We have a lot of problems with "skip" from other departments (one even in another state). One of the Motorola service technicians advised us a few years back to change to a different frequency (we even got approved for one in the 155.--- range). No further action was taken at the time b/c it wouldn't have been feasible to change all our equipment over to something new. But, if our FEMA grant for all new radios comes through, it would be the perfect time to change. We are a volunteer fire dept. We do our own dispatching over the radio, via a phone patch and keypad radios. So we need a good radio system.
Would it be advisable to change? If so, which would be the best to change: RX or TX? Providing that the new frequency isn't being used by any nearby agency (which it supposedly isn't), changing the RX would eliminate the skip we currently receive, right?
how close can the TX and RX frequencies be?
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hsfdchief200
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how close can the TX and RX frequencies be?
Last edited by hsfdchief200 on Tue May 10, 2005 4:22 am, edited 1 time in total.
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thebigphish
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hsfdchief200
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I forgot to describe the types of skip that we get. Some is strong enough to hit the repeater. (In this case we would need the change the TX frequency to eliminate picking this up, correct?)
Sometimes we get "simplex" skip - it's from a district in the next parish. It doesn't hit our repeater. So their RX must be the same as ours.
Sometimes we get "simplex" skip - it's from a district in the next parish. It doesn't hit our repeater. So their RX must be the same as ours.
...
300khz is the closest you can go with commercially available duplexers while keeping the transmitter and receiver at the same site.
you could transmit combine and receive multicouple on different antennas at the same site at that split... but that's $$$$$$$
as for "skip" - this is why one day god smiled upon Motorola and said
"THOU SHALT INVENT PL(tm) TONES AND ALL WILL BE BETTER"
(doesn't eliminate it - it just masks it so you don't hear it)
doug
you could transmit combine and receive multicouple on different antennas at the same site at that split... but that's $$$$$$$
as for "skip" - this is why one day god smiled upon Motorola and said
"THOU SHALT INVENT PL(tm) TONES AND ALL WILL BE BETTER"
(doesn't eliminate it - it just masks it so you don't hear it)
doug
BRAVO MIKE JULIET ALPHA
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psapengineer
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Separtion
Chief,
Is your antenna on a Tower, Wood Pole, Building? What is the height?
I'd need to know to make any suggestion.
Bob
Is your antenna on a Tower, Wood Pole, Building? What is the height?
I'd need to know to make any suggestion.
Bob
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hsfdchief200
- Posts: 66
- Joined: Tue Feb 01, 2005 9:28 am
We have PL codes (or tones) and still receive a lot of skip. Our PL is 167.9. Most of the "local" skip from a few neighboring depts 50-75 miles off doesn't hit the repeater, but we can hear it on our pagers and radios (sort of like simplex). For the pagers, all we have to do it take them out of the charger, and usually we don't hear any more of this skip.
We are in Louisiana, and we occasionally hit skip from a county in Texas - this DOES hit the repeater. So, there is no taking the pager out of the charger to stop hearing this.
Our antenna is on a tower, approximately 200-210 ft. Our primary coverage area is approximately 400 sq. miles.
The tower we are looking at putting the antenna on is about 450 ft. If we can't get on that one, we are looking at possibly adding another repeater to put on top of an elevated water tower close to the very dead areas we have.
What do you think?
We are in Louisiana, and we occasionally hit skip from a county in Texas - this DOES hit the repeater. So, there is no taking the pager out of the charger to stop hearing this.
Our antenna is on a tower, approximately 200-210 ft. Our primary coverage area is approximately 400 sq. miles.
The tower we are looking at putting the antenna on is about 450 ft. If we can't get on that one, we are looking at possibly adding another repeater to put on top of an elevated water tower close to the very dead areas we have.
What do you think?
Skip getting into our radios
If you can, i would change your frequency to use the wider split. It will make the repeater work better and not require such an expensive duplexer. Plus it's hard to find a radio service tech that really knows how to tune up duplexers these days. Most of the service guys you see are lucky if they know which end of the key to put into the ignition slot of the service van.
Change your tone code to something else if you have a neighbor using the same thing.
You can have two repeaters on the same frequency. Just use a different tone code in the input of the second repeater. Use the same output code for both of them. That way it doesn't matter which repeater your using, your radio will hear both. It just requires programming another channel into all your radios. This way channel one is say the east repeater and channel 2 is the west repeater. Both use the same RF frequencies and the same PL tone on the output. It is just the second repeater will use say 136.5 tone for the PL to get into the repeater receiver.
Jim
Change your tone code to something else if you have a neighbor using the same thing.
You can have two repeaters on the same frequency. Just use a different tone code in the input of the second repeater. Use the same output code for both of them. That way it doesn't matter which repeater your using, your radio will hear both. It just requires programming another channel into all your radios. This way channel one is say the east repeater and channel 2 is the west repeater. Both use the same RF frequencies and the same PL tone on the output. It is just the second repeater will use say 136.5 tone for the PL to get into the repeater receiver.
Jim
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psapengineer
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Antenna Separation
Chief,
The input to output separation needed between input and output frequencies varies by manufacturer. In general 80dB to 120dB must be obtained to protect the receiver from the transmitter.
Your receiver will also need to be protected against any other VHF transmitters on the same tower or in the immediate vicinity. Conversely, others will need to protect their receivers against your transmitter. So, if you're alone (VHF's only user) in the area, your life it golden. If not, it requires a significant amount of coordination and devices.
There are two effects that you have to worry about. You have to prevent strong nearby VHF transmitters from swamping the Rx front-end amplifier; this is called desense. You also have to prevent weak signals (on the Rx frequency) from being radiated into your receiver; this is called Transmitter side band noise. Side band noise is generated in all transmitters and usually extends a few MHz each side of the transmitter's assigned frequency; it must be notched out at the offending transmitter and cannot be dealt with at the receiver.
You can get about 60dB of isolation of isolation by having two antennas separated by about 60 feet or more on a steel tower. You then need to get the other 20 to 60 db by using cavities. See: http://www.guerrilla.net/reference/quic ... t_sep.html
Telewave, in California, is one of the few firms that do a really good job building custom RF protection systems for VHF.
Basically, you will need to install a pass cavity that protects your receiver from the general world around it. You then also will need to install notch or pass-notch cavities in the Rx line to protect against certain strong nearby Tx signals, including your own Tx. You should then also install a dual isolator and pass cavity on your Tx to protect the rest of the world from your system. You will also need a notch or notch-pass cavity on your Tx to notch out the low level side band noise that your Tx produces on your receiver’s frequency. You will need to also notch out any other folk's Rx channel that are in the area.
More Info: http://mrtmag.com/mag/radio_transmitter ... r_desense/
I know what I’ve written is not a simple answer but its not a simple problem. The more separation between your actual frequencies makes the job significantly easier and significantly cheaper. Again, remember to consider all other VHF frequencies on the tower or in the area as also needing to be dealt with.
You do also need to change PL to get rid of the skip (likely its ducting)
from the co-channel agency in Texas. Normally one puts a device on the raw Rx to count the “hits” for all PLs seen on the channel and chooses a “quiet” PL for their use. DPL may also be an option for you.
Good luck in your challenge. Bob
The input to output separation needed between input and output frequencies varies by manufacturer. In general 80dB to 120dB must be obtained to protect the receiver from the transmitter.
Your receiver will also need to be protected against any other VHF transmitters on the same tower or in the immediate vicinity. Conversely, others will need to protect their receivers against your transmitter. So, if you're alone (VHF's only user) in the area, your life it golden. If not, it requires a significant amount of coordination and devices.
There are two effects that you have to worry about. You have to prevent strong nearby VHF transmitters from swamping the Rx front-end amplifier; this is called desense. You also have to prevent weak signals (on the Rx frequency) from being radiated into your receiver; this is called Transmitter side band noise. Side band noise is generated in all transmitters and usually extends a few MHz each side of the transmitter's assigned frequency; it must be notched out at the offending transmitter and cannot be dealt with at the receiver.
You can get about 60dB of isolation of isolation by having two antennas separated by about 60 feet or more on a steel tower. You then need to get the other 20 to 60 db by using cavities. See: http://www.guerrilla.net/reference/quic ... t_sep.html
Telewave, in California, is one of the few firms that do a really good job building custom RF protection systems for VHF.
Basically, you will need to install a pass cavity that protects your receiver from the general world around it. You then also will need to install notch or pass-notch cavities in the Rx line to protect against certain strong nearby Tx signals, including your own Tx. You should then also install a dual isolator and pass cavity on your Tx to protect the rest of the world from your system. You will also need a notch or notch-pass cavity on your Tx to notch out the low level side band noise that your Tx produces on your receiver’s frequency. You will need to also notch out any other folk's Rx channel that are in the area.
More Info: http://mrtmag.com/mag/radio_transmitter ... r_desense/
I know what I’ve written is not a simple answer but its not a simple problem. The more separation between your actual frequencies makes the job significantly easier and significantly cheaper. Again, remember to consider all other VHF frequencies on the tower or in the area as also needing to be dealt with.
You do also need to change PL to get rid of the skip (likely its ducting)
from the co-channel agency in Texas. Normally one puts a device on the raw Rx to count the “hits” for all PLs seen on the channel and chooses a “quiet” PL for their use. DPL may also be an option for you.
Good luck in your challenge. Bob