Two conversations using one antenna???

[poltergeisty]

I was wondering how can this be. I can hear an officer talking on dispatch and in the background of his radio, I can hear an officer on tac-3 which I know for sure because a press of my scanner and I can hear it as well. Now I know the cruisers out here only have one little antenna and that is it, How is this done?

Maybe he has a scanner or there is something else to trunking I don't know yet? However, I can not verify that this particular cruiser had the one antenna, But from all my observations of cruisers from my town's police department there seems to be one stuby antenna on all of them, except for county and state patrol. Which brings up another question. What are all the antennas for on the county cruisers and state cruisers? One for regular coms, one for mdt, one for a repeater thing? thats what I know. Any insight to this?

 

[N_Jay]

I was wondering how can this be. I can hear an officer talking on dispatch and in the background of his radio, I can hear an officer on tac-3 which I know for sure because a press of my scanner and I can hear it as well. Now I know the cruisers out here only have one little antenna and that is it, How is this done?

Maybe he has a scanner or there is something else to trunking I don't know yet? However, I can not verify that this particular cruiser had the one antenna, But from all my observations of cruisers from my town's police department there seems to be one stuby antenna on all of them, except for county and state patrol. Which brings up another question. What are all the antennas for on the county cruisers and state cruisers? One for regular coms, one for mdt, one for a repeater thing? thats what I know. Any insight to this?

He could have been talking on his portable and you wre hearing his mobile.

In general on a moble it is one antenn per radio.

I don't know what your local guys have but they can have antennas for.
Mobile
Portable charger/converter
MDT
GPS
Scanner
Bank Money Bad tracking (Usulaly 4 in a square)
Lo-Jack (3 in a triangle)
GPS (puck)
Cellular
Second Radio (Other district, orther band, etc.)
Etc. Etc. Etc.

 

[scanfan03]

An MDT requires two antennas.

 

[N_Jay]

An MDT requires two antennas.

Some do, and some don't (most don't)! :wink:

 

[scanfan03]

Why would Harris County be stupid and have two per MDT? If they could have a system which only requires one?

 

[WayneH]

What MDT format is it? I haven't seen a dual setup myself yet.

-Wayne

 

[N_Jay]

Why would Harris County be stupid and have two per MDT? If they could have a system which only requires one?

Why would you say it is a stupid choice?

The installation of a second antenna is such a minor issue, it would not be considered in the decision.

Two antennas could be for;

1 Diversity receive
2 Full duplex
3 Mutiple band opperation

 

[poltergeisty]

Way cool thanx much. :wink: I never thought of his or her hand held. Ok my mind just began to brew some more questions now. Here they go.

What is the GPS for?

Does it report back to the station?

Or is this for Law enforcement out in the middle of no man's land?

Can this signal also be joined to a digital conventional or trunked system if for reporting back to a station?

And could I decode this? That would be way cool.

how does two antennas for MDT work without cross over and all that stuff?

What is diversity receive?

And I was wondering now about MDT on a digital system either conventional or trunked. I guess the MDT data is encoded again as a p25 or C4FM signal . I have one of my search ranges in my 296 set up for searching 820-866 and to trunk any signal it gets that it is able to, one frequency will stop and p25 flashes and it is a constant digital sound which sounds like MDT.

Can a system be set up to be able to TX-RX and have MDT on one antenna? Or are two required for this particular setup?

The state and countys antennas are arranged on the trunk. I believe a set of 4, and one or two on the top of the vehicle. It's been a while sense I have seen one of these cruisers. You know I think I will go to there web site and see if they might have some pitchers. Just accrued to me writing this, will update. :idea:

Sorry about all these questions but what better place to ask than right here at Radio reference.

 

[N_Jay]

Way cool thanx much. :wink: I never thought of his or her hand held. Ok my mind just began to brew some more questions now. Here they go.

What is the GPS for? Location (Duh) :roll: (J/K)

Does it report back to the station? Depends on the Software

Or is this for Law enforcement out in the middle of no man's land? ???

Can this signal also be joined to a digital conventional or trunked system if for reporting back to a station? Yes

And could I decode this? No
That would be way cool.

how does two antennas for MDT work without cross over and all that stuff? The antennas have NOTHING, (or very little) to do with intersystem interference


What is diversity receive? A system with two receiver front ends to tolerate fading and multipath better


And I was wondering now about MDT on a digital system either conventional or trunked. I guess the MDT data is encoded again as a p25 or C4FM signal . I have one of my search ranges in my 296 set up for searching 820-866 and to trunk any signal it gets that it is able to, one frequency will stop and p25 flashes and it is a constant digital sound which sounds like MDT. Can be P25, or more likely Proprietary. Could be on the trunked system, but more likely on a conventional channel.


Can a system be set up to be able to TX-RX and have MDT on one antenna? Or are two required for this particular setup? You could combine the antennas (with restrictions) but it would very rarely be done. There is NO advantage, except maybe on undercover cars.


The state and countys antennas are arranged on the trunk. I believe a set of 4, and one or two on the top of the vehicle. It's been a while sense I have seen one of these cruisers. You know I think I will go to there web site and see if they might have some pitchers. Just accrued to me writing this, will update. :idea: OK


Sorry about all these questions but what better place to ask than right here at Radio reference.

Yes

 

[poltergeisty]

:lol: :lol: :lol:

You could combine the antennas (with restrictions) but it would very rarely be done. There is NO advantage, except maybe on undercover cars.

This is what I was thinking about. One antenna for all purpose use.

I know what GPS is for. :roll: Just was not sure how they used it and why. What does (j/k) mean? Don't you mean X, Y, Z. If you find this funny cool. But really, am not sure what (j/k) means. You say it depends on the software. So this means it may be for location reporting and/or telling the station where you are at. If so, Why the telling the station where you are at capability?

Is this in case one steals a squad car and goes for a joy ride? :lol: Or a better way of knowing where an officer is at?

I would not want that if I where a police officer for that department. Strange but I think me and my gun would work just fine without my boss knowing where I was. I don't know, I am not a police officer so I guess I can't say much about it. Just seems like mommy wants to know where you are at at all times is all.

Why can't I decode a GPS signal from the system? Technically, not legally. If there is a law :lol:

 

[N_Jay]

:lol: :lol: :lol:

You could combine the antennas (with restrictions) but it would very rarely be done. There is NO advantage, except maybe on undercover cars.

This is what I was thinking about. One antenna for all purpose use.

I know what GPS is for. :roll: Just was not sure how they used it and why. What does (j/k) mean? Don't you mean X, Y, Z. If you find this funny cool. But really, am not sure what (j/k) means. You say it depends on the software. So this means it may be for location reporting and/or telling the station where you are at. If so, Why the telling the station where you are at capability?

J/K Means "Joking", don't take my "Duh" personally

Is this in case one steals a squad car and goes for a joy ride? :lol: Or a better way of knowing where an officer is at?

It "Can" do all these things. What any particular system "does" depends on what ity was designed to do. Most with GPS use it for AVL (dispatch knowing where you are) capability. Some use it with incar mapping. There is no "standard" set up.

I would not want that if I where a police officer for that department. Strange but I think me and my gun would work just fine without my boss knowing where I was. I don't know, I am not a police officer so I guess I can't say much about it. Just seems like mommy wants to know where you are at at all times is all.

Yep, most officers (and unions) hate it at first. It has to be sold as a office safety issue, and then with concesions that it can not be used to punish an officer. (Political pain in the butt!)

Why can't I decode a GPS signal from the system? Technically, not legally. If there is a law.

You would have to be able to decode the system it is on, and then decode the data that is being sent, and then hope it is not encrypted. Basically "no" unless you have lots of time and lots of money to waste for no good reason. (If you do please send some of the money to me)

:lol:

 

[poltergeisty]

All right great response. Thanx

Yep, most officers (and unions) hate it at first. It has to be sold as a office safety issue, and then with concesions that it can not be used to punish an officer. (Political pain in the butt!)

I could not agree more! I'm not an officer but would be my take.

You would have to be able to decode the system it is on, and then decode the data that is being sent, and then hope it is not encrypted. Basically "no" unless you have lots of time and lots of money to waste for no good reason. (If you do please send some of the money to me)

What I was thinking about is that it will be on the trunked system an agency would perhaps use. Or on a digital conventional system. Which ever be the case. I actually don't really care about trying to decode a GPS signal from a system that has it, just like to know how all of this is done.

Is fascinating to me and I guess it goes with the hobby. If there was a GPS signal on a system would it sound like an MDT signal? Or I guess be in bursts like packets of information during a time period?

 

[nd5y]

It depends on the system. For example the EMS service in Fort Worth
runs their AVL on 45.64 and it is a bunch of data bursts. From what I can
tell the base polls each mobile every few minutes. It sounds exactly
like 2400 baud packet radio that hams use in some areas but I can't
decode it.

Dallas and Fort Worth both have multi channel 800 MHz data systems
where each channel transmits continously. They sound different from
eachother and totally different than a trunking control channel.

I'm pretty sure Dallas has AVL on all the PD & FD vechicles and run it
on the data channels along with the MDTs, or maybe the MDTs send
the AVL data. I don't think Ft Worth has AVL yet.


Tom

 

[poltergeisty]

I figured this might be the case. AVL? What does it stand for? is 45.64 a frequency of the system they use? Could Hoka code 3 gold or other app. be able to identify perhaps and/or figure out how to decode such a data burst?

You say that Fort worth has AVL and then at the end of your post you say " I don't think Fort worth has AVL yet. Did you mean that PD and FD dose not have it. :?: :?: :?:

 

[nd5y]

AVL = Automatic Vehicle Location

You can have a GPS receiver that feeds location info to a data radio
which is what ambulance service runs on 45.64 MHz.
http://www.radioreference.com/modules.php?name=RR&action=fcc&cs=WPPB292
I don't know what kind of equipment or protocol they are using
and I don't know what a Hoka is or what it can or can't decode.
All I know is it is AFSK data that sounds just like 2400 baud AX.25 packet
with the standard Bell modem tones.
I use mostly use MixW http://www.mixw.net for my digital communications.
I'm not sure if the police and fire dept have AVL, I have not seen
any GPS antennas on their vehicles.

Tom

 

[BANDIT]

If it were 2400bps AX.25 packet you could track the cars on a map with APRS software. APRS is what the hams are using for tracking. GPS receiver data is fed to a two meter transciver thru a TNC and transmited over packet and through repeters. The base stations use a TNC to decode the data and feed it to the APRS software for tracking.

 

[nd5y]

If it were 2400bps AX.25 packet you could track the cars on a map with APRS software. APRS is what the hams are using for tracking. GPS receiver data is fed to a two meter transciver thru a TNC and transmited over packet and through repeters. The base stations use a TNC to decode the data and feed it to the APRS software for tracking.

Thats what I was thinking the first time I heard it but it is apparently not
AX.25 becase I can't decode it with MixW or a TNC.

Actually that is only partially correct. APRS software will only print
stuff that is sent in the APRS format. You can send about any kind of
data in AX.25 packets.

I remember several years ago the Irving, TX PD had some kind of data
terminals on one of their UHF channels that was 1200 baud AX.25.
I could copy the numbers they were using for callsigns in the
headers but the rest of the packet was either encrypted or compressed
or something that just came out as gibberish.

Tom

 

[BANDIT]

Yes I know about the other packet streems. I also have a PK232 MBX and also a registered copy of MixW that I use all of the time on the HF bands. It does more than the PK232 hi hi.....