How does a multi-channel system work?

[kb0rpj]

I am trying to visualize in my head how a VHF Trucked system works.
I have grasped the following

TX -> Combiner -> Antenna
RX -> MultiCoupler -> 2nd antenna

What am i not understanding is, how do they avoid the desense? Several of our MOSWIN sites here are VHF and in some cases the TX and RX Antennas are sitting stacked one on top of the other., which I would think would cause horrific desense, what I am missing?

My only thought was something like a high/pass filters, but with some channels being higher than the TX and some being lower than RX, how would that work.

can someone help me be a little smarter and draw me a mental picture of how this all works?

 

[10-43]

I am not involved in that part of the industry, but my first though is tuned cavity filters. They can be designed with multiple cavities to have a very narrow bandwidth.

 

[chief21]

This may be an oversimplification... But for a typical 20+ channel trunked tower site, the TX antennas (which often use combiners to minimize the number of antennas) are engineered to be stacked in a particular way. The single RX antenna, usually at the top of the antenna support structure, feeds all associated receivers through a multicoupler (and possibly other filtering devices).

 

[kayn1n32008]

Transmitters are combined using either hybrid combiners or cavity filters. They will feed a transmit antenna. The receivers are connected to a receiver multicoupler. There will be band pass filtering to keep the transmitters from causing desense in the receivers. The receivers are connected to a receive antenna.

Antennas can be either stacked or at the same elevation. Our province wide 700MHz system, the antennas are at the same elevation, BUT the repeater pairs use the same offset in the same direction.

On VHF stacked makes sense for better RX/TX isolation because there is no standard offset on VHF.

 

[kayn1n32008]

I am trying to visualize in my head how a VHF Trucked system works.
I have grasped the following

TX -> Combiner -> Antenna
RX -> MultiCoupler -> 2nd antenna

What am i not understanding is, how do they avoid the desense? Several of our MOSWIN sites here are VHF and in some cases the TX and RX Antennas are sitting stacked one on top of the other., which I would think would cause horrific desense, what I am missing?

Filtering. Filtering is how you avoid desense as well as vertical separation.

 

[mmckenna]

Yeah, its filtering.

I have an 800MHz trunked system. It's easier there since TX/RX is using a 45MHz split. VHF gets challenging since there is no standard split, as @kayn1n32008 said above. But the VHF frequency choice is done very carefully to make sure there is enough separation. One of the drawbacks of the mess VHF frequency coordination is.

My RX antenna is at the top of the tower, to get better RX from the 3 watt hand held radios. Under that is a tower top amplifier that boosts the received signal. In the tower top amp is some bandpass filtering. RX signal is sent down to a multicoupler with no additional filtering in it.

TX antenna is below RX by about 8 feet. Hybrid combiner is used for the 5 channels.

 

[UAT_978]

We just did a three channel VHF system where there is only 1 antenna for TX/RX. One hell of a combiner to be able to combine RX and TX to one antenna on 3 repeaters. Amazing engineering behind it. Around 3db of TX loss, and they used preamps to cover the losses on RX.

 

[kayn1n32008]

We just did a three channel VHF system where there is only 1 antenna for TX/RX. One hell of a combiner to be able to combine RX and TX to one antenna on 3 repeaters. Amazing engineering behind it. Around 3db of TX loss, and they used preamps to cover the losses on RX.

It's probably a daisy chain of 6 multicouplers. Each multicoupler is made up of 3x band pass cavities and 1x reject cavity. Each individual receiver and transmitter is attached to a multicoupler. 24 cavities in total. If they is the case, you are lucky you had adequate frequency separation for multicouplers and didn't need hybrid combines, ~7Db of loss is pretty typical for a hybrid combiner, BUT you can get very tight frequency spacing.

 

[Ubbe]

It's often easier to get good performance using the same antenna for both TX and RX than on separate antennas if they are omni. For digital RX diversity systems you usually have one TX omni antenna and several panel RX antennas that each cover a smaller sector to give a high gain figure. The RX combiners for each antenna have good bandpass filters and often a notch filter for the TX frequencies and have several outputs that goes to each channels RX diversity input.

To use many TX channels with one antenna there are different types of combiners for but they all will have a huge attenuation needing high power transmitters in the 100W range to have something left to go out from the TX antenna that usually will be a 6dB or more gain antenna.

/Ubbe

 

[KevinC]

It's often easier to get good performance using the same antenna for both TX and RX than on separate antennas if they are omni. For digital RX diversity systems you usually have one TX omni antenna and several panel RX antennas that each cover a smaller sector to give a high gain figure. The RX combiners for each antenna have good bandpass filters and often a notch filter for the TX frequencies and have several outputs that goes to each channels RX diversity input.

To use many TX channels with one antenna there are different types of combiners for but they all will have a huge attenuation needing high power transmitters in the 100W range to have something left to go out from the TX antenna that usually will be a 6dB or more gain antenna.

/Ubbe

Depends on how you define "many TX channels" and "huge attenuation". I routinely combine 12 channels at 7/800 MHz with less than 4 dB of loss.

 

[talviar]

Depends on how you define "many TX channels" and "huge attenuation". I routinely combine 12 channels at 7/800 MHz with less than 4 dB of loss.

We're doing 10 Channels on 800.... lose a little more than 3db with the combiners.... not sure on cable loss to the antenna for total loss.
Transmit antennas high on the tower, receive antennas anywhere from 25-50 lower.

 

[GTR8000]

We're doing 10 Channels on 800.... lose a little more than 3db with the combiners.... not sure on cable loss to the antenna for total loss.
Transmit antennas high on the tower, receive antennas anywhere from 25-50 lower.

You don't find that putting the TX antennas above the RX antennas results in imbalanced coverage? Subscribers receive a strong control channel, while inbound coverage can be a challenge in some fringe areas?

 

[talviar]

You don't find that putting the TX antennas above the RX antennas results in imbalanced coverage? Subscribers receive a strong control channel, while inbound coverage can be a challenge in some fringe areas?

Our coverage is balanced with that set up. Back when we ran an analog Smartnet system inbound quality and outbound quality was balanced well.

Same goes now with our P25 system.

 

[Ubbe]

For inbound/outbound balance it is usually high quality filters and equipment like the receivers used at a site and you measure with practically no desense of a receiver, but the mobile and portable radios at street level are affected much more from RF sources and interferencies in their lower quality receiver configurations that require at least 2-3 times higher outbound from a site. In a repeater configuration where you have a weak signal in you do not want to add too much noise to the signal due to a low RF level for the signal that gets repeated out. It also are more important to hear the dispatcher clearly than the mobiles.

For simulcast systems it isn't important to have a balance, there's no roaming between sites that goes on. For other systems where users roam to the best signal strength site you balance by the control channels signal strength but voice channels can have a higher RF output level.

/Ubbe

 

[kayn1n32008]

You don't find that putting the TX antennas above the RX antennas results in imbalanced coverage? Subscribers receive a strong control channel, while inbound coverage can be a challenge in some fringe areas?

You're over thinking it. These sites are engineered to have balanced coverage. Some things used to achieve that balance are(not exhaustive):
-tower mounted pre amps
-reduced transmitter power
-antenna gain
-electrical down tilt(to limit coverage)
-using multiple transmit antennas to limit combiner losses.

If the antennas are already on a tall tower at a site that has some HAAT, a couple wave lengths(on VHF) is not going to result in unbalanced coverage. The only real issue is going to be shadowing caused by the tower itself.

Fringe areas will ALWAYS pose problems, proper system design, and reasonable overlap help mitigate these problems. In theory, in a multisite trunk system, portables should be roaming to a better site.

The biggest concern is if you are trying trying to use a portable outside portable coverage.

My provincial system was designed for 95/95 coverage based on a high powered mobile, on our provinces 1, 2 and 3 digit highways. Some urban areas were spec'd for hand held portable coverage. Outside these urban areas portable coverage isn't promised, and is incidental.

 

[GTR8000]

You're over thinking it.

I'm really not, but thanks for your input.

 

[speedway_navigator]

-electrical down tilt(to limit coverage)

I have never seen down tilt in 2 way, is it used much? Is it more driven by coverage for rolling hills and terrain issues or to reduce co-channel RFI to a distant system?

 

[kayn1n32008]

I have never seen down tilt in 2 way, is it used much? Is it more driven by coverage for rolling hills and terrain issues or to reduce co-channel RFI to a distant system?

It is on our provincial system.

 

[GTR8000]

I have never seen down tilt in 2 way, is it used much? Is it more driven by coverage for rolling hills and terrain issues or to reduce co-channel RFI to a distant system?

Down tilt (mechanical and/or electrical) is very common in modern trunked systems, in particular when using panel antennas on mountaintop/high elevation sites where 99% of your subscribers are well below the height of the antennas.

 

[KevinC]

Down tilt (mechanical and/or electrical) is very common in modern trunked systems, in particular when using panel antennas on mountaintop/high elevation sites where 99% of your subscribers are well below the height of the antennas.

No mountains near me, but down tilt is quite common in my area as well.