(@&$%*#)@ Paging Overload!!!!

[nmfire10]

I have a feeling there is nothing I will be able to do about this, but I figure I will toss the problem out anyway.

Where we have our town's EOC, there is a tower with paging transmitters within about 2/3 of a mile. We have our VHF antenna on our tower out back as well. Both that one and any portables, mobiles, or roof-top antennas (all also VHF) there get totally blasted by this paging. Yea, we have a PL but that doesn't help when you are recieving a legit signal and it is getting blanketed over by by the paging. We also have a ham radio there without a PL decoder and that is totally messed up by it too.

Now, is there anything that we can do about it? I would love to jsut shut the paging down but that won't happen. How about filtering? ANYTHING??

Matt

[Pj]

I take it that's the tower that's owned by the cell company? If so, I would have the tower owner (wouldn't be american tower by any chance?) or the radio owner to correct the problem.

We have a tower at our building owned by American Tower, and we have no problems with all the other antennas that have sprung up there (Sprint, NEXTEL, AT&T and I believe Verizon). All our channels except three are VHF. Also, all our antenna's are on the very top of the tower.. Where are yours?

[ARZ902]

If you can't get them to check their transmitter,
try a suck out filter in your receive line at their transmit frequency.

Dennis

[radioconsult]

Intermod is a often mis-used term. First you need to determine what the exact frequency of the paging transmitter is. It may be LB, VHF, UHF or 931 MHz. Then work backwards from your most interfered with receiver and determine if there is a numeric mix, i.e. 2A-B, etc. IM is a mixture of several discrete RF frequencies mixing and generating additional frequencies that are the sum and difference of the mix. This mix can happen in the PA state of a transmitter, or in the front end of a receiver or it can be caused by such things as rusted hardware on the radio tower. Sounds like you may have receiver fundimental overload rather than IM. A internet search can sometimes turn up a simple computer program that will run all the math combinations for a particular IM problem. Year ago, before pc's Decibel Products published a handbook with hard tables for determining IM solutions. A quick test for fundimental overload would be to put an in-line attenuator about 10DB in the antenna line of the receiver, if the interference go away or is reduced, then you need to look for RX overload. Good topic, keep us posted on your findings.

[nmfire10]

They are on a DIFFERENT tower than the one we are on. They are about 2/3 of a mile away. Perhaps I did mis0use the term intermod. I think based on the distance (really close) and frequency (really close), it is the overload thing.


Per the FCC Database:

Service: Verizon Wireless Messaging
Call Sign: WNDF394
Frequency: 152.48
Class: FB6C
ERP: 1000 Watts

Is this the kind of "harmful interference" that the FCC says they have to do something about? If it is, how do I go about having something done about it?

[RKG]

What is your freq?

What kind of equipment are you using?

The first thing to do is to set up a spectrum analyzer at your receiver site, listen for the paging xmissions, and see if they are emitting spikes, and if so, where. Assuming not, calculate the off-frequency signal strength of their freq at your freq, and then consult the manual for your equipment for sensitivity information. Assuming that they are within technical limits but because of their power and the closeness of their freq to yours, consider investing in a good filter for your receive antenna feedline.

[nmfire10]

Them: 152.480

Us: 153.800 simplex & 155.295 out / 153.77 in repeater.


I guess I will have our radio service place do the spectrum thing since I don't have one of them little toys.

[RKG]

1. That is a good idea. You really cannot complain about someone else causing harmful interference without first acquiring evidence that they are operating out of spec. However, harmful spikes on paging transmitters are not uncommon, in part because of their high power and in part because they tend not to be regularly PM'd.

2. Are you broadcasting high power simplex on 153.800 from the same site as a repeater with an input of 153.770? If so, I'd expect the simplex transmitter to open up the repeater's receiver.

[nmfire10]

Yep. Same tower with the simplex 153.800. It doesn't open the repeater though. I have never tried to use the repeater at the same time as the simple 153.00 though. It is a new repeater (within the last 6 months). The 153.800 simplex is not used a lot so it may not have even been a thought when the put that repeater in. Either that or they went cheap and are using the same antenna as the 153.800 for the recieve on the repeater.

The ERP of the 153.8 simplex? I have no idea. The FCC license doesn't indicate that, I will have to check the radio service guys to find out. I would imagine it is at least 50 watts though, if not more.

[mhaynes41]

Intermod is a often mis-used term. First you need to determine what the exact frequency of the paging transmitter is. It may be LB, VHF, UHF or 931 MHz. Then work backwards from your most interfered with receiver and determine if there is a numeric mix, i.e. 2A-B, etc. IM is a mixture of several discrete RF frequencies mixing and generating additional frequencies that are the sum and difference of the mix. This mix can happen in the PA state of a transmitter, or in the front end of a receiver or it can be caused by such things as rusted hardware on the radio tower. Sounds like you may have receiver fundimental overload rather than IM. A internet search can sometimes turn up a simple computer program that will run all the math combinations for a particular IM problem. Year ago, before pc's Decibel Products published a handbook with hard tables for determining IM solutions. A quick test for fundimental overload would be to put an in-line attenuator about 10DB in the antenna line of the receiver, if the interference go away or is reduced, then you need to look for RX overload. Good topic, keep us posted on your findings.

Yep I agree most if not all intermod is caused by two thing a mix of two or more frequency either in the PA decks or at the antenna. Also most recivers today use single conversion instead of the old double conversion receiver that due to cost. Most intermod happens in the VHF range between 150 to 153 mhz. Also another high power transmiter a few miles away like a FM radio station can cause the same afect. What happens is say you take a 150 mhz freg and say a 152 mhz freg and they mix you can at times get a freg any where in the band sometimes more then one. Also today most receivers have a wide front end that's so they can do a spreed from 146-162. So everything comes in the receivers front end. When it comes down to it there is not much you can do about it unless someone can be traced to having a drity transmiter and that is very hard to do most are really clean.

[April]

I would try to contact the Verison radio shop responsible for the maintce on that paging transmitter and ask for their help to reduce the interference.

[N4SZS]

As a field tech for a national paging company, I would agree with contacting the owner of the offending station and ask for their assistance in correcting the problem. Most major companys are shy of any bad press concerning paging interferance and will work very hard to correct a condition. The statement you can't fix-it if you do not know that it is broke really applies. The local maintenance tech most likley does not know they have a issue. MOST major companies have invested a lot of money into their systems and follow a rigorious preventive/corrective maintenance policy. With the downturn in the paging industry, companies avoid any bad press like the plague. Give them a call and involve your local radio maintenance company as well. Chances are that they will pay the costs to reduce or eliminate the problem. Good luck.

[Jim202]

Depending on what the paging company transmitter is, the problems could be made worse by which transmitter they are using. Bear in mind, that all the new paging transmitters are of the solid state PA variety. They older tube PA units were not as prone to the wide band noise these all solid state transmitters generate.

Having worked for a paging company for about 10 years, I have seen this kind of issue before. It is not always the paging transmitter that causes the problems.

One of the first things that I did, was to install a pass cavity on the output of the paging transmitter. This will clean up the pass band signature of the paging transmitter. In short, it will reduce the sidband white noise that all transmitter generate. Because the newer paging transmitters are all solid state, there is no tuned circuit on the input or output. This causes any low level noise and spurs to be amplified and put out on the antenna.

Another trick that can be used and was already suggested by someone, is to put a notch filter on your receiver input. Problems here are that the paging frequency is real close to your operating receiver frequency. You may actually cause the receive signal to also be reduced. A notch filter only can be made just so sharp. It too has a pass responce that could reduce the signal you want. Inserting it into your receive coax line will introduce signal losses.

You could have just plain front end overload in your receiver. Not much you can do about this except find a better, sharper passband radio receiver.

You might also look at the cables your using. Now comes the break of the boys from the men in installing coax cables. I always like to see heliax or double shielded coax used. You might also look at using the solid outer jacketed "Heliax" style coax that doesn't use braided wires for the shield.

As for your simplex operation, with the frequencies you have given here, there is no way your operating without seeing some degration of the repeater input signal. You probably have just never noticed it with a weak input signal to the repeater and keyed up the simplex transmitter at the same time.

Try to locate someone from the paging company to talk with. Contact the local sales office for them and ask how to get in contact with someone from the engineering department. They might not have the correct phone number, but if you keep asking around, someone will be able to point you in the right direction.

Jim

[Will]

I am going to visit a Verizon radio tech tomarow, and will ask him for a contact number that I can post.