Alex,
You have a nice list of frequencies but I wished they were not so evenly
spaced with both the TX and RX frequencies separated by exact equal amounts.
This way if any IM exists, it will be right on an RX frequency. I did an IM
analysis with the good news that there is only one 9th order IM but 69 11th
order IM's. If I include 13th order IM's there are as many as 639.
The bad news is that every IM falls exactly on an RX frequency of one of the
six channels. However, 13th, and higher order, IM products are usually not
causing too much trouble. But if they are right on frequency as in your
case they may show up depending on antenna selection and antenna
combination. We have had special antennas made with all the Aluminium
junctions welded as much as was possible. In other words, as few clamps and
bolt/nut type hardware that mechanical integrity allowed. ANY loose
hardware on the tower/antenna structure is a candidate for creating IM
problems. All cables should be new and of the double shielded variety.
Proper connectors, specifically designed for the cable to be used, are a
must. All connectors should be as tight as possible without creating
mechanical stress on them.
We have trunking systems with as many as 7 channels combined with 3 or four
other conventional VHF repeaters and no IM and/or desensing. TX RX spacing
is usually 4.02 MHz and TX/TX spacing is as little as 190 kHz. They are all
multi coupled with Sinclair C2027 and C2037 multi coupling units over two or
three antennas and where TX/TX spacing is less than 200 kHz Isolators are
used. Great care is taken to have TX frequencies that may create IM products
separated on different antennas. Generally two antennas such as the
Sinclair 210C4 on one mast above each other should have an isolation of at
least 25 dB. This will be different if one is on the top and the other one
is side mounted on the tower. Greater vertical separation will help.
I have attached a file that shows the relationship of the 9th and all 11th
order IM products with the TX frequencies that are involved. For example
(161.125 X 5)-(160.125 X 4) creates a 9th order IM on exactly 165.125 MHz.
It does not take a strong IM signal to be heard this way. And there is no
way to filter that out other than preventing the IM from being created.
I wish you good luck and hope that you will be able to solve this annoying
issue.
73,
Tony Lelieveld
VE3DWI
-----Original Message-----
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo WN3A
Sent: August 3, 2005 11:21
To: [email protected]
Subject: RE: [Repeater-Builder] Help needed with TX combiner problem
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: [email protected]
> [mailto:[EMAIL PROTECTED] On Behalf Of
> vintageaudio2004
> Sent: Wednesday, August 03, 2005 10:52 AM
> To: [email protected]
> 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
>
>
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
> --
> Internal Virus Database is out-of-date.
> Checked by AVG Anti-Virus.
> Version: 7.0.338 / Virus Database: 267.9.5/58 - Release Date:
> 7/25/2005
>
>
--
Internal Virus Database is out-of-date.
Checked by AVG Anti-Virus.
Version: 7.0.338 / Virus Database: 267.9.5/58 - Release Date: 7/25/2005
Yahoo! Groups Links
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
F-INTERMOD R3.3
Frequency Intermodulation Report
Printed On August 4, 2005 at 21:26:20
C:\Documents and Settings\tlelieve.BELL\My Documents\IM new
Tony\Alex print.txt
Output from 165.09500 to 166.15500 MHz
Less than or equal to Intermodulation Order 11
Channel Tx Frequency Rx Frequency Type H.Bandwidth
T001 Ch 6 161.12500 166.12500 Duplex .03000
T002 Ch 5 160.92500 165.92500 Duplex .03000
T003 Ch 4 160.72500 165.72500 Duplex .03000
T004 Ch 3 160.52500 165.52500 Duplex .03000
T005 Ch 2 160.32500 165.32500 Duplex .03000
T006 Ch 1 160.12500 165.12500 Duplex .03000
Ord I.Mod Freq Channel(R/T) Difference T001 T002 T003 T004 T005 T006
9 165.12500 Ch 1 R .00000 5 0 0 0 0 -4
11 165.12500 Ch 1 R .00000 1 5 0 0 0 -5
11 165.12500 Ch 1 R .00000 2 3 1 0 0 -5
11 165.12500 Ch 1 R .00000 2 4 0 0 -1 -4
11 165.32500 Ch 2 R .00000 2 4 0 0 0 -5
11 165.12500 Ch 1 R .00000 3 1 2 0 0 -5
11 165.12500 Ch 1 R .00000 3 2 0 1 0 -5
11 165.12500 Ch 1 R .00000 3 2 1 0 -1 -4
11 165.32500 Ch 2 R .00000 3 2 1 0 0 -5
11 165.12500 Ch 1 R .00000 3 3 0 -1 0 -4
11 165.12500 Ch 1 R .00000 3 3 0 0 -2 -3
11 165.32500 Ch 2 R .00000 3 3 0 0 -1 -4
11 165.52500 Ch 3 R .00000 3 3 0 0 0 -5
11 165.12500 Ch 1 R .00000 4 0 1 1 0 -5
11 165.12500 Ch 1 R .00000 4 0 2 0 -1 -4
11 165.32500 Ch 2 R .00000 4 0 2 0 0 -5
11 165.12500 Ch 1 R .00000 4 1 0 0 1 -5
11 165.12500 Ch 1 R .00000 4 1 0 1 -1 -4
11 165.32500 Ch 2 R .00000 4 1 0 1 0 -5
11 165.12500 Ch 1 R .00000 4 1 1 -1 0 -4
11 165.12500 Ch 1 R .00000 4 1 1 0 -2 -3
11 165.32500 Ch 2 R .00000 4 1 1 0 -1 -4
11 165.52500 Ch 3 R .00000 4 1 1 0 0 -5
11 165.12500 Ch 1 R .00000 4 2 -1 0 0 -4
11 165.12500 Ch 1 R .00000 4 2 0 -1 -1 -3
11 165.32500 Ch 2 R .00000 4 2 0 -1 0 -4
11 165.12500 Ch 1 R .00000 4 2 0 0 -3 -2
11 165.32500 Ch 2 R .00000 4 2 0 0 -2 -3
11 165.52500 Ch 3 R .00000 4 2 0 0 -1 -4
11 165.72500 Ch 4 R .00000 4 2 0 0 0 -5
11 165.32500 Ch 2 R .00000 5 0 0 0 1 -5
11 165.12500 Ch 1 R .00000 5 0 0 1 -2 -3
11 165.32500 Ch 2 R .00000 5 0 0 1 -1 -4
11 165.52500 Ch 3 R .00000 5 0 0 1 0 -5
11 165.12500 Ch 1 R .00000 5 0 1 -1 -1 -3
11 165.32500 Ch 2 R .00000 5 0 1 -1 0 -4
11 165.12500 Ch 1 R .00000 5 0 1 0 -3 -2
11 165.32500 Ch 2 R .00000 5 0 1 0 -2 -3
11 165.52500 Ch 3 R .00000 5 0 1 0 -1 -4
11 165.72500 Ch 4 R .00000 5 0 1 0 0 -5
11 165.12500 Ch 1 R .00000 5 1 -1 0 -1 -3
11 165.32500 Ch 2 R .00000 5 1 -1 0 0 -4
11 165.12500 Ch 1 R .00000 5 1 0 -2 0 -3
11 165.12500 Ch 1 R .00000 5 1 0 -1 -2 -2
11 165.32500 Ch 2 R .00000 5 1 0 -1 -1 -3
11 165.52500 Ch 3 R .00000 5 1 0 -1 0 -4
11 165.12500 Ch 1 R .00000 5 1 0 0 -4 -1
11 165.32500 Ch 2 R .00000 5 1 0 0 -3 -2
11 165.52500 Ch 3 R .00000 5 1 0 0 -2 -3
11 165.72500 Ch 4 R .00000 5 1 0 0 -1 -4
11 165.92500 Ch 5 R .00000 5 1 0 0 0 -5
11 165.12500 Ch 1 R .00000 6 -1 0 0 -1 -3
11 165.32500 Ch 2 R .00000 6 -1 0 0 0 -4
11 165.12500 Ch 1 R .00000 6 0 -1 -1 0 -3
11 165.12500 Ch 1 R .00000 6 0 -1 0 -2 -2
11 165.32500 Ch 2 R .00000 6 0 -1 0 -1 -3
11 165.52500 Ch 3 R .00000 6 0 -1 0 0 -4
11 165.12500 Ch 1 R .00000 6 0 0 -2 -1 -2
11 165.32500 Ch 2 R .00000 6 0 0 -2 0 -3
11 165.12500 Ch 1 R .00000 6 0 0 -1 -3 -1
11 165.32500 Ch 2 R .00000 6 0 0 -1 -2 -2
11 165.52500 Ch 3 R .00000 6 0 0 -1 -1 -3
11 165.72500 Ch 4 R .00000 6 0 0 -1 0 -4
11 165.12500 Ch 1 R .00000 6 0 0 0 -5 0
11 165.32500 Ch 2 R .00000 6 0 0 0 -4 -1
11 165.52500 Ch 3 R .00000 6 0 0 0 -3 -2
11 165.72500 Ch 4 R .00000 6 0 0 0 -2 -3
11 165.92500 Ch 5 R .00000 6 0 0 0 -1 -4
11 166.12500 Ch 6 R .00000 6 0 0 0 0 -5
Total of 69 listings, no other solutions found.
