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Some Radio Questions! Please :)

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SCPD

QRT
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Feb 24, 2001
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0
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1. OK, so what is the point of a radio system, where the base and mobile tranmit on a different frequency? I understand that a repeaer system HAS to do this, but I am just talking a simplex system. Why not just license one frequency, it's cheaper and all the units can hear the other traffic from cars? How does it work? I mean, how do the units hear the mobile unit if they're RX on a different frequnecy!

2. Sometimes with a radio system, the base radio or coucil will hit the repeater with other agencies however sometimes the repeater just shuts of at the end of their TX. Why is this? And what is the difference? I never notice this on trunking systems, just conventional.

3. Why do agencies still use frequencies without reapeaters? I can never hear the mobiles, only the dispatcher so does that mean the other mobile units can't hear them either?

sORRY, just net to all this radio stuff! thanks soooo much! :)
 
K

kb0nly

Guest
If they are using a duplex pair without a repeater, operating simplex, then its the benefit of the dispatch. I know a few counties that used to do this years ago. The dispatcher could listen to the units calling in and still be able to talk to the others at the same time. However the mobile units would have to switch to another channel, or basically talkaround, to talk to each other. Its odd, but some did it this way for various reasons.

Also some licensed a pair instead of a simplex frequency so they could just put a repeater in place later on. I know it doesn't make sense but many did it when Analog was king.
 

RKG

Member
Joined
May 23, 2005
Messages
1,094
Location
Boston, MA
1) "Two-frequency simplex" (un-modifed, "simplex" usually implies all parties sending on the same frequency) has been explained by kb0nly. In addition, for some radio services, you don't want subscribers to be able either to talk to one another or hear the other unit: e.g., taxicabs.

2) Not sure of your question, but you make be asking why the repeater has a "tail" when keyed in response to subscriber input and usually does not have a "tail" when keyed by the console wireline. To some extent, this is just an accident of history: back in the days when the SpectraTac comparator was the norm, transmitter keying in response to subscriber input was handled by the Tone Keying Module, while transmitter keying in response to a wireline command from the console was handled by the Tone Priority Module. Two different pieces of hardware, and they worked differently. There are some who believe that a subscriber tail was a design feature related to "roll off", which, if true, would not apply to console generation (as roll off is not a wireline issue). I don't know if that is true or rationalization.

3) Why not? It depends on the intended function of the system. Bear in mind that a repeater in a typical voted system does nothing to extend the talk range between dispatch and subscribers; the repeater's function is to extend the range of unit-to-unit traffic. In some applications, you might expect 99% of all traffic to be either between subscribers and dispatch or between subscribers who are close to one another. (This, in fact, describes most of the traffic in the fire service.) In other applications, unit-to-unit traffic-over-a-distance may be important (as it is in most PD applications).

In my neck of the woods, repeating in the PD and voted simplex in the FD were always the norm, until recently (when the FDs all went to repeating). Some folks suggest that the "new" norm is so that Chief's can monitor what is going on at the fireground without leaving home, but I don't believe that for second.

Note, though, that repeating in the fire service requires additional radio training for the guys. If a jake gets himself in trouble in a situation where he can't get out to a receiver (e.g., a sub-basement in a building), no one is going to hear his call for help unless he remembers to switch to "direct."
 
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