Users may not create topics, posts, or private messages containing or relating to the following material (especially pertaining to Motorola copyrighted software, unless you want Motorola to come along and shut this site down):

  • Listing for sale or trade of, or links to sites offering for sale or trade of, or giving away, Radio
    Service Software (RSS) or Customer Programming Software (CPS)
  • Profanity, pornography, defamation, or slanderous remarks directed towards any individual or entity
  • Commercial advertising (except in the Batboard Vendors forum, as approved by the Admin/Mod Staff)
  • Any other items which may be deemed as offensive

If any topics, posts, or private messages containing or relating to the aforementioned material are brought to the attention of the Admin/Mod Staff, they will be deleted.

Additional FAQ items appear here in Forum Rules. Please review them for posting guidelines and further clarification.

MTR2000 RF/audio feedback when remotes unkey

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

Moderator: Queue Moderator

k7kcn
New User
Posts: 8
Joined: Sun Oct 31, 2010 11:34 pm

MTR2000 RF/audio feedback when remotes unkey

Postby k7kcn » Tue Oct 01, 2019 8:52 pm

Trying to figure out a feedback issue with an MTR2000.

With a properly tuned duplexer some odd feedback is getting into the RX from the TX.

Disconnecting the RX and leaving it as an open port fixes the issue, so does putting the TX side into a dummy load.

One odd thing I've noticed is that with DPL it doesn't seem to happen, as well as on a handheld changing the PL while keyed to an incorrect PL - the repeater stops passing audio and TX drops as it should.

It seems to be something like a slow RX shutdown but I cant put my finger on it. I have two duplexers connected with about 100+dB of isolation - yet the issue still happens.

Sound file can be heard at: https://soundcloud.com/mrksvideos/mtr2k-feedback

Any help would be greatly appreciated.

-K

fineshot1
Posts: 1032
Joined: Wed Oct 12, 2005 5:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby fineshot1 » Wed Oct 02, 2019 5:55 am

have you tried connecting a high quality dummy load on the duplexer antenna port
to see if it happens again in that configuration? It seems as if it could be a near field
issue that may be caused by an issue with the RF PA itself.
fineshot1
NJ USA

k7kcn
New User
Posts: 8
Joined: Sun Oct 31, 2010 11:34 pm

Re: MTR2000 RF/audio feedback when remotes unkey

Postby k7kcn » Wed Oct 02, 2019 11:09 am

I did, just an hour or so after posting, and you know what.....no issue. It has to be the feedline or antenna. Not sure what it is but at least this it not the rigs final location, just a test location. I did look up some issues possibly with [PIM] and it may be the issue. Taking the rig (in pieces) up to the site to test to see what happens. Thank you for the reply, it solidifies and proves the system from the duplexer inward is not the issue.
Last edited by alex on Thu Oct 03, 2019 4:05 am, edited 1 time in total.
Reason: Fixed PIM for you.

RFguy
Posts: 1349
Joined: Wed Dec 21, 2005 6:17 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby RFguy » Thu Oct 03, 2019 7:18 am

In the past I have used a different PL on the RX and TX frequencies of a repeater to head off issues on installs that were at risk for intermod. It was a bit of preventive medicine, but more of masking an issue rather than solving it.

k7kcn
New User
Posts: 8
Joined: Sun Oct 31, 2010 11:34 pm

Re: MTR2000 RF/audio feedback when remotes unkey

Postby k7kcn » Thu Oct 03, 2019 6:28 pm

Thank you both for input on the oddball issue. I took the MTR and duplexer up to the site today and proved everything out to be working as intended on a real base station antenna. It is indeed the feedline or antenna here at the house that was causing this weird issue. I have an offer from a friend to bring a TDR over before I rip out the coax to determine what the issue is exactly. But for now I will rack it up to the LMR400 causing....something. I'm pretty sure it isn't the antenna as the feedback has been noted elsewhere online as PIM. I still am a true believer in LMR400 but now only in semi-duplex operations.

The MTR2000 was benched before shipment and was given a clean bill of health...good to have proven that today.

And thanks Mods for the correction on PIM above :)

-K

Karl NVW
Posts: 131
Joined: Wed Sep 03, 2003 7:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby Karl NVW » Fri Oct 04, 2019 1:03 pm

Motorola's field organization doesn't recommend use of ANY cabling in the duplex RF path with a braided shield. That goes double when folks are using a notch-only duplexer instead of a properly selected true Bp-Br or bandpass-only model, especially on a multi-user RF site. I'm not saying that you have committed either transgression, because there's not enough info provided for anyone to pass judgement. Just sayin' there's no reason to be turning passive IM into an active headache...
Karl - WA8NVW AFA5VB
SHARES + NCS

Jim202
Posts: 3566
Joined: Sun Sep 09, 2001 4:00 pm

Re: MTR2000 RF/audio feedback when remotes unkey

Postby Jim202 » Fri Oct 04, 2019 6:32 pm

k7kcn wrote:Thank you both for input on the oddball issue. I took the MTR and duplexer up to the site today and proved everything out to be working as intended on a real base station antenna. It is indeed the feedline or antenna here at the house that was causing this weird issue. I have an offer from a friend to bring a TDR over before I rip out the coax to determine what the issue is exactly. But for now I will rack it up to the LMR400 causing....something. I'm pretty sure it isn't the antenna as the feedback has been noted elsewhere online as PIM. I still am a true believer in LMR400 but now only in semi-duplex operations.

The MTR2000 was benched before shipment and was given a clean bill of health...good to have proven that today.

And thanks Mods for the correction on PIM above :)

-K


It is a very well known fact that LMR type coax is very prone to causing all sorts of problems in duplex or as most of us call it repeater service. New cable may not show up a problem for some time, but it will develop a problem given enough time.

The problem stems from 2 types of shielding used in the LMR cable construction. The aluminum foil and the copper braid develop corrosion over time and given enough moisture intrusion. This will cause a diode action to develop over time. The end result is intermod, noise you can't get rid of and hours trying to locate the source of the noise. It gets worse with multiple radios at the same location. Pay attention to changes in the noise and the kind of weather when the noise gets worse. Like it being windy and having rain in the past.

The solution is to use some good coax cable for your feedline. Normally I would suggest the use of heliax type coax. Problems go away and you don't have to worry about them down the road.

Another area to focus on is your jumper cables used on your filter system. These should be double copper shielded coax or super flex coax cables.

Let the rest of the group on here know how you make out resolving your problems. It is always nice to know that problems of this type have been solved and what you did to solve them. It helps others in their quest to resolve the same kind of problems.

Jim

fineshot1
Posts: 1032
Joined: Wed Oct 12, 2005 5:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby fineshot1 » Mon Oct 07, 2019 5:24 am

Karl NVW wrote:Motorola's field organization doesn't recommend use of ANY cabling in the duplex RF path with a braided shield. That goes double when folks are using a notch-only duplexer instead of a properly selected true Bp-Br or bandpass-only model, especially on a multi-user RF site. I'm not saying that you have committed either transgression, because there's not enough info provided for anyone to pass judgement. Just sayin' there's no reason to be turning passive IM into an active headache...


This is a totally bogus statement.
Motorola commonly uses duplexers from TXRX, EMR, Telewave, and Sinclair
all of which use interconnecting cabling that uses silver covered double shields.
Cabling such as RG-142U, RG223U, RG316DS, and RG400U. All you have to do is
look at the specification sheets from these duplexer suppliers above.
fineshot1
NJ USA

Karl NVW
Posts: 131
Joined: Wed Sep 03, 2003 7:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby Karl NVW » Mon Oct 07, 2019 11:59 am

Fineshot1's comment is itself incorrect. For background, I spent 43 years at Motorola Comm/C&E/Solutions, along with other fine RF engineers who contributed to the well-known R56 site standards and practices. Motorola still recommends that collection every day to dealers, consultants and customers alike. If they elect to build and install a communications system to lesser performance levels they are free to do so. They may later discover that it does not meet the performance levels they had previously anticipated.
Unlike standard braided copper shields and aluminum foils, the double silver braided cabling that fineshot1 mentions is highly resistant to moisture corrosion, a source of PIM. The equipment suppliers that he named are aware of Motorola's PIM level requirements for their RF devices. You'll also find that even those 'economy' notch-only mobile duplexers using semi-rigid cable between cells of the filter casting still don't pass a Moto PIM test.
Karl - WA8NVW AFA5VB
SHARES + NCS

fineshot1
Posts: 1032
Joined: Wed Oct 12, 2005 5:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby fineshot1 » Thu Oct 10, 2019 12:06 pm

Karl NVW wrote:Fineshot1's comment is itself incorrect. For background, I spent 43 years at Motorola Comm/C&E/Solutions, along with other fine RF engineers who contributed to the well-known R56 site standards and practices. Motorola still recommends that collection every day to dealers, consultants and customers alike. If they elect to build and install a communications system to lesser performance levels they are free to do so. They may later discover that it does not meet the performance levels they had previously anticipated.
Unlike standard braided copper shields and aluminum foils, the double silver braided cabling that fineshot1 mentions is highly resistant to moisture corrosion, a source of PIM. The equipment suppliers that he named are aware of Motorola's PIM level requirements for their RF devices. You'll also find that even those 'economy' notch-only mobile duplexers using semi-rigid cable between cells of the filter casting still don't pass a Moto PIM test.


Karl - you can take all the pot shots you want at me but nothing I posted above is incorrect.
You may want to take this chance to become current with turnkey offerings from the moto
of today and not the yesteryear you seem to be stuck in. I have not seen a mobile duplexer
offered by moto in decades for turnkey offerings.
fineshot1
NJ USA

Karl NVW
Posts: 131
Joined: Wed Sep 03, 2003 7:13 am

Re: MTR2000 RF/audio feedback when remotes unkey

Postby Karl NVW » Fri Oct 11, 2019 7:44 pm

I'm not taking pot shots at anyone, just pointing out some incorrect information in your comment. These are discussion groups and I don't mind discussing this topic. I've only been retired a bit over three years, and still have most of the vendor supply catalogs and links to their sites.
For the past decade Motorola has not manufactured their own cavity filters or duplexers in house. Instead they supply various models built by the suppliers you listed to Motorola specs.
If you are looking for semi-rigid interconnect cabling on duplexers, take a look at the RFS Celwave HB 636-6 and UHF 633-6 offered with the economy repeater packages.
Karl - WA8NVW AFA5VB
SHARES + NCS


Return to “Base Stations, Repeaters, General Infrastructure”

Who is online

Users browsing this forum: No registered users and 1 guest