At 12/20/2004 08:35 PM, you wrote:
>We have 146.04/.64 repeater on a nearby mountain top. It worked
>great for years with a range of 100 miles or more. Since the phone
>company and a pager company installed their high power transmitters
>near the site of the repeater (within 100 yards) the repeater is
>virtually useless. After much head scratching I believe that the
>difference in frequency of the pager transmitter of 600 khz is the
>problem but have no idea how to solve the problem without going to
>an odd split. The repeater coordinator for this area of Virginia
>won't even consider that as an option.
>
>The equipment that we are using is excellent. The transmitter and
>receiver on the repeater are both Motorola Micor and were modified
>with to the repeater frequencies by FCC 1st class licensed hams
>using Motorola parts. This is not an equipment problem. We are
>running a set of Wacom cavities which were bought new and are
>correctly tuned and the antenna is a Phelps-Dodge Stationmaster.
>When the intermod occurs it is dependant on BOTH pagers transmitting
>at the same time. If only one pager is transmitting there is no
>problem. This may at first sound unusual but the pagers are in the
>150 mhz band and they are exactly 600 kc apart. These transmitter
>are both 250 watts or more output.
I've dealt with similar problems involving two VHF HB TXs that were 5 MHz
apart. They mixed with every 5 MHz split UHF repeater at the site to
convert the output to input (A+B-C mix). At first one of the VHF TXs was
in another building a few hundred feet away, & at that time the
interference was just a few dB above the noise floor & only present after
long periods of dry weather. But then that TX moved into the same building
as the other VHF TX & our repeaters. That caused it to be present all the
time & much stronger.
I tried DFing the actual source of the interfering signal(s). Sometimes
they came from a tower joint, other times it was from an air conditioning
unit, then another spot on the tower a minute later. It just came from
everywhere. The site manager tried painting all the suspect tower joints
(which had no signs of rust or corrosion - this was a fairly new tower)
with some sort of metallic paint. This more or less cured the problem for
about 2 months, then it came back until it rained or the joints were
painted again. The problem finally went away when one of the TXs (a 25
watt paging TX) went off the air.
Bottom line is that short of getting one of the TXs involved in the mix to
move, there is no good solution. Remoting your TX to another location is
probably the best bet. You can always run more power if the alternate
location offers less coverage, but you can't make up the difference in RX
if you put your RX there instead.
>After much head scratching I believe that the
>difference in frequency of the pager transmitter of 600 khz is the
>problem but have no idea how to solve the problem without going to
>an odd split. The repeater coordinator for this area of Virginia
>won't even consider that as an option.
An odd-split will solve the problem but should only be a last resort, as
opposed to going off the air completely. Among other disadvantages, it
makes your repeater harder to find by travelers not familiar with your
bandplan. Your coordinator may resist the idea, particularly if there are
no odd-split repeater pairs currently in your bandplan. However, it's up
to the coordinator to accomodate you the best they can given their current
activity levels, channel loading & your own technical
constraints. Refusing to consider odd-splits on the basis of "we don't
have any & don't want them" or "they're not in the ARRL bandplan" is not a
valid reason.
Bob NO6B
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