MD Transportation Authority Troubles

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TinEar

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The Transportation Authority police have had trouble the past couple of days with at least two of their freqs - 453.1 (McHenry) and 453.575 (Port/Key Bridge with the PL 167.9). There has been a cycling tone on the freq blocking communications. I didn't hear the tone on the other users of 453.575 with different PL tones but I didn't hear any operational traffic either.

Radio maintenance is working on the problem today but can't seem to solve it - yet. Meanwhile, both those entities that have been displaced have moved to 453.475 and both are using a PL of 167.9 while there. I haven't heard Bay Bridge or Hatem Bridge yet so I don't know if they've also moved. This is obviously an unidentified channel they had in their radios. I haven't heard a reference to a channel number for this new freq up to this time.
 

ka3jjz

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Hmm, I wonder if this has anything to do with what I noticed yesterday on the Port frequency, where someone was evidently keying up a repeater and really hosing up the frequency with noise and junk? I reported it on a different thread.

Interesting, too, that they'd put all their ops in our area on a single frequency with the same PL. Some poor planning there, for sure.

73s Mike
 

TinEar

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Mike, that noise hitting the repeater yesterday was caused by something internal to their system. The radio techs were talking about it all morning on the 453.1 freq. They were trying to connect and disconnect certain things from the network to see if it would go away but couldn't find any of the obvious things as the cause. It's funny though that it would only affect the 453.575 freq on one of the three PL tones used there and not the other two.

[Edit] I just found your post from yesterday. It never did "light up" indicating a new post which is why I missed it.

{Edit2] Bay Bridge , PL 127.3, is still using 453.575 without problem.
 

ka3jjz

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I guess none of those guys are hams; we've had that same kind of problem on the BRATS 147.03 repeater on and off. In many cases, it turned out that one or more commercial user's transmitters were dirty (putting out spurs); another time it was one of those Radio Shack TV amplifiers that was coupling into the TV antenna and rebroadcasting hash up and down the spectrum. Of course, my first thought, given the weather we had yesterday, was skip, but I guess that's not quite the issue here.

I know from experience how difficult it can be to track down a source of interference in an urban area. There are so many possibilities.

73s Mike
 

ka3jjz

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A quick check this morning shows that the Port/Key Bridge and McHenry freqs are still down, and 453.475(167.9) is still being used

73s Mike
 

Metro174

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I was talking with an MDTA officer at CBIF tonight abouth their radio troubles. He said that management has advised them they have no clue how long this problem with their radios will be continuing. Apparently the radio techs think it is external interference. (I have no idea) and they have contacted the FCC. For the forseeable future, all MDTA Baltimore area radio traffic will be on this channel.
 

lawman349

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That explains alot...

On Wednesday MdTA Police at the Harry Nice Bridge in southern Charles County could be heard on CCSO Ch. 3. I thought maybe this had something to do with the new Charles County P25 digital system going online soon, but it makes sense that MdTA's system is down...
 

ka3jjz

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The most logical culprit for all the interference troubles would be the former 450 mhz City frequencies, many of which are now used by city services. 5 will get you 10 that one or more of those transmitters is out of alignment. When's the last time they were serviced? Probably quite a while ago.

73s Mike
 

TinEar

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The problem with that theory Mike is that the interference and tones that are keying the repeaters on 453.1 and 453.575 are doing so with the correct PL tone included. The 453.1 interference is using a 127.3 tone and the 453.575 interference is using a 167.9 tone. That's why the problem is confined on 453.575 to just the Port/Key Bridge portion and is not bothering Bay Bridge or Hatem traffic on the same freq. If my old records are correct, Baltimore's old 453 freqs used neither of those tones. The inclusion of the correct PL tones within the interfering signal(s) makes it look very much like it is a piece of MdTA's own equipment that is causing the problem.

I would start looking for a discarded MdTA radio or old console that is still drawing power from either a battery or direct electrical source and is shorting the mic connection or output section of the transmitter which, in turn, is keying the repeater. Just a wild ass guess on my part but there is some logic behind it based on the inclusion of PL tones in the interference.
 

ka3jjz

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TinEar said:
The problem with that theory Mike is that the interference and tones that are keying the repeaters on 453.1 and 453.575 are doing so with the correct PL tone included. The 453.1 interference is using a 127.3 tone and the 453.575 interference is using a 167.9 tone. That's why the problem is confined on 453.575 to just the Port/Key Bridge portion and is not bothering Bay Bridge or Hatem traffic on the same freq. If my old records are correct, Baltimore's old 453 freqs used neither of those tones. The inclusion of the correct PL tones within the interfering signal(s) makes it look very much like it is a piece of MdTA's own equipment that is causing the problem.

I would start looking for a discarded MdTA radio or old console that is still drawing power from either a battery or direct electrical source and is shorting the mic connection or output section of the transmitter which, in turn, is keying the repeater. Just a wild ass guess on my part but there is some logic behind it based on the inclusion of PL tones in the interference.

Yep, quite logical Mr.Spock :D Of course, that means that tracking this problem down gets a little easier. I guess the FCC Laurel office will be involved in tracking this one down. I wish I was a fly on the wall in that office - all those neat toys....73s Mike
 

TinEar

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Several years ago I had the opportunity to play in one of their signal tracking vans. I bet the drool marks I made are still on the floor of that van wherever it is.
 

aristotle73

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Insert Idiot here ---> me:

I understand what a repeater means and what keying up a repeater means, but what are PL tones and how do they play a part in keying up a repeater?
 

TinEar

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PL tones are discrete signals that are included in the signal sent by a transmitter that specifically identify an individual entity. (That's an overly broad description but it will suffice for now.) By including the PL tone in the transmission, a receiver can discriminate between signals coming from the intended transmitter and those from someone else using the same frequency. The tones come in two varieties - analog and digital. Sometimes they are called PL and DPL tones, other times they are called CTCSS and DCS tones. If your scanner is capable of distinguishing these tones, your manual will have a section that describes how to find them and then how to set them.

In this case, the frequency 453.575 is used by three distinct elements of the MdTA. They are the Port of Baltimore/Key Bridge detachment, the Bay Bridge detachment and the Hatem Bridge detachment. All of those units can use the same frequency and not interfere with each other.

Others in this area that duplicate frequencies are the Town Of Aberdeen and Greenbelt City, both using 453.80. There are also BWI Airport police and the City of Bladensburg using 453.90. They all use distinct PL tones to differentiate themselves. None interfere with the other. Actually, there is a long list of people using the same frequency as someone else in this general area but this is just an example of some that do.

In the case of the MdTA 453.575 frequency, I have it entered in my scanner three times, each with a different PL tone and each with its own Alpha tag to identify and label it correctly.

It all comes down to a way that multiple agencies can use the same frequency without interference. Before trunked systems came along, this was a very big problem since there were more units than there were available frequencies.

Finally, there is nothing at all dumb about asking the question or not knowing about PL tones. There was a time we all asked that question.
 

TinEar

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The interference on 453.1 and 453.575 seems to have disappeared. However, the two affected units - McHenry and Balt Port/Key Bridge - are still using that backup freq of 453.475 [PL 167.9]. Today, they are complaining of interference on that one from the Laurel Police Department. Laurel transmits on 453.4375 [PL 146.2] so that tells me the MdTA radios have terrible adjacent channel rejection performance. I wonder if they are using replacement radios rather than their normal radios during this frequency change period. Signals that far apart in both frequency and physical location, both operating with discrete tones, should not be interfering or bothering each other. But they are - at least on the MdTA end. I have not heard a word of complaint from Laurel.
 

ka3jjz

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TinEar said:
The interference on 453.1 and 453.575 seems to have disappeared. However, the two affected units - McHenry and Balt Port/Key Bridge - are still using that backup freq of 453.475 [PL 167.9]. Today, they are complaining of interference on that one from the Laurel Police Department. Laurel transmits on 453.4375 [PL 146.2] so that tells me the MdTA radios have terrible adjacent channel rejection performance. I wonder if they are using replacement radios rather than their normal radios during this frequency change period. Signals that far apart in both frequency and physical location, both operating with discrete tones, should not be interfering or bothering each other. But they are - at least on the MdTA end. I have not heard a word of complaint from Laurel.

Which leads me to think that the MdTA folks have not had the PL receive turned on, even though they're likely to be transmitting it. If they did, it's likely they wouldn't get quite as much slop from Laurel. 73s Mike
 

Dispatcher308

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Why hasn't the 453.475 frequency been put in the Database???? (I was told it was submitted.) If it is being used by MDTA. We know they use it and it could be used for other opertaions when they get the main channels back up and running. Even though we don't have a channel # doesn't mean we can't post it.
 

TinEar

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As of last night, the Baltimore Port/Key Bridge guys were back in operation on 453.575 but McHenry was still on the alternate .475 freq. Let's see if they go back to their original freq today.

[Edit] Nope...McHenry is still using 453.475 today.
 

ka3jjz

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Still the same yesterday, Al. .475 had McHenry on it; it almost looks as though it's a permanent change.

73s Mike
 
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