We run a low band repeater Xmit on 48.XX Receive on 44.XX, Today I noticed that when two trucks are parked near each other one will transmit and the other cannot receive the transmission, but yet the two vehicles about a half a block apart function normally.
Why is this? The Transmit & Receive Freqs are just about 4MHz apart, could the lower freq from the transmitting portable be affecting the other mobile from receiving the higher receive freq?
I have never seen anything like this before, then again I never worked on a LB repeated system before either.
Your input is appreciated.
Vince
I am soooo confused
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raymond345
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This is very normal.How much power are u using?
The radio rx is getting to-much power and it does not work.I worked on a group of radios with 25 watt outputs with the same trouble.
the vech. were up to 40 feet to each other. I turned the output down to 10 watts no more trouble.
Raymond [email protected]
The radio rx is getting to-much power and it does not work.I worked on a group of radios with 25 watt outputs with the same trouble.
the vech. were up to 40 feet to each other. I turned the output down to 10 watts no more trouble.
Raymond [email protected]
You are not alone! When I first started dealing with low-band about 5 years ago, it didn't seem to follow any sort of rules. Turns out it does, but it's a set all to itself. What you're experiencing is absolutely normal, 4MHz offsets mean very little to a receiver in low-band. If you're used to dealing with regular VHF & UHF bands, you're in for some culture shock. Some of the things I've learned over time:
Do not try to tune a low-band mobile antenna inside your install bay, the VSWR will never appear good enough. I find you have to be at least 30 feet away, in the middle of the parking lot. Even then, if a large truck drives by on the street, you can watch the VSWR meter move up & down as it passes! Craziness!
Do not cut it to the length recommended for your particular frequency in any cutting charts! Start about 4 inches too long & work your way down in 1/4 inch increments. Every antenna on every vehicle seems slightly different. Some vehicles will simply not provide a large enough ground plane to work properly at all.
Do not let the metal hang-up clip on the back of the microphone touch your palm when you're checking for VSWR, if it's very high (like in an open or shorted cable, or simply a mis-cut antenna), you'll get a nasty stinging sensation in your hand, similar to touching an electric fence! At least, this happens using M208/216's, I don't know about other models. Give it a try if it sounds crazy, you'll soon be a believer.
Get used to people complaining that FM radio stations are coming through loud & clear in certain spots. The "higher" low-band frequencies are 1/2 harmonics of these stations.
Get used to atmospheric conditions playing havoc with low-band coverage, especially on a repeater. I've seen systems go in the toilet for a couple of days. All the hardware checks out beautifully with the service monitor, but still it sucks out in the field. The next day, everything's perfect. Makes me want to quit RF altogether.
There may be others on this board who are far more proficient at it than me, but if you have to change a repeater frequency, especially if the offset changes, get the duplexer retuned by the factory. It'll save you a lot of headaches. I can do the other bands no problem, but can never eliminate the de-sense in low-band.
Hope some of this helps, I've ranted long enough!
Todd
Do not try to tune a low-band mobile antenna inside your install bay, the VSWR will never appear good enough. I find you have to be at least 30 feet away, in the middle of the parking lot. Even then, if a large truck drives by on the street, you can watch the VSWR meter move up & down as it passes! Craziness!
Do not cut it to the length recommended for your particular frequency in any cutting charts! Start about 4 inches too long & work your way down in 1/4 inch increments. Every antenna on every vehicle seems slightly different. Some vehicles will simply not provide a large enough ground plane to work properly at all.
Do not let the metal hang-up clip on the back of the microphone touch your palm when you're checking for VSWR, if it's very high (like in an open or shorted cable, or simply a mis-cut antenna), you'll get a nasty stinging sensation in your hand, similar to touching an electric fence! At least, this happens using M208/216's, I don't know about other models. Give it a try if it sounds crazy, you'll soon be a believer.
Get used to people complaining that FM radio stations are coming through loud & clear in certain spots. The "higher" low-band frequencies are 1/2 harmonics of these stations.
Get used to atmospheric conditions playing havoc with low-band coverage, especially on a repeater. I've seen systems go in the toilet for a couple of days. All the hardware checks out beautifully with the service monitor, but still it sucks out in the field. The next day, everything's perfect. Makes me want to quit RF altogether.
There may be others on this board who are far more proficient at it than me, but if you have to change a repeater frequency, especially if the offset changes, get the duplexer retuned by the factory. It'll save you a lot of headaches. I can do the other bands no problem, but can never eliminate the de-sense in low-band.
Hope some of this helps, I've ranted long enough!
Todd
Ok, So Now I'm Really Really Confused.
I tried to transmit on a mobile with a LB MT1000 in one hand and the transmission came through on the HT with no problems. I expected that the 100 watt maratrac would squash the MT1000 and nothing would come through on it lik it dose from truck to truck like I stated earlier. The only difference I can see in the mobile setups is that the ones we have the most trouble with are the trucks with the 1/4 wave ball & whip antennas. On newer installations we started using the base loaded NMO antennas instead of the monster whips.
I wonder if this could be the cause of all our problems.
I tried to transmit on a mobile with a LB MT1000 in one hand and the transmission came through on the HT with no problems. I expected that the 100 watt maratrac would squash the MT1000 and nothing would come through on it lik it dose from truck to truck like I stated earlier. The only difference I can see in the mobile setups is that the ones we have the most trouble with are the trucks with the 1/4 wave ball & whip antennas. On newer installations we started using the base loaded NMO antennas instead of the monster whips.
I wonder if this could be the cause of all our problems.
One point that you might want to look at is the condition of the spring. If the spring has been in service for any period of time, you need to check it. In the better made springs, there is a braided wire that shorts out the spring. This wire is put there to make it behave RF wise.
As the spring gets old, the wire will begin to rot out. If you are in salt country, the winter road salt will eat the braided wire out in about a year or so.
To check the spring, bend the spring over and look at the wire. It is easy to see the condition of the wire.
The same thing happens on the small springs that some radio shops use to put on the base loaded ASP coils. These cause just as much transmitter problems. Only they show up much more easily. Put your watt meter on the cable. Take a piece of wood and bend the antenna over a little and look for a jump in the refected power. Sometimes just causing the spring to move will show the problem.
Jim
As the spring gets old, the wire will begin to rot out. If you are in salt country, the winter road salt will eat the braided wire out in about a year or so.
To check the spring, bend the spring over and look at the wire. It is easy to see the condition of the wire.
The same thing happens on the small springs that some radio shops use to put on the base loaded ASP coils. These cause just as much transmitter problems. Only they show up much more easily. Put your watt meter on the cable. Take a piece of wood and bend the antenna over a little and look for a jump in the refected power. Sometimes just causing the spring to move will show the problem.
Jim
The pheonomimn (I know I spelled it wrong!)is called Front End Overload. The newer receivers are real broadbanded which does not help. The receiver front end is getting so much energy from the closeby Low Band transmitter that the energy converts to a large voltage accross the first RF amplifier stage upseting the device's bias and it quits amplifing the smaller signal from the repeater's output.
Also there can be enough energy from another transmitter, even a broadcast/TV, close by to cause the transmit/receive diode switch to flip to transmit, killing the siganl going to the receiver. I have had the same problem when arriving at a heavealy RF populated mountain top with a lot of transmitters' rf getting into my receivers front end and blocking the receiver or much worse having to listen to all the intermod being generated in my own receiver's RF amp. Some radio receivers do it more than others. This is what seperates the real receivers from the poor ones!!!! The ability to reject high level signals.
<font size=-1>[ This Message was edited by: Will on 2001-12-14 01:05 ]</font>
Also there can be enough energy from another transmitter, even a broadcast/TV, close by to cause the transmit/receive diode switch to flip to transmit, killing the siganl going to the receiver. I have had the same problem when arriving at a heavealy RF populated mountain top with a lot of transmitters' rf getting into my receivers front end and blocking the receiver or much worse having to listen to all the intermod being generated in my own receiver's RF amp. Some radio receivers do it more than others. This is what seperates the real receivers from the poor ones!!!! The ability to reject high level signals.
<font size=-1>[ This Message was edited by: Will on 2001-12-14 01:05 ]</font>
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Glen W Christen
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Also, be aware that the sometimes the length of the coax can upset your VWSR. I have had to shorten it to get an acceptable reading.
The strangest call that I ever had with LB was "When I key the radio, the dash starts to smoke." It did.
Solution: The glass in the windshield of the old Chevy with in-glass radio antenna had broken out at the connector. When keyed, 100 watts of RF made the wire a heating element and caused the rubber gasket to get hot enough to smoke.
The strangest call that I ever had with LB was "When I key the radio, the dash starts to smoke." It did.
Solution: The glass in the windshield of the old Chevy with in-glass radio antenna had broken out at the connector. When keyed, 100 watts of RF made the wire a heating element and caused the rubber gasket to get hot enough to smoke.
One of the worst questions I get from repeater customers is "I'm right next to the other truck and can't talk to it,there must be something wrong!"
I try to explain their repeater and mt top site with all the co-located transmitters and receivers working at the same time, how there are many expensive filters and schemes to make them all work together and a little about desense and such. Then I explain the truck radios are pretty much naked and the rx radio is getting overwhelmed and this is normal. Their next question is "Why can't I talk to the truck sitting right to me?"
I give up. I try not to get myself on a hook anymore trying to explain.
I would not give your problem any more thought and move the trucks farther apart!
I try to explain their repeater and mt top site with all the co-located transmitters and receivers working at the same time, how there are many expensive filters and schemes to make them all work together and a little about desense and such. Then I explain the truck radios are pretty much naked and the rx radio is getting overwhelmed and this is normal. Their next question is "Why can't I talk to the truck sitting right to me?"
I give up. I try not to get myself on a hook anymore trying to explain.
I would not give your problem any more thought and move the trucks farther apart!
I Sincerely Hope you customer base is not in the Public Safety Sector,I am quite sure that they would not go for the response you give.On 2001-12-15 11:58, 57Shasta wrote:
One of the worst questions I get from repeater customers is "I'm right next to the other truck and can't talk to it,there must be something wrong!"
I try to explain their repeater and mt top site with all the co-located transmitters and receivers working at the same time, how there are many expensive filters and schemes to make them all work together and a little about desense and such. Then I explain the truck radios are pretty much naked and the rx radio is getting overwhelmed and this is normal. Their next question is "Why can't I talk to the truck sitting right to me?"
I give up. I try not to get myself on a hook anymore trying to explain.
I would not give your problem any more thought and move the trucks farther apart!
The problem as was brought up earlier is the Ball & Whip Antenna, The braid in the springs are coroded. We replaces a few antennas & the problem is all gone.
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powerlineman
- Posts: 14
- Joined: Sat Dec 01, 2001 4:00 pm
I am a lineman for a rural electric co-op and we have a low-band repeater. We have the same problem but all of our employees are aware of what causes it and we deal with it. Everyone understands talkaround. Our office control station runs 110 watts (for some reason?) with a 3 element yagi to talk to the repeater. If you are sitting in a truck in the path of the yagi, the moble receiver is wiped out 2-3 blocks away. You are ok if you are in the parking lot under the antenna. Desense can be detected 8-10 blocks away in certain spots.
I used to like LB but now I thinks it sucks, especially with portables. Our co-op thinks that low band is the only way to go....I don't think it has any advantages over high-band
I used to like LB but now I thinks it sucks, especially with portables. Our co-op thinks that low band is the only way to go....I don't think it has any advantages over high-band
Low band has many advantages over the higher bands as far ad mobile to mobile distance it is also the way to go since a lot of companies are getting off thus leaving the channels clear. As for portables I couldnt agree with you more the coverage sucks, you have to install a miriad of remote receivers, voters etc..... We use portables but as extenders of our mobiles (X band repeaters).On 2001-12-16 12:48, powerlineman wrote:
I used to like LB but now I thinks it sucks, especially with portables. Our co-op thinks that low band is the only way to go....I don't think it has any advantages over high-band
The repeater has been up for years without a hitch and I am not about to tell my company that we should go trunked. We have a few UHF repeaters on line as well and the LB blows them away with the mobile to mobile coverage any day.
<font size=-1>[ This Message was edited by: vcaruso on 2001-12-16 13:48 ]</font>
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/\/\y 2 cents
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Hey Idiots.....you are overloading the front end of the reciever that is sitting right next to you......if the radios are at +5 MHZ(or less if lowband) apart they tend to interfere when you are putting out like 25 or 45 watts....try TXing on a handheld next to the truck and then you may be able to hear something....Notice why most public saftey never have a problem with this is because Most of them are 800mhz and are 20 Mhz apart and also they TX at 3 watts on 800 and they usually dont have mobiles cause those nifty techs and sales guys at Motorola Spec enough recieve satellites to let tehm get out pretty much anywhere on a portable. This is not 100% true but at least in most of the new 800 Motorola systems that cover 1 city or county it is.
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/\/\y 2 cents
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oh yeah...I re read the post to be sure..if you are using 100 or 110 watt radios on lowband..you are definitely overloading the front end of the reciever.......quit doing it or you can screw up the radios..try programming them so that they have an option button you can program to so they put out like 10 or 12 watts when close and 110 when out and about...I know this may sound like a pain in the ass fot the customer, but it is needed unless they want to ruin the recieve side of the radio....either that, or switch to nextel...im sure they would rather hit the hi/lo power button and save a few thousand a month.
YO ! 2 cents The standard spacing for
800 mhz is 45 mhz not 20 and the vast majority of mobiles in public safety or
business are at least 12 watts more likely the 25-35 watt models. I understand the passion in the post but think about what you put there. You are absoutely correct though
about simple front end overload. That is exactly what is happening. Phrawg
800 mhz is 45 mhz not 20 and the vast majority of mobiles in public safety or
business are at least 12 watts more likely the 25-35 watt models. I understand the passion in the post but think about what you put there. You are absoutely correct though
about simple front end overload. That is exactly what is happening. Phrawg
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Sorry about that....now that I have sobered up I do remember that it is 45 MHZ apart for 800MHZ But yes....Its pretty obvious that it is fron end overload...........And yes most 800 mobiles are 12 to 15 watts, but most of the time 800 public safety systems that cover 1 city stricly work off portables unless it is a Motorcycle (Motorcycle spectra) or Fire truck...and the portables put out 2.5 to 3 watts...