Have you tried common earthing of the feeders? About to set them up
Do the radios have good shielding between them (metal cases no plastic).All metal case radios
Have you looked at the site with a good specan? Will look into it
Is the sound you are hearing antenna flutter?
80 MHz Cols what brand are they? Antenna Agencies and RFI
We found over here in the west we replaced cols with 4 stack dipoles because of flutter and bad joints internally.
Space doesn't permit stacked dipoles
I really think it is time to check with a good Spectrum Analyser. You may be surprised what you see coming done the feeders from the other txers.
Make sure you look at the rx freqs when they are all in rx only, you may well find that you have a sprogy rxer basically txing into you other rxers. Will check
----- Original Message -----
From: Ian Wells
Sent: Thursday, March 30, 2006 6:33 AM
Subject: [Repeater-Builder] desnse on community sites

 Hi guys .I have a small problem that i have been trying to sort out for a while now .I have 5 repeaters operating on one of my sites
3 vhf low band and 2 uhf band.The frequencies are as follows
A-TX 70.300MHZ RX 72.800MHZ       fitted with 4 1.5 meter  by 4 inch meter tins-most likely a notch diplexer    3db colinear ant   FM828E
B-TX 70.5875MHZ RX 73.0875MHZ   fitted with 4 1.5 meter by 4 inch meter tins-most likely a notch diplexer    3db colinear  ant   FM828E
C-TX 70.725MHZ RX 73.225MHZ      fitted with a new band pass/band reject possibly a varinotch     3db colinear ant    FM828E
D- TX 473.575 RX 478.775MHZ         fitted with mobile notch diplexer 6mc450s  6db colinear ant   MAXON PM150/SM4450SC
E- TX 489.1MHZ RX 483.900MHZ      fitted with 6ld450s diplexer 6db colinear ant     MAXON PM150/SM4450SC
All coaxes are Heliax from antennas to Diplexers with N connectors
Interconnecting cables between diplexers and radios  are rg223u
A and B are a 1.5 meter apart with C in the middle
All antennas are mounted  aprox 50 cms or more away from C or each other .
D and E  have a 1.5 meter between them
ie-                                           D
                                     A        C       B
I am at times having desense (ie scratch ) especially in windy weather occuring on repeaters B and D.It doesn't seem to affect the desense wether the transmitters are txing or not .There isnt any problem of the repeaters affecting each other ,they all work great just the problem of scratch coming and going .A,C and E do get scratch on occasion but no where as bad as D and B.D and B works great sometimes (clear with no scratch)and then reception will slowly develope major scratch and then slowly clear up -possibly electrostatic or wind moving the antennas ?.I have been fixing loose  bolts on the windmill tower to stop problems incase of tower twist and this affecting service.I am wondering if there are any futher sugestions . 
Thank You,
Ian Wells,
Kerinvale Comaudio,
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