Hello,
I just installed a new 6-ch VHF MPT1327 system using MTR2000 Motorola
repeaters (100W) with optional preselectors installed, a Telewave
M101-150-6TRM combiner, and Sinclair RM201-112S1B RX multicoupler.
Antenna system is two DB228, one RX, one TX. RX antenna in on top of
tower, RX is a bit lower from about half the tower down. Tower is
90FT.
Frequencies used are:
CH1 - T160.125 R165.125
CH2 - T160.325 R165.325
CH3 - T160.525 R165.525
CH4 - T160.725 R165.725
CH5 - T160.925 R165.925
CH6 - T161.125 R166.125
Yesterday we finished putting together the system, and coverage is
currently more than was expected. But we are having a big problem (I
suspect) with IM products, as when I for example have the control
channel on CH1 (160.125) and key repeater CH6 (161.125) my carrier
indicator on CH1 opens up, and this also happens when doing other
combinations by keying the other channels in different combinations
and see other carrier indicators opening up on other channels. The
interfering signals are very weak in most cases, barely opening the
squelch of the receivers, but even so they are a problem. When I look
at the spectrum analyzer with a small antenna I see all kinds of
spikes left and right of the main carriers (most of them 40 or 50db,
or more, bellow carrier), but if I hook up the combiner output into a
dummy load and sample it into the analyzer display it looks much
cleaner, and of course the interference problem disapears. I don't
want to jump into conclusions, but it seems the IM (if it is) is being
generated somewhere outside the equipment. Furthermore, if I scan the
160-170MHz segment (with a Icom IC-2GAT) while transmitting with more
than one repeater channel, I can pick up a lot of signals, so it seems
to be all over the place. I know they come from the system as I can
hear the control channel signaling.
BTW, this is a very remote location, and the only other system that
operates anywhere near ours is anothe VHF conventional repeater (a
plain GR300) at 157.700 and 162.700. The other tower is located about
half a mile away.
I'm really a bit concerned about this problem, as it disrupts
communications between channels when for example two groups are using
separate channels at the same time, as audio from one group will show
up on the others, and so forth.
Please if you have any suggestions as to where to start looking for
solutions, they would be very appreciated. So far as a test I tried to
lower the TX power of the repeaters, but even at 25W I still see some
interference as described above. Much less but still it is there,
besides there are still numerous combinations of TX that I even have
tried so I'm sure the solution is not there. Since the interfering
signals are weak, as another test I tried to raise the squelch setting
of the repeater receivers to the maximum, but still some interference
gets trough.
Thanks very much in advance for any help.
Alex
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