a > device like the MFJ-1026 noise canceller or any other tricks to improve the > receive situation? Or is this just "how it is" on 50MHz? > > Secondly, when the wind blows, which it frequently does on this exposed > site, I am affected by the good ol' rusty bolt effect (or duplexer hash) > which I am aware of the causes / reasons for but wondered if anyone had any > experience of tracing this kind of problem to find the offending "bolt" > whatever it turns out to be! There's a load of disused antennas on the mast > and some with their cut off feeder tails flapping so that was where I was > going to start first but it would be great to be able to positively identify > the offending item somehow. > > Thanks in advance for any replies. > > Best wishes to all, > > Matt > G4RKY. > > > > > > > > > Yahoo! Groups Links > > > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
