Continually losing RX in 396-series scanners

[tfhphoto]

My base station installation of Uniden handheld scanners has an ongoing issue and I can't figure it out.

Here is the CURRENT situation: I have a Diamond V2000A antenna mounted on a eight-foot mast on the roof a two story office building. The scanner is fed with about 100' of LMR 400. Consistently, and within about 30 days of attaching either a 396T or 396XT (it's happened with different units of both models) the scanner will lose almost all usable RX.

When sent into Uniden for repair, they'll replace some diodes/resisters and return the scanner with a note advising to keep the scanner away from transmitters.

The problem is that there are are no known VHF/UHF transmitters close by except for a 50-75" railroad comm tower about 12 blocks away. Currently, there are no transmitters the building other than the usual personal communications devices (cell phones and tablets).

In a PAST RF configuration in my building, a yagi antenna fed by a 900 MHz Max Trac base station was moved to be co-located below the V2000A. The two coax feeds (to the scanner and to the Max Trac) were partially co-located.

Thinking that RF from the Max Trac was getting into to the scanners, I purchased a custom Stridsberg notch filter centered at the base station's TX frequency and installed that between the scanner and coax.

The filter made no difference and the attached scanner went deaf..

The company I work has since ended use of the Max Trac and I removed the base station and its antenna, resulting in the current configuration.

I made the assumption that decommissioning the Max Trac would solve my problems. Nope.

The only other possibility I can come up with is that my antenna is across the street form a series of auto dealerships have occasional radio station "live remote" broadcasts. Could it be the FM stations' remote link equipment?

At this point, I'm tired of sending scanners out for repair and would welcome any assistance.

 

[ecps92]

other RF thoughts you may have overlooked
a. Is there a Cellsite on your Bldg ? / Near-by
b. Is the Fire Alarm system [fire box to Fire Dept] RF or hard-wired?
c...

My base station installation of Uniden handheld scanners has an ongoing issue and I can't figure it out.

Here is the CURRENT situation: I have a Diamond V2000A antenna mounted on a eight-foot mast on the roof a two story office building. The scanner is fed with about 100' of LMR 400. Consistently, and within about 30 days of attaching either a 396T or 396XT (it's happened with different units of both models) the scanner will lose almost all usable RX.

When sent into Uniden for repair, they'll replace some diodes/resisters and return the scanner with a note advising to keep the scanner away from transmitters.

The problem is that there are are no known VHF/UHF transmitters close by except for a 50-75" railroad comm tower about 12 blocks away. Currently, there are no transmitters the building other than the usual personal communications devices (cell phones and tablets).

In a PAST RF configuration in my building, a yagi antenna fed by a 900 MHz Max Trac base station was moved to be co-located below the V2000A. The two coax feeds (to the scanner and to the Max Trac) were partially co-located.

Thinking that RF from the Max Trac was getting into to the scanners, I purchased a custom Stridsberg notch filter centered at the base station's TX frequency and installed that between the scanner and coax.

The filter made no difference and the attached scanner went deaf..

The company I work has since ended use of the Max Trac and I removed the base station and its antenna, resulting in the current configuration.

I made the assumption that decommissioning the Max Trac would solve my problems. Nope.

The only other possibility I can come up with is that my antenna is across the street form a series of auto dealerships have occasional radio station "live remote" broadcasts. Could it be the FM stations' remote link equipment?

At this point, I'm tired of sending scanners out for repair and would welcome any assistance.

 

[ofd8001]

Any chance of a static electricity spark occurring when you connect the antenna to scanner?

How about other types of RF energy, including lightning strikes nearby, infiltrating to the antenna/coax system? Do you have this system well grounded/bonded to dissipate any such RF energy?

 

[tfhphoto]

Thanks Bill,

The nearest cell site is about 8 city blocks away and the fire panel is hardwired.

 

[jonwienke]

+1

If there are no nearby transmitters, then the only other plausible culprit is lightning / static buildup due to inadequate or improper grounding.

 

[tfhphoto]

The antenna mast is attached to a metal structure that is purpose built to mount/support items (such as HVAC units) on the top of commercial rubber-roofed buildings. The antenna/mast is the only item mounted on this particular structure. The coax enters the building through a hole drilled (and subsequently grommeted) in the side of a vent duct. The coax itself was assembled by a local radio shop and I have no reason to suspect bad coax.

I was under the impression that the structure IS grounded and I have run a copper braid from the antenna mount to the structure, but have not run my own ground from the structure to a ground buss.

I do like the idea of double checking for proper ground.

 

[tfhphoto]

This may or may not matter as far as grounding, but since the last 396XT "went down," I put a Bendix-King EMH5992 (RX only) on this setup with no - knock on wood - issues.

I do understand there is a VAST difference between the front ends of a professional single-band radio and a scanner.

 

[jonwienke]

but have not run my own ground from the structure to a ground buss.

I do like the idea of double checking for proper ground.

Never make assumptions when it comes to that sort of thing.

 

[cellphone]

This may or may not matter as far as grounding, but since the last 396XT "went down," I put a Bendix-King EMH5992 (RX only) on this setup with no - knock on wood - issues.

I do understand there is a VAST difference between the front ends of a professional single-band radio and a scanner.

This leads me to believe that you have some RF interference that is overloading the front end of your 396XT.

 

[phask]

Beg, borrow or steal a spectrum analyser.

I'd check grounding as well - you need the coax grounded and lighting arrestor for code.

Make sure someone is not throwing a big static spark directly to the radio itself.

In the early PC days, I used to lose installs from static.

 

[Robinsmark]

RE: Antenna

The coax enters the building through a hole drilled (and subsequently grommeted) in the side of a vent duct. The coax itself was assembled by a local radio shop and I have no reason to suspect bad coax.

I would never assume that someone else installed the connectors properly. Always check it yourself for looseness, or poor solder connections. Disconnect both ends of the LMR400 and check with an ohm meter for short to center pin. If you read foil and or braid to center pin you have a problem. LMR 400 is not the easiest cable to work with. I hope you sealed the outdoor connection to keep moisture out.

 

[mm]

https://www.minicircuits.com/WebStore/dashboard.html?model=ZFLM-43-5W+


I had a similar problem with some sensitive ADS-B and XM weather radio receivers on a distributed feed at a airport installation.

The head end receivers LNA's were damaged due to close by avionics transmissions.

The above mini-circuits limiter fixed my issue, 60 dollars is not a lot to spend on insurance.

 

[tfhphoto]

I will check and add grounding to the coax and antenna.

I'm not questioning the coax because it's performing really well RX-wise while hooked up to the Bendix-King mobile. But, to be sure, I'll hook it up to an antenna analyzer. And, Coax Seal is my friend.

Again, I appreciate all the suggestions.

 

[tfhphoto]

I confess I'm not familiar with the limiters. It will cut off a very high strength signal, but allow lower strength signals to pass?

Thanks

 

[cellphone]

Beg, borrow or steal a spectrum analyser.

A $20 SDR works well as a poor man’s spectrum analyzer if you can’t get ahold of the real thing.

 

[pinballwiz86]

Perhaps you might want to stop attaching your scanners to the outdoor antenna. Use the stock antenna if you wish to stop sending in your scanners for repair.

I also have a 396XT. If you are experiencing drop outs, make sure Weather Alert is not on. Or Priority Scan. It will cause the radio to have "drop outs" while it checks the weather tone or the priority channels every 2 seconds.

 

[Ubbe]

Any chance of a static electricity spark occurring when you connect the antenna to scanner?

For some reason there's a capacitor in series with the coax that makes it possible to have a static buildup in the coax. The antenna is DC grounded. If the coax are left unconnected for a while it could get a static charge. Always connect to a terminating shortcircuit or resistance to prevent that from happening. That Mini-Circuit protection seems to be a good solution if there are high powered transmitters nearby.

EA4EOZ, an amateur radio electronic enthusiast: V2000 teardown

/Ubbe

 

[jonwienke]

Perhaps you might want to stop attaching your scanners to the outdoor antenna. Use the stock antenna if you wish to stop sending in your scanners for repair.

Not a feasible option inside a multistory building in most cases.

I also have a 396XT. If you are experiencing drop outs, make sure Weather Alert is not on. Or Priority Scan. It will cause the radio to have "drop outs" while it checks the weather tone or the priority channels every 2 seconds.

That has nothing to do with the OP's problem, which is something frying the front end of his radios, causing a permanent loss of reception until the radio is repaired.

Either there is a powerful transmitter nearby, or static electricity or stray voltage is frying the radio. Grounding needs to be checked, and also whether the coax runs parallel to any AC power cables, which would induce a current in the coax when power flows through the AC cables.

 

[Ubbe]

I wouldn't think AC power lines could be an issue. Thers's always two leads next to each other that have the current going in opposite directions cancelling out each others magnetic field?

/Ubbe

 

[jonwienke]

I wouldn't think AC power lines could be an issue. Thers's always two leads next to each other that have the current going in opposite directions cancelling out each others magnetic field?

Wrong. Most of the magnetic field is cancelled out, but not all, becase the wires are not in the same location, and are not twisted or shielded like RF feedlines. When the AC cables are carrying a heavy current, several volts can be induced in nearby wires. There is a reason why network, thermostat, and other low-voltage / low power wiring is not allowed in the same conduit as AC power wiring, and running them parallel in close proximity is always a bad idea.