Not knowing what the Telewave and Sinclair part numbers translate into in
terms of the actual hardware in use, you'll need to provide some details.
First of all, what kind of filtering is done on the tx combiner and rx
multicoupler? Is the tx combiner hybrid-ferrite or cavity-ferrite? What is
the measured isolation between antennas (if known)?
Since all of the tx's are spaced at 200 kHz and all of the rx's are also
spaced at 200 kHz, if there is any nonlinearity in the receive portion of
the system (due to preamp overload, front end overload, etc.), there is a
good chance that the desired signal is mixing with the transmitters,
yielding new products that fall onto the other receive channels. Not
knowing any other details, that would be my best-guess as to what's
happening. If this is, in fact, the case, then there needs to be additional
isolation between transmitters and receivers, either by increasing the
antenna-to-antenna isolation, or adding additional filtering ahead of the
preamp/power divider in the rx multicoupler to further attenuate the tx
carriers.
The less-likely cause would be a passive (external) mix such as in a rusty
tower joint or guy wire hardware, but that type of mix is usually of the
variety of only the transmitter carriers mixing, i.e. the much weaker
receive signal transmitted by a user in the far field wouldn't contribute an
appreciable level to the mix.
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
> -----Original Message-----
> From: Repeater-Builder@yahoogroups.com
> [mailto:[EMAIL PROTECTED] On Behalf Of
> vintageaudio2004
> Sent: Wednesday, August 03, 2005 10:52 AM
> To: Repeater-Builder@yahoogroups.com
> Subject: [Repeater-Builder] Help needed with TX combiner problem
>
>
> 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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>
>
> Yahoo! Groups Links
>
>
>
>
>
>
> --
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> 7/25/2005
>
>
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