I have a high power pair of UHF frequencies operating on a tower. I am receiving traffic from another licensed user through our repeater when both our systems are keyed up. Our towers must have good line of site. We have different PL codes programmed into our system. In addition we are very limited in our area with attempting to obtain additional UHF frequencies. There is so much saturation in our area that other than 5 watt portable licenses we are out of luck. Even a 50 watt pair narrow banded is difficult to come by.
In light of all these problems why can't someone make a device maybe that goes in line before the receiver that can actively filter out unwanted RF on that frequency? Maybe the device uses the PL codes or some other means. Maybe there is already something out there that does this and I am just unaware. I do understand how PL codes work with transmitters and once you have an open carrier you will hear all RF on the frequency. Why couldn't there be some type of DSP that receives the radio transmissions and only allows the appropriate one to pass? That way the receiver will only receive and rebroadcast the desirable RF.
Obviously this would only be for repeaters. Mobiles and Portables could still be affected but in my situation we do not receive any interference on those, just the repeater when it is keyed up. Maybe there is just some RF theory that I do not understand. Any comments or thoughts on this subject would be appreciated.
Thanks,
Mark
Solution to RF availability??
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afterimage84
- Posts: 151
- Joined: Mon Apr 14, 2003 5:09 pm
Here is a bit more detail. I am involved with public safety (Fire and EMS) as well as planning long term communications for a regional dispatch center that dispatches fire and EMS. THe UHF frequency is a 90 Watt 180 Watt ERP license that is wide band. This license was obtained several years ago before the refarming. The other user is approximately 20 miles away. So mobile and portable traffic do not interfere. What we hear is their repeater when our repeater is keyed up. Also this is not an opertional frequency only administrative. While we would like to use this for an operational frequency due to this other co-channel user it would not be suitable. The other licensee is the state and they use if for public transportation. If there was some way to filter out their transmissions then we could use this frequency without any problems. If I could get something like this to work for our location I could implement this across our county as we attempt to build a UHF system. Our current attempts are being hampered by limited RF in the UHF spectrum. We are currently looking at T band frequencies becoming available as the local TV deadline to move to digital is nearing but that could be some time.
A filter based on the PL before the receiver would be no more affective than the PL squelch in the receiver. There is really no way to avoid interference from other stations on your frequency that have coverage into your receiver site.
PL, DPL, whatever, your receiver should be mute until the correct tone comes in. And a DSP solution owuld work no better as even if such an arrangement existed, it can't tell one carrier from the other any better than the PL you already have.
Perhaps you don't fully understand how your receiver, be it a mobile, portable or repeater, handles PL/DPL. Let me try to explain in sort of layman's terms.
Your receiver will hear everything on its frequency. Carrier squelch keeps the audio muted until a signal of sufficient strength comes in. That will unmute your receive audio. Obviously this would generally be unacceptable. With PL or DPL, the squelch keeps the audio muted until a carrier comes in with the correct tone. As long as the correct tone is present, the audio will unmute. When the tone goes away, the receiver audio mutes. This is good and bad.
The good obviously is that you will only hear desired signals. Your dispatcher will not be annoyed by other users, and your repeater will not repeat other frequency users. This is all good and is the intent. The bad is that if one of your units is transmitting, the receiver at the repeater (or at the base if not using a repeater) will unmute. If another signal from another system comes on and is stronger to your repeater than your unit, your repeater will no longer hear the correct PL and will mute. A non-radio savvy user will not know WHY your unit is no longer being heard; he'll just assume the transmission ended. All your receiver knows is that the desired PL went away so that means mute, and that's what happened.
With the "capture effect" of FM, the strongest signal totally blocks the other in a receiver. Only if both signals are nearly identical in strength will there be a heterodyne that will tip everyone off that there's an interfering signal. And the heterodyne between two FM carriers of nearly identical strength sounds like two horny cats in heat killing each other in an alley. In nearly all situations, your users will have no idea there is interference. All they'll know is the transmission ended and either reply or think (correctly but not understanding) something has gone wrong.
The only time a signal with an incorrect PL might get through your squelch would be during the hang time of the repeater during the squelch tail after the desired unit ended the transmission. I hear that a lot even though it shouldn't happen. That's a matter of proper PL set-up and usage in the receiver.
Hopefully I've helped you here. I'm not quite sure I understand your problem, but it sounds like it's covered by my explanation.
Good luck.
PL, DPL, whatever, your receiver should be mute until the correct tone comes in. And a DSP solution owuld work no better as even if such an arrangement existed, it can't tell one carrier from the other any better than the PL you already have.
Perhaps you don't fully understand how your receiver, be it a mobile, portable or repeater, handles PL/DPL. Let me try to explain in sort of layman's terms.
Your receiver will hear everything on its frequency. Carrier squelch keeps the audio muted until a signal of sufficient strength comes in. That will unmute your receive audio. Obviously this would generally be unacceptable. With PL or DPL, the squelch keeps the audio muted until a carrier comes in with the correct tone. As long as the correct tone is present, the audio will unmute. When the tone goes away, the receiver audio mutes. This is good and bad.
The good obviously is that you will only hear desired signals. Your dispatcher will not be annoyed by other users, and your repeater will not repeat other frequency users. This is all good and is the intent. The bad is that if one of your units is transmitting, the receiver at the repeater (or at the base if not using a repeater) will unmute. If another signal from another system comes on and is stronger to your repeater than your unit, your repeater will no longer hear the correct PL and will mute. A non-radio savvy user will not know WHY your unit is no longer being heard; he'll just assume the transmission ended. All your receiver knows is that the desired PL went away so that means mute, and that's what happened.
With the "capture effect" of FM, the strongest signal totally blocks the other in a receiver. Only if both signals are nearly identical in strength will there be a heterodyne that will tip everyone off that there's an interfering signal. And the heterodyne between two FM carriers of nearly identical strength sounds like two horny cats in heat killing each other in an alley. In nearly all situations, your users will have no idea there is interference. All they'll know is the transmission ended and either reply or think (correctly but not understanding) something has gone wrong.
The only time a signal with an incorrect PL might get through your squelch would be during the hang time of the repeater during the squelch tail after the desired unit ended the transmission. I hear that a lot even though it shouldn't happen. That's a matter of proper PL set-up and usage in the receiver.
Hopefully I've helped you here. I'm not quite sure I understand your problem, but it sounds like it's covered by my explanation.
Good luck.
Chris,
Hamming 31 years
http://www.wa2zdy.com
Wesley Chapel, Pasco County, Florida
Snow? What's that?!
The human race is proof that Darwin was wrong.
Hamming 31 years
http://www.wa2zdy.com
Wesley Chapel, Pasco County, Florida
Snow? What's that?!
The human race is proof that Darwin was wrong.
Two repeaters 20 miles apart? There is no solution for that one. However your mobiles should not be hearing their repeater unless the PL squelch in your radios is not properly set up. And your repeater should not be hearing the other system's units.
Either the PL squelch is not set correctly in your mobiles/portables or in your repeater. Or they're using the same PL you are using.
Other than that, the interference issue I described above is nearly inevitable. Two repeaters 20 miles apart - that's insane.
Either the PL squelch is not set correctly in your mobiles/portables or in your repeater. Or they're using the same PL you are using.
Other than that, the interference issue I described above is nearly inevitable. Two repeaters 20 miles apart - that's insane.
Chris,
Hamming 31 years
http://www.wa2zdy.com
Wesley Chapel, Pasco County, Florida
Snow? What's that?!
The human race is proof that Darwin was wrong.
Hamming 31 years
http://www.wa2zdy.com
Wesley Chapel, Pasco County, Florida
Snow? What's that?!
The human race is proof that Darwin was wrong.
Thanks for your explanation on the PL side of things I do appreciate it. However I actually have a pretty good understanding of how it works and why with existing technology I can hear the other system. Actually you bring up a good point that I did not think of earlier. We are only hearing the other use when we end our transmission and the repeater has not muted. There is a tail of a second or two where the repeater is unmuted after we end our transmission which we can hear the other user. When we are transmitting we do not hear them spashing over us. I guess our signal is so strong that it does not allow them to interfere. In retrospect I guess this is a pretty important piece of information that I neglected to include.
My idea is about something completely new. I know that I am able to see both discrete signals on a service monitor and detect each specific PL. What I am wondering is if we could have a device that could look at both those signals and litteraly stop one from entering the reciever. Maybe it is a computer based device that has software programed to evalute RF signals and based on some predetermined condition entered in by the user block the unwanted RF. Maybe like I said earlier that there is no way to separate out and filter unwanted RF on the same frequency. It was just an idea I had been thinking about. I was curious what other people might think. Imagine the possibilities if you could actually block RF on the same frequency. Maybe I am off in La La land but who knows.
My idea is about something completely new. I know that I am able to see both discrete signals on a service monitor and detect each specific PL. What I am wondering is if we could have a device that could look at both those signals and litteraly stop one from entering the reciever. Maybe it is a computer based device that has software programed to evalute RF signals and based on some predetermined condition entered in by the user block the unwanted RF. Maybe like I said earlier that there is no way to separate out and filter unwanted RF on the same frequency. It was just an idea I had been thinking about. I was curious what other people might think. Imagine the possibilities if you could actually block RF on the same frequency. Maybe I am off in La La land but who knows.
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jackhackett
- Posts: 1515
- Joined: Tue Jun 10, 2003 8:52 am
First, what you are seeing there is the 'capture effect', that wa2zdy was talking about.dittrimd wrote: We are only hearing the other use when we end our transmission and the repeater has not muted. There is a tail of a second or two where the repeater is unmuted after we end our transmission which we can hear the other user. When we are transmitting we do not hear them spashing over us. I guess our signal is so strong that it does not allow them to interfere.
My idea is about something completely new. I know that I am able to see both discrete signals on a service monitor and detect each specific PL. What I am wondering is if we could have a device that could look at both those signals and litteraly stop one from entering the reciever
Secondly, there's no practical way to filter out one signal on a frequency without filtering out every signal on that freq. I suppose maybe there's some way to do it by using the phase difference between the two, but in the real world, just forget it.
Two observations: if what you're reporting is that the other side's traffic is coming over your repeater during the tail, then what is happening is that your repeater's receiver IS hearing their mobiles. However, while your own mobiles are keyed, the capture effect blocks the other side's mobiles. That effect goes away when your mobiles unkey, and the repeater now hears and repeats the other side's mobiles until your hang timer causes it to shut down (for lack of the proper PL).
There are two solutions to this. One one be to shorten your repeater's tail. Another would be to cause it not to send tone during the tail. Both are pretty straightforward progamming options in most controllers.
If the hypothesis is that your mobiles are hearing the other side's repeater output, this would happen whenever and for however long your mobiles were off-hook (that is, in monitor mode), and it would not affect your portables. Sounds like this is not the case. The only solution would be for you to cause your mobiles not to drop tone when off hook, which only a few radios can be programmed for.
There are two solutions to this. One one be to shorten your repeater's tail. Another would be to cause it not to send tone during the tail. Both are pretty straightforward progamming options in most controllers.
If the hypothesis is that your mobiles are hearing the other side's repeater output, this would happen whenever and for however long your mobiles were off-hook (that is, in monitor mode), and it would not affect your portables. Sounds like this is not the case. The only solution would be for you to cause your mobiles not to drop tone when off hook, which only a few radios can be programmed for.
Thanks to everyone for you input. As far as us hearing their mobiles that may be possible but I am pretty sure they are operating a fixed base station on the receive frequency or input frequency to our repeater. This is based on thier license information. It is possible that we could be hearing their mobiles but due to the clarity of the voice transmission we are hearing it is more likely a higher powered base.
Will as for notifying anyone about interference I am reluctant since their license is valid and was issued right about the same time ours was. In addition the other license holder is the State of Connecticut and I really do not want to get into a dispute with them as we will probably lose. I would rather that we just keep using this channel as an administrative channel which it has served very well and investigate other alternatives.
We actually have an additional 5 UHF low powered pairs in our town that are being used by the police, fire and public works for cross banding to lowband. I am looking at building a simulcast trunking system that would make better use of our existing RF and adding two additional tower sites to improve coverage. The intension of this post was to just investigate the feasibility of blocking unwanted RF better than just using PL tones.
Thanks again for this lively discussion it has been very interesting.
Mark
Will as for notifying anyone about interference I am reluctant since their license is valid and was issued right about the same time ours was. In addition the other license holder is the State of Connecticut and I really do not want to get into a dispute with them as we will probably lose. I would rather that we just keep using this channel as an administrative channel which it has served very well and investigate other alternatives.
We actually have an additional 5 UHF low powered pairs in our town that are being used by the police, fire and public works for cross banding to lowband. I am looking at building a simulcast trunking system that would make better use of our existing RF and adding two additional tower sites to improve coverage. The intension of this post was to just investigate the feasibility of blocking unwanted RF better than just using PL tones.
Thanks again for this lively discussion it has been very interesting.
Mark
If we define "base" station as per the rules (FB1, FB2, etc.), and you are on UHF, then they should not be transmitting on a base on your repeater input frequency, as that is strictly and explicitly forbidden. They could be using a control station (FX1, a fixed site station that only communicates to a fixed site repeater), but if that is the case, then the rules require that they employ minimum power and directional antennas so as to avoid precisely this situation.
I say the foregoing because it sounded to me from your posts that the errant transmissions were coming through your repeater, i.e.,. were being sent and received over your input frequency and being repeated by your repeater only during the tail period after your own mobile dropped key. Again, if that is the case, and if you program your repeater to drop tone on tail, you shouldn't hear their tailgating transmissions.
I say the foregoing because it sounded to me from your posts that the errant transmissions were coming through your repeater, i.e.,. were being sent and received over your input frequency and being repeated by your repeater only during the tail period after your own mobile dropped key. Again, if that is the case, and if you program your repeater to drop tone on tail, you shouldn't hear their tailgating transmissions.
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CuriousGeorge
- Posts: 124
- Joined: Wed Oct 16, 2002 6:22 pm
Two alternatives to consider instead of a formal interferance report...
Contact the state agency in question directly and maybe you can work it out with them. Instead of guessing as to their operations, and them about yours, work out the co-channel usage and maybe you'll solve a problem for them at the same time.
Or, last I checked, the state level coordinator for American Association of State Highway and Transportation Officials (AASHTO) works for CT DOT. We licensed a frequency that AASHTO coordinates for an EMS radio system and the gentleman from CT DOT was very helpful. Much more so than our local APCO person. (We're not in CT though so maybe your local contact is more responsive.)
On the other side of the coin, you don't have it that bad compared to some. A local PD has the same frequencies licensed as a large metro transit system about 25 miles away. And, they both use the same PL. So, early in the morning and depending on weather conditions, you can wake up to hear the transit mobiles and portables making it through the repeater in town. And neither agency wants to (can??) change PL tones, despite having this problem for years.
Contact the state agency in question directly and maybe you can work it out with them. Instead of guessing as to their operations, and them about yours, work out the co-channel usage and maybe you'll solve a problem for them at the same time.
Or, last I checked, the state level coordinator for American Association of State Highway and Transportation Officials (AASHTO) works for CT DOT. We licensed a frequency that AASHTO coordinates for an EMS radio system and the gentleman from CT DOT was very helpful. Much more so than our local APCO person. (We're not in CT though so maybe your local contact is more responsive.)
On the other side of the coin, you don't have it that bad compared to some. A local PD has the same frequencies licensed as a large metro transit system about 25 miles away. And, they both use the same PL. So, early in the morning and depending on weather conditions, you can wake up to hear the transit mobiles and portables making it through the repeater in town. And neither agency wants to (can??) change PL tones, despite having this problem for years.
Without exact information it is a bit difficult to guess correctly what is going on. What puzzles me is that you hear the other party coming through your repeater on the tail only and for the duration of the tail.
If the repeater is setup correctly, it will not pass any audio that doesn’t have the correct PL. As soon as your mobiles dekey, the repeater should close its audio path because it is no longer receiving correct PL. It should transmit the 2-second tail without audio unless a valid PL signal is received again during this time.
An exception to this is a timer in the repeater controller that allows audio to pass without PL for a short time. Typically this timer is set around 200mSec. It prevents the audio from closing when the RX signal goes down and up in weak signal areas.
My guess is that the audio gate in the repeater controller is passing audio all the time regardless of PL while repeater keyup is controled by PL. This either is because of a defect, incorrect setup or wiring.
If this repeater is a MSF5000 repeater, you should look at the different qualifiers for repeater keyup and repeat audio.
Nand.
If the repeater is setup correctly, it will not pass any audio that doesn’t have the correct PL. As soon as your mobiles dekey, the repeater should close its audio path because it is no longer receiving correct PL. It should transmit the 2-second tail without audio unless a valid PL signal is received again during this time.
An exception to this is a timer in the repeater controller that allows audio to pass without PL for a short time. Typically this timer is set around 200mSec. It prevents the audio from closing when the RX signal goes down and up in weak signal areas.
My guess is that the audio gate in the repeater controller is passing audio all the time regardless of PL while repeater keyup is controled by PL. This either is because of a defect, incorrect setup or wiring.
If this repeater is a MSF5000 repeater, you should look at the different qualifiers for repeater keyup and repeat audio.
Nand.
Seems the other system is NOT operating per their technicnal parms. on their FCC license.
The other co-channel repeater system is too short spaced given all the info.
Short spaced? TO dammed close, 24 miles and 100 watts ERP from a hill top..... blame the dxxxxx so-called frequency coorrdenators.
The other co-channel repeater system is too short spaced given all the info.
Short spaced? TO dammed close, 24 miles and 100 watts ERP from a hill top..... blame the dxxxxx so-called frequency coorrdenators.
Last edited by Will on Tue Nov 15, 2005 3:28 pm, edited 1 time in total.
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thebigphish
- Posts: 1477
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- What radios do you own?: AM/FM