[Repeater-Builder] Re: The utimate PIM test site discovered in Brussels…

[Rodney Baker]

On Tuesday 27 January 2009 21:48:19 "on7wp"  pedro.w...@telenet.be  wrote:
>  The utimate PIM test site discovered in Brussels…
>
>  Part two of my quest… by Pedro M.J. Wyns ON7WP AA9HX
> [...]
>  
>  A VHF NBFM repeater station into a half wave collinear (Big disaster)
> [...]
>  
>  The problem:
>
>  All works fine but the VHF repeater. We start by disabling the
>  transmitter, as the problems are purely receive related.
>
>  Tracking down the problem:
>
>  [...]
>  So apparently the noise and mixing products are generated OUTSIDE the
>  DAB transmitter system.
>
>  Then we went to the roof with a Yaesu FT817 on the VHF RX frequency
>  using the same filter setup. A three element antenna is used for
>  pinpointing the problem. Noise is coming from multiple sources…
>
>  A very large airconditioner air outlet (6x8 meter) on a lower roof 50
>  m from the tower, maximum (S9) INSIDE the very large cover ????
>
>  Metal shieldings around the building.
>
>  The lightning protection connection (S6) on the major coax cables,
>  small airco outdoor units (S8) straight under the tower, the mount of
>  the VHF antenna (S7), the DAB antenna (S8)…
>
>  Levels are off course not absolute but it is very clear that multiple
>  objects are radiating in this very strong field. I have no indication
>  of the actual radiation value but the lowest FM antenna is only 10
>  meters above the walking area. DAB is at 20m, DVB-T at 25m.
>
>  The next step ???
>

Pedro,

In my experience (and that of others from whose previous mistakes I've 
learned) collinear antennas on high-rf sites are a recipe for disaster; if not 
when first installed then certainly down the track. Most collinear antennas 
have joints inside between elements that change properties with age, vibration 
and temperature cycling and probably contain dissimilar metals (so any 
moisture/condensation inside the radome will encourage electrolytic 
corrosion).

My recommendations would be as follows:

1. Replace the collinear with a dipole or dipole array (if you need gain) that 
is of fully welded construction. Anodised aluminium is OK if you're away from 
corrosive atmospheres but if you want absolute longest life go for one that is 
all stainless steel construction. Use appropriate mounting hardware (beware 
using different grade s/s to that of the antenna - it will corrode like 
crazy). 

2. If you are able to, with the cooperation of the site and building owner(s), 
see if you can clean/refix/bond all metallic junctions that appear to be 
radiating (or as many as you are physically able to). For antenna mounts, 
remove, clean all surfaces and apply conductive grease on all mating surfaces. 
Some may call this overkill but I don't think so on this type of site. It 
should be a regular part of site maintenance to inspect and maintain all 
mounting hardware, connectors, lightning suppressors/earth connections etc.

3. You may not be able to do anything about the air-conditioning housings but 
try anyway. Make sure the housings are bonded to earth (check your local 
electrical/building codes/regulations before doing anything and employ 
suitably qualified contractors as required). 

>  I would like to try other VHF RX antennas as I have the idea that
>  maybe the biggest IMD is made inside my own repeater antenna (Brand
>  new Procom ½ wave radialless).

I think you're right. As a rule of thumb we only every used collinear-type 
antennas on sites with a single repeater or base station. Any time there was 
more than 1 TX or a likelihood of high RF from nearby sites we used either a 
single dipole (if gain was not required) or a fully welded metal dipole array 
(e.g. those manufactured by RFS or RF Industries).

>  The UHF antenna is not bothered by this noise, nothing is observed and
>  it is located at the same height and distance from the FM panel
>  antennas ????
>
>  Please your input if you have any idea how to handle this case, and
>  hopefully solve it…
>
>  Pedro ON7WP-AA9HX

Good luck. I hope this is of some help to you.

Regards,
Rodney.
-- 
===================================================
Rodney Baker VK5ZTV
rodney.ba...@iinet.net.au
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