You're blessed with close spaced frequecies... at least
their in some type of order.
Bypass the multi-coupler preamp if possible to see what
happens to the imd. You need a good service monitor
or equivalent to measure the isolation between the
antennas.
I'd be more of fan of breaking the 6TRM into two 3TRM
units and feeding a second TX antenna. You could
interlace every other frequency and seem much better
preformance from the system.
More later... it's supposed to be my day off....
skipp
> "vintageaudio2004" <[EMAIL PROTECTED]> wrote:
> 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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