LMR 400 antenna line too long?

[SVFC462]

Looking for some assistance from you all.

There was an old GPS antenna with LMR 400 line going from the roof to the radio equipment room at my work. the Boss said i could change it over for a scanner antenna. Fast forward - Diamond Discone D130NJ has been installed, and Uniden BCD996P2 scanner has been programmed and is up and running.

ISSUES - only hearing some transmissions (UHF, VHF) that are close by. Cant hear any of the trunk systems (700/800) near by. The antenna line is LMR400 (dont know how old it is) and is probably over 100ft from scanner to antenna. I installed a Pre-amp into the antenna line and it barely helps.

I will update this soon with some pictures. I am bringing a multi meter with me tomorrow.

thanks in advance.

 

[737mech]

Check connectors again, think about adding fm trap. My guess it's a bad connector. Use volt meter to check shorts to shielding?

 

[mmckenna]

At 100 feet long, LMR-400 will have 1.5dB of loss at 154MHz. Not bad at all.
At 460MHz, 2.7dB of loss, a bit less than 1/2 your signal lost in the cable. Not really a big deal.
At 855MHz, 3.7dB of loss. Not great ,but a bit over 1/2 your signal lost.

None of those are really a big deal if you have a decent signal outside. One way you can test this is to take your scanner on the roof and connect directly to the antenna and see if you get a strong signal.

I agree with 737, I'd suspect a bad connector. Check everything well, any corrosion at all will be an issue and might indicate water intrusion into the cable jacket. Don't assume that the person who installed the connector originally did a proper job.
Also, check to see if there is a lightning suppressor where the cable enters the building (there should be), bypass that temporarily to see if it's the issue. Inspect the connectors that are attached to that.
Check the full length of the cable if you can access it. Wouldn't be heard of for someone to cut it.

Sounds like this may have been for a Network Time Protocol system (NTP) at one time. GPS receivers (would have been inside the building) send 5 volts DC up the cable to the antenna. The GPS antenna uses the 5 volts DC to run an internal amplifier to boost the signal before sending it back down to the NTP box. I've got a few of these at work. On shorter runs, I have used LMR-400 and LMR-600. On some longer runs, I've used 1/2" heliax. A bit of cable loss isn't a big deal for these systems. But it still should work fine for what you are doing.

 

[n5ims]

A few other things to think about and check out.

Was there some kind of filter on the old cable to filter out some strong signals that wasn't noticed and is now filtering out your 700/800 MHz signals? Don't rule out other things like the power injector or old amps (just a few examples) that may be blocking the signals. As others have said, also check the entire run of cable (as much as possible anyway) to verify that nothing unexpected exists (bad splices, damaged cable, etc.). I've seen coax that was cut and repaired using wire nuts instead of coax splices because the electrician that damaged the cable knew power wires not coax.

Are the agencies you're having issues with running simulcast systems that are giving you simulcast distortion due to the strong(er) signals with the new antenna? Your new discone will pick up signals from all areas and could easily make it so you're mixing the signals from multiple sites and killing the signal by one site cancelling out another site leaving you with little to no signal.

 

[cmdrwill]

Diamond Discone D130NJ has been installed,
ISSUES - only hearing some transmissions (UHF, VHF) that are close by. Cant hear any of the trunk systems (700/800) near by.

Discones and other so called discones do not work at 700/800. Try a real 700/800 antenna.

 

[SVFC462]

Discones and other so called discones do not work at 700/800. Try a real 700/800 antenna.

The same antenna worked perfect at my parents house .. CT State Police and MA State Police sounded fantastic.


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[Ubbe]

If you added a preamp and it didn't made things worse, it actually improved reception a little, then it can't be an overload issue that de-sense the scanner. Must be something with the antenna connectors. If the GPS where used for time sync then it could have set signals at a 10Mhz frequency and that works even if the coax and connectors are bad. But if it sent the 1.5GHz signal down the cable then it must be perfect coax for it to work.

With the multimeter you should measure mega ohm to the antenna and then make a short circuit at the scanner end and go up to the antenna and measure that it is a short of maybe 2-3 ohm. You can measure that between the top element and bottom element of the discone.

/Ubbe

 

[Peerlessk]

Is use a comet ds-150 antenna , lmr-240f ,125 feet of it 45 of it used . I could care less what I read about discones not working . I get Albany and Buffalo 700-900 fantastic coverage . Covers the whole scanning spectrum here in Lima NY . Must be a magician standing on the roof top along side my antenna

 

[DeepBlue]

Discones and other so called discones do not work at 700/800. Try a real 700/800 antenna.

Discone antennas work perfectly fine up to 3gHz (model and build quality depending). Used all the time by a great many agencies for monitoring. I follow the others in stating to check the connectors especially the outside one at the antenna. If it wiggles at all, there may be shielding disconnected if you can twist the base of the connector. tarnish on the center pin will show if moisture has gotten in the cable/connector but is nothing to totally freak out about however, I would change the connector. The old chrome connectors were horrible. Also look for chafing where the LMR may have come into contact with the roof line or sharp bends or may have been pulled through tight walls, etc. If you are going to use a pre-amp, go with one for the freq range you will use, NOT a broad band preamp. Unless you know what you are doing with additional filtering they are more hassle than they are worth, AND they should be placed at the antenna end of the feed line. A good AM and FM "trap" or braodcast band filter never hurt anyone, unless you are looking to receive the low end of the Air band.

Good luck. Your discone is a fine antenna. I have one on my system at home as well.

S

 

[radioman2001]

Make sure you have removed anything being used for the GPS antenna, some GPS antenna are an active antenna that runs power up the coax, and that needs to be gone even if unplugged. Comparing one location to another is useless, elevation is King with buildings and other structures affecting reception. I have had good luck using a TV amps, but with less than 100 ft of coax I would look somewhere else for the problem.

 

[SVFC462]

I added a picture below to show the building and surrounding structures.

Red (antenna)
Yellow (antenna line; LMR 400)
Blue (cell tower in back of property)

The facility sits but off the river and only at about 190ish feet for elevation (per the maps)


I am most likely going to go with the fact that it is the connectors. I will change them out and see if that helps.

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[Ubbe]

That cell tower are awfully close and judging by its shadow it has a high number of antennas. Anyone of those could have 10dB gain and several hundred watts of power in your direction. Are you sure that signal quality improved when using the amplifier?

Simple test, add the attenuator feature on signals that seem weak. If you notice no change in signal strenght or it increases then you have overload issues.
Only if the coax where not secured and moved around in the wind and it wasn't water proofed should I suspect the connectors. If that ohm meter measurement where fine I wouldn't mess with the connectors.

/Ubbe

 

[mmckenna]

The cellular base station is likely causing you some issues on 700 and 800MHz, but unlikely it's causing the issues on VHF. It's likely you have more than one issue to deal with. Connectors would be the first place to look. Getting that fixed should resolve VHF coverage issues. If 700/800 still is a problem, you might need to look at filtering out the 700MHz LTE band.

Any other transmitting antennas on the roof of the building that could be causing issues?

 

[krokus]

Have you gone to the roof, with a handheld scanner, to see if you can detect the wanted signals?

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