Narrowband adjacent channel users ????

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zerg901

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"co channel user spacing for narrow band channels" - I just Googled for this and found nothing

Does anyone know what the spacing rules will be for "co channel" and "adjacent channel" users after narrowbanding?

AFAIK - the legacy VHF high band systems - which were using channel centers that were .015 Mhz apart (ie 154.31, 154.325, 154,34, etc) - had spacing requirements of approx 20 or 30 miles for co channel users - and 5 to 8 miles for adjacent channel users. (But Everett MA FD used to have a full power repeater on 154.31, and the adjacent Chelsea FD had a full power repeater on 154.325, and they never seemed to interfere with each other).
 

nd5y

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I read a few years ago that 7.5 kHz adjacent channel geographical separation was supposed to be the same as co-channel but I can't find where I read that and don't know for a fact if it is true.
 

studgeman

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For co-channel users there is no geographical separation requiremnt. Mileage separation hasn't been used in years. Coordination is done on coutour studies for co-channel users, propagation studies can also be used. Long story short, Interference contour cannot intersect with someone elses service contour.
 

radioman2001

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There is no specific portion of the regs that outlines this. The radio frequency co-ordinators are the ones who has decided this to meet the no inteference issue to incumbents. So in essence there will not be a floodgate of available VHF channels as a result of narrow banding. Another wasted government program.
 

studgeman

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In VHF after round 1 of narrowbanding the channels still overlap creating adjacent channel issues, praticurally for Analog. In UHF this isn't a problem. There are rules governing how to coodinate, but it is still up to the judgement of the coordinators to aprrove or disapprove.
 

RKG

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An interesting question.

Start by recognizing that the width of spectrum that is "occupied" (to use a timely term) by an FM transmitter is different from both the channel spacing and the maximum permitted deviation.

The accepted rule of thumb is that occupation equals two times max deviation plus 6 KHz.

Thus, a "wide" channel, licensed for and using 5 KHz as max deviation, would "occupy" 16 KHz of spectrum. (5 * 2 + 6 = 16)

A "narrow" channel, using 2.5 KHz as max deviation, would "occupy" 11 KHz of spectrum. (2.5 *2 + 6 = 11).

The foregoing calcs assume the transmitter is centered exactly on its licensed frequency. This is seldom the case: one of the beauties of FM (and the reason it is used for LMR voice) is that it tolerates off-frequency transmissions without altering tonality of encoded voice. FCC regulations tolerate off-frequency operation of 2.5 PPM (about 1.1 KHz @ 450 MHz nominal) for MO stations and 0.5 PPM (about 0.25 KHz) for a high power FB station. Since this tolerance could be on either side of nominal center frequency, it should be doubled in considering possible "occupation" of spectrum.

Now, remember that power doesn't drop immediately an absolutely at the edges of "occupation" (known in the trade as the "skirt"); it is required to fall off rather sharply, but it doesn't go to zero.

So we design a frequency allocation system to allow some width of spectrum between adjacent channels that is expected not to be occupied, in order to reduce the probability and magnitude of adjacent channel interference. Nominally, for the "wide" channels, this "guard band" of unoccupied spectrum 9 KHz (25 - 16 = 9), which is 36% of nominal channel spacing. But for the "narrow" channels, the guard band is only 1.5 KHz (12.5 - 11 = 1.5), only 12% of nominal channel spacing.

(In both cases, the guard band is less if allowed frequency tolerance is included.)

Thus, the probability and magnitude of adjacent channel interference is much higher for "narrowbanded" channels than it was for the old "wide" channels.

Not too long ago, I was asked to be part of a project to assign a whole bunch of "new" narrowband UHFchannels to a bunch of formerly VHF fire departments. We went to great lengths to be sure that channels that were adjacent in terms of frequency were not assigned to departments adjacent geographically.
 

zerg901

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What if we had a flat community that had 1 VHF highband narrowband repeater with an antenna on a 100 foot tower. And the mobile and portable radios had typical operating characteristics (whatever that may be for transmitters and receivers). How close could an adjacent channel repeater with equivalent characteristics be located to the first repeater?

One radio tech has told me that adjacent channel narrowband (11K0F3E) repeaters can actually be colocated on a common site - but there are some extensive calculations that have to be done (maybe in regards to exact antenna placement)./
 

jim202

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What if we had a flat community that had 1 VHF highband narrowband repeater with an antenna on a 100 foot tower. And the mobile and portable radios had typical operating characteristics (whatever that may be for transmitters and receivers). How close could an adjacent channel repeater with equivalent characteristics be located to the first repeater?

One radio tech has told me that adjacent channel narrowband (11K0F3E) repeaters can actually be colocated on a common site - but there are some extensive calculations that have to be done (maybe in regards to exact antenna placement)./
What ever radio tech told you that needs to be fired from his job. It is obvious that he doesn't have a clue on what he is talking about. It is obvious he has never had to deal with solving problems of co site location interference problems.

I sure wouldn't want to be the tech that had to solve interference problems with what your trying to tell us. I wouldn't even allow such a system to be installed. That is like having a repeater and putting a high powered base on the adjacent channel of the input frequency at VHF. It just isn't going to play. There is no way you could filter out the strong signal and still be able to hear your weak portables on the input.
 

zerg901

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Jim202 - you might have misunderstood

Consider this setup for 1 antenna tower -

154.31 repeater output with 153.77 repeater input

plus

154.3175 repeater out put with 153.7775 repeater input
 

dljosephson

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What ever radio tech told you that needs to be fired from his job. It is obvious that he doesn't have a clue on what he is talking about. It is obvious he has never had to deal with solving problems of co site location interference problems.

I sure wouldn't want to be the tech that had to solve interference problems with what your trying to tell us. I wouldn't even allow such a system to be installed. That is like having a repeater and putting a high powered base on the adjacent channel of the input frequency at VHF. It just isn't going to play. There is no way you could filter out the strong signal and still be able to hear your weak portables on the input.
It's not quite that bad, but a few essential pieces were missed. Colocating close-spaced transmitters is actually a valid approach to squeezing more users in a given piece of spectrum and no doubt it will be used more as people try to push narrowbanding to its limits. If you transmit on adjacent channels *at the same power, with the same antenna gain and from the same location* then both signals will arrive at the distant receiver with very similar strength. This is how MotoTRBO gets two channels in a 12.5 kHz bandwidth, 6.25 kHz for each. Even though they are adjacent channels, receivers have no problem sorting them out because they are at the same strength.
 

RKG

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It's not quite that bad, but a few essential pieces were missed. Colocating close-spaced transmitters is actually a valid approach to squeezing more users in a given piece of spectrum and no doubt it will be used more as people try to push narrowbanding to its limits. If you transmit on adjacent channels *at the same power, with the same antenna gain and from the same location* then both signals will arrive at the distant receiver with very similar strength. This is how MotoTRBO gets two channels in a 12.5 kHz bandwidth, 6.25 kHz for each. Even though they are adjacent channels, receivers have no problem sorting them out because they are at the same strength.
Not quite: MotoTRBO transmits on a single frequency. It achieves the "two channel" capability by time division multiplexing of digital data. (This is why Motorola calls MotoTRBO "6.5 KHz equivalent", not 6.5 KHz compliant.)
 
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