MSF 5000 Broadcast Band Interference

[krazybob]

I own several MSF 5000's. Only one is installed at a commercial site that is next to another commercial site with a couple Megawatt transmitters. I've been led to believe that the MSF was capable of operating on heavy RF sites without skipping a beat but this one may be an exception. It was previously at a private site using a DB224 side-mounted antenna at 85 feet and awesome coverage. It is now at a commercial site with a Station Master at 85 feet and greater coverage, but with devastating interference / white noise / decreased signals. Most VHFers on the other site mount their antennas low at the base of the tower to stay out of the main lobe of the broadcasters. I'm on a different hill 1.07 miles away with my antenna right in the middle of their main lobe.

To be specific I live 1/2 mile from the site. Using an Agilent HP 8924C service monitor and a Hustler G7-144 tuned for the repeater I have a received signal strength of -13dBm. After hearing a broadcast station riding my hang timer I went and found the station on the air. In spectrum analyzer mode I located two broadcasters that were also at -13 to -15dBm listening with the same now untuned Hustler G7-144. I have a pair of Angle Linear bandpass cavities inline without a preamp but it isn't cleaning up the mess. I am looking at adding additional cans but Chip isn't doing ham anymore. He's ready to retire and I hear and doing government stuff. Good for him! He's earned a break. Moving along, all cables inside are RG-214 with crimp N connectors, but I can switch to 1/4" SuperFlex with solder connectors. The interference seems to be more front-end overload than intermod and I don't see crimp versus solder connectors adding to the issue. It doesn't appear to be IM is my point.

So then I have choices that I am aware of and maybe some that you will offer. Do I use individual cavities for the particular frequencies (expensive and insertion loss), or do I consider something like a DCI Bandpass filter that covers +/1 2MHz centered on 146MHz that appear to have greater than 85dB rejection down in the FM band? http://www.dci.ca/pdf/DCI-146-4H.pdf? My thinking is that these filters are tanks, I've seen a couple in our new commercial site (I wonder why LOL), and they cover VHF paging noise from the same hill top next to us.

I sincerely appreciate any help you might offer.

[desperado]

If you are hearing the transmitter in the repeater, then it's getting deep into the receiver section of the repeater. With those signal levels, first thing I would do is pull all the 214 and go to hardline with properly installed connectors. You mentioned superflex. If you are referring to LMR400/600 then you will increase your problems rather than decrease them. During that process I would pull the bandpass cans and install between the duplexer and receiver a single frequency pass can. Something with a very tight pass frequency and also install a circulator between the TX port on the repeater and the duplexer. If you can find two frequency specific pass cans then I would consider pulling the duplexer and just going with the two pass cans one for TX the other for RX and installing a second antenna. You will need 40 foot of vertical separation between the two antennas to keep the TX out of the RX. I would also look at the broadcast TX frequency and all other frequencies that are at or above -60 db received and the repeater site and do an inter-mod study. If you have either 3rd harmonic or direct inter-mod product's landing on your receive or transmit frequency then you may have to change frequencies on your repeater. We had a 28 channel SMR trunked repeater system at a TV broadcast site with 3 TV transmitters, 6 or 8 FM radio transmitters and numerous other commercial repeaters at the site and it all worked (other than the one ham repeater that was intermoding into one of the commercial repeaters and was forced from the site). It can be made to work. You are gonna need to do some leg work and spend some money to make it happen but it can be a workable solution even in these adverse conditions.

[FMROB]

Moving along, all cables inside are RG-214 with crimp N connectors, but I can switch to 1/4" SuperFlex with solder connectors. The interference seems to be more front-end overload than intermod and I don't see crimp versus solder connectors adding to the issue. It doesn't appear to be IM is my point.

Agreed, it sounds that the broadcasr signal ( which is most likely high power) is just making a mess. You might want to try adding additonal pass cans in line with the duplexer to try to squeeze down the offending signals, looking at the noise floor with a spec an prior to attacking this would also be in order. We have some UHF equipment co located with VHF broadcast and we have to use what we call broadcast filters to get rid of the nonsense. Its just a giant blob of spectral noise that blasts the front end and desenses the heck out of it.

What worries me is that you are actually hearing the boradcast on your carrier, which could mean some mixing issues, but is it getting into the rx or mixing in the tx section?


As far as removing the RG142, why would you want to do that is beyond me? RG142 has superior shielding and is a very good quality interconnect cable, hence its vast use in almost every duplexer, combiner, and filter system. That is just my opinion. As far a crimp or solder connections, well ok. I agree with you, crimp or solder shouldn't make a difference, although I prefer a soldered connection. Captivated hardline connectors are pretty much the norm even on 1/2", so what would be the advantage to a soldered connection.

Just my 2 cents
Rob

[krazybob]

Thank you for taking the time to reply. I sincerely appreciate you each doing so.

When I referred to SuperFlex I was referring to Andrews SuperFlex and not LMR400. They are night and day and world's apart. I also said that besides the 6-cavity duplexer, which has no bearing here, I am using a dual cavity Angle Linear bandpass solution that is not doing the job. They are quoted by Chip Angle as having 90dB of rejection outside the 1MHz window. It clearly isn't enough. I also see no need to change the RG-214 because this does not appear to be an IM issue. It appears to be a front-end overload issue. This is why I asked about using the DCI 4 cavity helical resonator. It has a hot roll off and rejects 89dB at 126MHz. If it's FM broadcast overload I suspect it would be gone. I suspect that I am close enough where this could be a reality. Or do I use individual cans of low rejection? That was my question and it hasn't been addressed.

I am going to the site this weekend to see if I can see anything specific on my frequency. If I find anything directly on my input I'm done. But I wonder if there are other suggestions you might have for me? We have other members searching the Internet for solutions and not finding solutions...

One caution my Elmer gave me was to FIRST and ALWAYS attach the station antenna to a Bird watt meter connected to a dummy load on the right side. Using a 5-10-20 watt slug rotate it to reverse and measure the wattage coming down the pipe. My SA is rated at 200mW and it is very likely I am getting far more than that!

[d119]

Are you using an external controller at all? If so, it's possible if you're hearing the FM broadcast transmitter that it's entering on your controller cabling, if you're using one.

You said your duplexer has no bearing - but it potentially does. What kind of duplexer is it?

I'd also try swapping out your 214 jumpers. You may have something that is a "magic length" and causing you all sorts of hell. It's worth a try.

The MSF 5000, as you know, is a very robust product and should NOT be experiencing this kind of interference. You've over a mile away from the offending transmitter - something's not right in your antenna network. You shouldn't need that kind of hellacious filtering in this situation... Look closer at the cabling.

[escomm]

I will attest to D119's comments about the MSF5000. A few of my customers had them running at Mt Wilson for eons with no ill effect and some ham's too with the boss's permission. Don't think you'll find a more RF infested site in the world.

[krazybob]

I too have heard stories of the MSF 5000 being used on Mt. Wilson. I'm next to a similarly hot site east of Wilson called Heap's Peak. It has three megawatt FM stations and is rumored to be RF hell. My Elmer in fact ran a box on Wilson with no trouble. I didn't think the MSF would be a problem either. Most 2 meter guys run their antennas at the bottom of the towers to stay out of the lobes of the broadcasters. Now I'm on another hill nearby and right in the middle of the lobes! I am at 85 feet and they are at 134 feet aiming down towards the cities below.

I'm not running any controller yet. I was planning on running one later. I do know that my Elmer left a pigtail hanging from the main board while attempting to add a controller but this box has been used for nearly two years without issue. Could this be acting like an antenna? The symptom is that the PL opens but audio doesn't pass except on strong signals.

The duplexer is a Sinclair Q2330e 6-cavity. All cables are Belden 8268 RG-214, 3 feet each. I am not using any preamp. I am using Andrews 1/2" hard line.

What could I possibly do different? I can switch to Andrews 1/4" SuperFlex cables. I have other MSF's that I can swap out but I agree that these are like bullets. Could this be a tuning issue? I didn't tune the unit.

Lastly, it it may be too soon to ask, I AM going to connect a controller. Will I need to put ferrite beads on each line or take other measures? I know the MSF to be an excellent unit. All I can say to describe this is like a blanket covering the signals. Squelch opens for stronger signals but not HT's and weaker mobiles. This repeater was used at my home 1.6 miles from the FM broadcasters with excellent results but using a side-mounted DB224e pointing away from the broadcasters.

I'd like to think that I'm pretty sharp and that I've learned a lot along the way. I guess not enough. I am eager for your help.

Humbly,

[Jim202]

Let me ask one simple question here. What do you have for a ground on the equipment? A small # 14 wire will not be of much good in this location.

You might have to use a wide copper braid strap to get a low resistance ground with a low inductance to help you with the problem. If the site your at was designed properly, you should see a copper strap running around the building to ground all the equipment cabinets to. If that is there, make use of it.

Jim

[krazybob]

Thank you for asking, Jim. I sincerely appreciate the help.

The ground in the building is crap, although it is a 12ga. wire running around the baseboard of the building. I personally installed a new 8 foot copper-clad ground rod, an Alpha-Delta PolyPhaser clamp, and a PolyPhaser. We have 12ga. solid copper wire grounding the equipment with the plan of changing this out to 10ga. stranded copper. If it makes a difference I will either buy ground straps from DX Engineering or strip RG-214 and grab its braid.

But I am concerned, as in my Elmer, with the fact that he left a few short wires in place when attempting to connect a RC-210 controller to the MSF. It failed on the Marty connector and test points, but that is an issue for another time. At any rate, they are inside the sealed main board case and inside the MSF cabinet, which is grounded.

Is there anything else that you would recommend?

The symptoms again for clarity are: a blanket-like attenuation on all signals, PL opens but audio passes only on stronger signals. In spite of these anomalies stations are getting in from our fringe areas but with white noise that was not there before the move. They may be heard cleanly on the input much of the time from my home 1/2 mile lower than the repeater site using a Hustler G7-144. The repeater uses a new commercial repeater antenna above a new Verizon installation.

Humbly,

[Bill_G]

You're colocated with a cellphone carrier, and you're working 2M, and you're in the beam path of an FM broadcaster?

It's time to move your box.

[krazybob]

According to all the posts before yours the MSF is more than capable. Couple you explain your comment? Thank you for taking the time to write one.

[Bill_G]

You have a crashproof car that is getting crushed in traffic.

You're co-lo'd with Verizon. Despite the money they spend to reduce noise and PIM in their band, they generate enough low level broadband noise to raise the VHF noise floor 10db or more if you are within a few hundred feet. From switching noise in the tower marker lights, to processor noise from the backhaul links, to low cost security cameras, to tiny amounts of hash from their array that is well within legal limits, it all adds up to trash for your receiver that you cannot filter out.

You're in the main beam of the FM broadcasters whom you say are pumping megawatts. Even if they went above and beyond the legal requirements for proper filtering to an easy number of 80db down from a +90dbm carrier, that still throws .01 watts of RF rocks at your receiver. Subtract approx 100db free space loss, that leaves a -90db of broadband hash saying howdy to your front end that you cannot filter out no matter which filter you apply.

Combine the two possible sources of noise at the site you've chosen, and you're fighting against the wind. Even the MSF is not that big of an oak tree. This hurricane will knock it over. Cut your losses, and find a new home if possible.

[Bill_G]

Let me put it this way - In my experience, when I hear cellphone carrier and VHF in the same sentence about a site, I grab a Bible, not a tech manual.

[krazybob]

So you are telling me that as long as I am on a Verizon tower (located above the triangle in between the panels) I will never cure this problem?

[krazybob]

I absolutely understand that I need to read the spectrum analyzer from the site -- and we will be doing that in a few hours. But from my home where I am now the SA is showing me that with a 1MHz spread I just have a noise floor (that could be lower.) But if I switch to 1356MHz spread to include the FM Broadcast Band I see a tick right on my frequency.

The repeater used to be here at my home with an antenna on an 85 foot tower. We enjoyed excellent coverage and excellent HT performance. I wonder if we could have done better. This was with no Angle Linear or additional cavity at all.

I wonder what these will show in a few hours...

http://controlservers.net/home-sa.html

[Bill_G]

Yes. If you stay at the Verizon site, you will be fighting this for a long time because you're 2M, and you're physically very close to a bunch of equipment that throws off VHF hash well with legal limits. Being in the beam path of the FM broadcasters just ices the cake.

But, if you're curious as to how bad it is, you'll need a service monitor to perform a receiver desense test. You should also place your spectrum analyser on your rx line after any filters, and probably with a 10db pad in line to protect it.

[mike m]

I have to agree with Bill on this a new location may be in order.

We went thru this 3 years ago with a Verizon site on a 8000 foot hill near Prescott AZ but in our case it was a 6 meter repeater which was an order of magnitude worse than our 2 meter repeater.

Between the noise emitted from the Verizon switches and similar Verizon security cams that Bill talks about, all the negatives made the effective sensitivity go from -122 dbm when we shut the Verizon system down for a few minutes, reduced back to -99 dbm with Verizon back up and running.

Since this was a private site owned by a friend of the 6 meter repeater, The Verizon tech actually shut their equipment down on a snowy Feb morning at 3 A.M. for a couple of minutes to run our test while he also installed some new equipment on a adjacent mtn backup site.

I doubt you'll get this kind of help at your site from Verizon however.

[krazybob]

I want to thank you all for your advice and everything has been considered. My elmer is a broadcast engineer and spent the day on site with me. Some of it was used today along with other advice. EVERYTHING is grounded to a new ground rod, the hard line extra was cut off and a new N connector attached. The interconnect cables remain 3' RG-214 and I see no need to switch to 1/4" SuperFlex since we have no desense. If you say it matters I'll change it. People talk about the magic length of the interconnect cables and humbly I don't know how to calculate that or if it even relates to FM broadcast band or paging interference.

Upon arrival we bypassed everything and plugged the antenna straight into the service monitor (HP Agilent 8924c, NIST certified). On RF In it can handle 100 watts; on Antenna In it can only handle 200mW. We used the wire whip antenna for many of the tests. With a 2MHz window it was clear but with a 20MHz window it revealed what I already knew was there: 152MHz paging. With a 135MHz span it clearly showed all of the grunge in between and the -1dBm broadcasters. Powerful stuff. My associate uses a Kenwood TK-790 commercial with encode only and whenever we talked in with our HT's (Motorola's) up his radio would bleep, bleep, bleep. Obviously we have paging interference coming in when strong signals hit th4e machine. I can get just an intermod filter from Dale at PAR Electronics for $70 and get ~ 50dB attenuation. That will be enough to make the paging disappear.

The noise floor was about 5dB, which I had here at the home. We still have an issue with the signal opening PL but no audio passing while I can hear it from my home station. I think that we may need to change the setting of the squelch and my luck I did not have the MSF programming cable with me. At the end of the day we had a flat noise floor with no obvious spikes WITH the duplexer and Angle Linear twin cavities. Without them we had grunge all over the screen. Reflected was less than 1 watt of at the duplexer out. Not bad. I am very, very tired but I will try and update and I hope that I don't miss answering a question or that I followed specific advice.

The old cabling for the controller has been removed. Unknown if any affect. I don't know how to get the magic length but all interconnecting cables are RG-214. If I am correct the RG-214 might only be an issue for ingress noise because it only has 95% braid. It is Belden 8268 but I have Times Microwave that is also only 95%.

We did remove all cavities at the beginning and everything looked fine as I mentioned with a 2MHz span but going out to 20 revealed the paging junk and going out 135MHz revealed the monster FM stations. The MSF has its circulator intact and it is working as expected. The MSF remains in its cabinet, although we plan on removing it when we put the rack in for the link radios, etc. First we lick this problem! We also ran a complete new ground system using 8ga. multi-strand copper and replaced the Polyphaser for good measure. No, I won't sue you and if you knew the truth you might have agreed

Did this accomplish anything? I'm not sure. We still have the problem of mobile and HT station opening the PL but no audio is passing. Yet I can usually go to the input from my home and hear them fine. I assume that we still have an interference issue that is acting up. Even with the Angle Linear cavities. I have a DCI coming in that others talk highly of and I was warned that when one plays in a harsh environment things like this take time to workout.

[wavetar]

Posts which had nothing to do with the actual topic as posted by the OP have been removed. I'm keeping the thread open as it's an informative thread, but if it goes off the rails again, it will be locked.

[WPXE839]

I found your thread by searching for MSF interference. If you look at the top-right of the display on your Agilent what is the dBm reading? That is your useable floor after your filtration. In a perfect world it will be above 100 but I suspect that you are seeing considerably lower. You may be correct that PL is opening but the squelch is too high and it is causing a popping sound. You wrote that within a 2MHz span your floor was flat but what was the dBm reading? What was your reference level? 0dB? I would not tell you to jump sites just yet but I wonder... I've never worked with crystals before and I understand that they have like a 7dB insertion loss and require a preamp to try to recover something that has already been lost. Maybe someone else that has experience with crystal filters can give you advice on using one. The DCI filters are good. I am fighting a similar situation so I am watching what you are doing for suggestions as well. Honestly I don't know if you can beat a -1dBm broadcast station. That is intense. From 1/2 mile away?

[krazybob]

Chris,

I do know the MSF to be a very robust repeater. That's why I own three and have two deployed. But can you suggest an alternative to RG-214? I am willing to do whatever I need to do to change this equation around. I don't mind getting my hands dirty.

Also, you asked about the duplexer. It is a Sinclair Q2330e 6-cavity BrBp and I love them. I do have a WaCom WP642 already tuned up but it uses PL-259's on the 8" cans.

The antenna system has been checked top to bottom without any anomalies found. It is new from the top down. A new antenna with direct DC ground, new Andrews 1/2" hard line with new positive contact connectors, a new PolyPhaser, new 8ga. grounding wire to a new 8' grounding rod. All equipment is grounded to a central point using the grounding lug on the MSF 5000. We are not yet running the RC-210 controllers but should we use bypass caps with ferrite cores for the wiring in and out?

Are you using an external controller at all? If so, it's possible if you're hearing the FM broadcast transmitter that it's entering on your controller cabling, if you're using one.

You said your duplexer has no bearing - but it potentially does. What kind of duplexer is it?

I'd also try swapping out your 214 jumpers. You may have something that is a "magic length" and causing you all sorts of hell. It's worth a try.

The MSF 5000, as you know, is a very robust product and should NOT be experiencing this kind of interference. You've over a mile away from the offending transmitter - something's not right in your antenna network. You shouldn't need that kind of hellacious filtering in this situation... Look closer at the cabling.

[Wowbagger]

I absolutely understand that I need to read the spectrum analyzer from the site -- and we will be doing that in a few hours. But from my home where I am now the SA is showing me that with a 1MHz spread I just have a noise floor (that could be lower.) But if I switch to 1356MHz spread to include the FM Broadcast Band I see a tick right on my frequency.
http://controlservers.net/home-sa.html

Make sure the SpecAn isn't lying to you - if it's front end is being overloaded by the FM signal you could see a spur on your frequency that is generated within the SpecAn's front end. If you have a 3dB pad, put it in line and see if the spur goes down by 3dB or 9dB - if the latter, it's the SpecAn lying.

[Bill_G]

Excellent advise. There should be a linear change. Whenever you're working in a high rf condition, always have a handful of pads to insert in your test equipment, and device under test. You can easily be chasing ghosts generated in the front end.

[krazybob]

Awesome advice from all three of you. I DO have a bag of N connector attenuators rated at 10 watts each. Since I followed your advice and checked what was coming down the antenna I know that only 1 watt is coming back. My attenuators are safe. I can pad then up to to about 40dB.

I saw -79dBm on the SA, so that is my floor after filtration? Ouch. That hurts then.

I did a radio check just a bit ago with a known station and the wind is so fierce today that he was not his usual self. The DCI should be here tomorrow as well. Sustained winds are about 20mph gusting to over 50. But I am not hearing 2 meter hash as I have on other machines.

[Bill_G]

Your noise floor is determined by a rcvr desense test.

[krazybob]

I am beginning to think that there may be a hardware failure but even that doesn't make sense. We had a regular user hitting me at S9+60 at my home 1/2 mile away from the repeater site and yet he would only occasionally pass audio. But he did seem to pass PL. It appeared that he was riding the hang timer and when it dropped so did he. That would normally suggest a problem on his end but so many other users have the same symptom exc3pt they don't sound like they are riding the hang timer. Another station closer but weaker using a Motorola HT could open PL AND pass audio. This was never the case before and I am pulling my hair out!

[WPXE839]

Your noise floor is determined by a rcvr desense test.

On the HP Agilent 8924c, which is the cell site CDMA version of the HP 8920B, it reads out the signal strength in dBm of what is being heard by the antenna. It is not perfect because the sensitivity of the service monitor is not stellar. Not many service monitors are. If it is reading -80dBm with an antenna connected that is about 35uV. If your MSF 5000 measures -119dBm for a reasonable signal to break squelch that is about .25uV. That's quite a spread. Once you add in the duplexer you may end up at around -108dBm, or close to 1uV. Returning to the spectrum analyzer reading about -80dBm off the antenna that's quite a difference. Unplug the antenna and plug the spectrum analyzer into a dummy load - what's the reading now in the top-right corner in dBm?

[Bill_G]

Your noise floor is determined by a rcvr desense test.

On the HP Agilent 8924c, which is the cell site CDMA version of the HP 8920B, it reads out the signal strength in dBm of what is being heard by the antenna. It is not perfect because the sensitivity of the service monitor is not stellar. Not many service monitors are. If it is reading -80dBm with an antenna connected that is about 35uV. If your MSF 5000 measures -119dBm for a reasonable signal to break squelch that is about .25uV. That's quite a spread. Once you add in the duplexer you may end up at around -108dBm, or close to 1uV. Returning to the spectrum analyzer reading about -80dBm off the antenna that's quite a difference. Unplug the antenna and plug the spectrum analyzer into a dummy load - what's the reading now in the top-right corner in dBm?

That's my experience too. Typically service monitors and spec analysers have a much higher rcvr threashold than a radio. They are general purpose devices meant to operate across a wider range than most radios. However, they are only good at seeing high level signals. If the interference is below the test equipment threashold, you'll never see it. That's why I recommended he perform the rcvr desense test. However, your point that he is using an 8924C is well taken. It is a CDMA test set, and not the correct tool to analyze this problem. It's nice it has a spec an to see things off the air, but it doesn't help him determine the degree of impairment his rcvr is having.

[krazybob]

I am at a loss as how to proceed. I have followed EVERY piece of advice given. I've shortened cables. I've removed controller jumper. I've installed a dual band pass cavity after the duplexer. It is supposed to create a 1MHz window at 90dB down on the center frequency. I have pounded in a new ground rod and connected everything with 8ga. multi-strand copper wire. I have grounded the duplexer to the repeater. I purchased a DCI filter that has not been installed yet but I am not holding my breath. The MSF is supposed to be a robust and capable public safety grade repeater. At the end of the day stations that previously were full quieting struggle to get through. It is like a force field surround me and keeps most signals down below a reasonable level. When I attach an antenna to the service monitor spectrum analyzer it reads -76dBm. Does that mean that a signal needs to be nearly 40uV to get through? I am doing the math. The offending FM broadcasters (there are three) all state that their antennas are at 126 feet and undoubtedly have down tilt to serve the communities below them. I am on the next mountain over 1 mile away and my antenna is at 85 feet - right in the path of the FM broadcasters that ring in at -1 to -3dBm. Then I have paging transmitters that when we key up on an HT to bring up the repeater we hear bleep, bleep, bleep on a nearby receivers. The paging transmitters ring in at -25dBm.

I have diligently followed all of your advice respecting each of you along the way. I am lost and stuck in a contract. I would gladly move it back to my home where we enjoyed outstanding performance and then use this site on 900MHz to link to our 2nd site on the other side of the mountain. Ideally I'd like to solve this site problem and enjoy the outstanding 150 mile range it provides.

When looking at the SA we don't see a hash in and around our site. Verizon's equipment is outside and our antenna is above their panels. When we remove the filtration the FM broadcasters and their harmonics instantly populate the SA screen along with the paging transmitters. Do I consider a crystal filter and preamp as has been suggested? I cannot see adding one cavity on top of another and getting no where. Should I switch to 1/4" SuperFlex or even 1/2" SF? I'll do whatever the experienced users tell me to do as long as it is based on fact and prior experience. I refuse to say that I am in over my head. I just have not experienced this type of interference.

<sigh>

[PETNRDX]

All that extra filtering will NOT remove a signal that is on your RX freq. Don't waste your money on more filters.
Do the above mentioned RX desense and effective sensitivity test.
Find out if the interferer is ON frequency.
Use an HT or something just wandering around the site.
If it is not on freq, then find where it is being developed (like intermod sources).
The MSF is a great radio, but if something is a signal, its a signal. The radio won't pass audio unless that signal is STRONGER than an interferer. If the interferer has no PL, it captures your receiver.
First find out if there really is what appears to be an "on channel" interferer.
Then work on whether or not that interferer is away from your radio and your site, versus IN your radio, or AT your site.
Don't give up.
I support a LOT of VHF systems that are co-located with Verizon.
It can be done. Just probably going to be some step by step work.

[PETNRDX]

Forgot to mention, I would not recommend any pre-amp if you are already having interference problems.
A pre-amp can make things worse.
Find the source of the problem first.

[krazybob]

Thank you very much for your reply, Steve. You're spot on, as they say! No, no preamp until the source is located.

S poke at length with an industry legend today that makes band pass filters, notches, preamps, etc. He has given me a set of marching orders that you have just touched on as well as a few others. I am doing my best to be a good student. I'm not new to radio but I am somewhat new to repeaters. I've been spoiled by having the repeater at my home with a DB224e antenna up the side of an 85 foot tower (top down.) It worked awesome! HT coverage was excellent.

Here is what I'll add in the hopes that it may help someone else.

First, I am supposed to start at the front; the duplexer. Prior to the duplexer the SINAD was a workable -118dBm, which I thought was a bit lower than I measures with my service monitor. The associate that tuned the duplexer measured -118dBm on his IFR and I measured -125dBm on my NIST certified HP 8924C. After tuning the Sinclair Q2330e 6-cavity duplexer he reported a sensitivity of -108dBm and I know that is way too low. Thats .891uV. I have been instructed to do it over again and not settle for anything less than a return loss of at least 20dB.

Next, I am to aim for 5" inter-connect cables of RG-214 or don't shoot the messenger, LMR 400. It has half the loss of RG-214 and is 100% shielded. When LMR 400 is used indoors it does not represent an issue. It is when it is used outdoors with dissimilar metals and corrosion. Remember, I'm just the student. I'll be prepared with RG-214 as well. If one cannot make it with 5" cables he recommended 24" and the length may vary. Then I am to insert the Angle Linear dual cavity band pass filters WITHOUT the preamp, just as you have said. The suspicion is that I am getting beat down by the 152MHz paging transmitters that come in at -25dBm and not the FM broadcasters that are down below 100MHz and hitting me at -1 to -3dBm. Phew. All these numbers.

You probably already know this stuff Once I have ensured that I have a properly tuned duplexer with a low SWR on RX and TX I should do a test using the tracking generator and make sure that everything likes one another. Does this make sense? I've never heard that one needs to worry about SWR on RX but he recommended a HT transmitting into the duplexer to a Bird and then a dummy load. Make sure I have a low SWR.

Once I have done all of these things I am supposed to take her for a test drive and not settle for a return loss of at least 20dB.

I am prepared to do whatever I need to do to fix the problem and that includes humbling myself in front of all of you. I don't know this stuff like yousse guys and I appreciate you taking the time to reply, Steve. I am sure that you would have said all of the above as well and remember folks that I have included it here in case someone else might benefit from it, along with the help each of you has offered.

Thank you again, Steve.

[WPXE839]

The information that Steve (and others) have given you, plus your new Elmer is dead on the money. I figured out who the Elmer was. We use his hardware on all of our commercial stuff at work. Follow the instructions and like Steve said make sure that the interference isn't exactly on your input. If it is you're hosed. 5" or 24" interconnecting cables is an attempt to get as close to a 1/4 wavelength as possible and to optimize the feeds. As you tried to hide from LMR 400 is not bad when used indoors for interconnecting cables. I don't recommend it for the main antenna line but otherwise it has half the loss of RG-214 and plenty of shielding. You need to be absolutely sure that your interference isn't coming in on bad crimp connections or poor soldering jobs. Other than that you've received excellent advice in this thread and from your Elmer. I hope that once you have fixed your issues you'll come back and give a report.

[PETNRDX]

A lot of people use LMR and have good luck with it. I don't use it at all on repeaters. Once in awhile on plain old base stations. (Simplex, semi-duplex). But, it might work for what you are doing.
Loss is not much of an issue. Inches of any good cable will not be something that you could measure "over the air". As one other person mentioned above, RG-142 or RG-400 are really good cables for this.
Yes, SWR matters on Rx. Maybe not as much as TX but it will detune your filters if way off.
One of the things you are VERY likely fighting is that while your duplexer is a very good one, it is NOT a true bandpass design. Note how the "connections" into each cavity are a Tee. Not one "input" and one "output". So what you wind up with maybe one Mhz or more AWAY from your two frequencies of interest, there is VERY little attenuation of out of band signals. That (and most) VHF duplexers are really just to keep your TX out of your RX and vice versa. Very little "protection" from other systems.
Adding the DCI or better yet, a TRUE pass cavity between RX and TX and the duplexer might help, or at least tell you something.
Yeah, find out what is wrong with the tuning of that dupe. Sounds like there is a problem.
I think you will find the problem eventually, and learn something in the process.
As for humbling, You are not the only one.
I have been "learning" radio for about 45 years. And I get humbled pretty regularly.
Most of my work the last dozen years is Vhf combining systems. Hence, being humbled....