Different PL/DPL but same frequency…

[n1chu]

Why would an agency such as a fire dept use the same frequency but different PL or DPL?

A fire dept has a fireground frequency listed twice, once with a PL and again with a DPL.
Wouldn’t this cause a problem if both were in use on the same FG or even two seperate calls running simultaneously but close to each other?

While we are at it, how many times have you heard of a FD using the dispatch frequency as a FG channel, dispatch using the repeater and FG using simplex?

 

[buddrousa]

Many small departments do that so when they are on the scene with minimal repeater coverage they can talk to command and command can still hear dispatch but use the same squelch tones.
I have also seen 1 repeater with 3 different Fire Departments on it all with different Tone Squelch Codes with a 4th Tone for all 3 on a Mutual Aid with each other.

 

[PACNWDude]

This is done a lot in the oil and gas industry for VHF, especially for portable repeaters and cleanup crews in remote sites. Channel use and frequency spectrum is minimal, and the PL or DPL lets you have more effective "channels" where everything is not hearing everything/everyone else.

Lots of simplex and use of PL tones to allow for better use of spectrum and lower cost for those that might muck up their radios, need cheaper radios, and for hazmat use.

Zetron even had their 38 (Repeater) Tone Panel system that used PL tones to mimic a trunked repeater, making sure each "channel" was not stomping on each other through PL tones, and having buffers to control audio processing.

 

[n1chu]

I would like to narrow my question to FD’s that use repeaters and have their pick of FG frequencies… and still choose to use the dispatch frequency for simplex FG.

Let’s say it’s a VHF High Band repeaterized system. Only one dispatch frequency, but multiple choices of different frequencies, all simplex mode for FG.

…Trying to make sense out of using EITHER of the dispatch repeaters input or output frequencies for FG usage.

 

[ofd8001]

That practice is frowned upon by the National Fire Protection Associations Standard for the Use of Emergency Services Communications. "Why they do that" is a question for those doing it.

I can see a real bad scenario: Firefighters get in a "Mayday" situation and call for help right about the time the dispatch center is sending other responders to a trash fire. Not good.

That said, while NFPA Standards are consensus standards and represent the input from a lot of different folks, unless a state/local government has adopted them "as the law", those failing to comply are not subject to punishment - just criticism.

About the only thing that would justify use in my mind is that all the other fireground channels are committed to other incidents and the dispatch channel is all that is available.

9.3.1.3* A communications radio channel, separate from the
radio dispatch channel, shall be provided for on-scene tactical
communications.

A.9.3.1.3 The communications center should have the ability
to monitor all radio communications, including those communications
on tactical radio communications channels, where
practical. The AHJ should carefully evaluate the various
communication solution alternatives available, providing the
incident commanders with the appropriate mix of communications
capabilities to address their specific scenarios, ranging
from a small rural residence to a mammoth concrete and steel
structure in an urban downtown area. The AHJ should provide
a simplex radio communications channel for use in locations
outside the coverage area of any installed radio infrastructure.
If the simplex frequencies selected for tactical use are the
same as the output frequencies of any repeaters used by the
system, a method of positive lockout of automatic system use of
that frequency should be provided, controlled from the responsible
telecommunicator workstation.

 

[n1chu]

Thank you for quoting the NFPA Standard for use of Energency Communications. The NFPA never misses a beat when it comes to addressing every aspect the fire service deals with. In fact, I believe when the Federal Govt enacted OSHA, they specified the inclusion of NFPA Rules and Standards. But that’s another story for another thread.

Your response lends itself to my take on how the pair of dispatch frequencies should or should not be used. It’s always nice to know your take on things has a solid support group like the NFPA to back you up!

And the idea of a single simplex fireground frequency being listed twice with differing PL or DPL settings probably also falls under the same NFPA Section because it relates to the sharing of a frequency in a manner that could possibly cause interference.