Hi Ian, 

A lot of us in the US have to sit back and look at the 
information you wrote and do the translation to values 
we have here in the US. (... he says laughing to himself)

There are a hundred plus questions that need to be asked 
to get the best picture of what's going on with your 
repeater system. 

Your mention of the "rx scratch" without a transmitter 
on is key issue. You don't say how much other rf is 
in the area?  ... at other sites?  ... or what else is 
in the area. 
A few things come to mind first..   First the antennas are 
omnis and have you tried changing them around to the other 

Do the antennas have a true DC ground return path? Are 
the antennas fiberglass radomes (covers) or are they 
the exposed metal type construction? 

The notch pass duplexers might be a larger issue if 
you hadn't said the tx doesn't have to be on for the problem 
to show up. But the duplexers might not have a true dc 
ground path through the probes/loops.  You might check for 

Does the feedline/coax have grounding kits on them? 

Regarding your tower leg clean up... many people will make 
up ground braid - short jumper straps that screw into the 
tower legs over the original tower joints. There's a lot 
about doing that well out of the scope of this chat, but 
it's another way of reducing problems in older tower systems. 

Static build up in wind might mean the antennas, duplexer or 
various combinations of...  might not have a dc path (aka dc 
bleed path) to ground. Static problems in dry or drier weather 
conditions would be a possible indication of this problem. 

.... but not exclusive to.    If the notch duplexers and the 
antenan(s) don't go to dc/rf/station/system ground, you could 
swap things around (antenna/duplexer type) to sort through the 

Your turn... 


> "Ian Wells" <[EMAIL PROTECTED]> wrote:
>  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
> VHF   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
> 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
>                                                 E
> 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
> the transmitters are txing or not .There isnt any problem of the
> 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
> will slowly develope major scratch and then slowly clear up -possibly
> electrostatic or wind moving the antennas ?.I have been fixing loose
> 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,
> www.kerinvalecomaudio.com.au

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