Fire VHF Direct Mode

[brendan3]

Question for the brain trust regarding direct mode on squelch protected repeater sites. Using cal fire for example the repeaters often require an input tone. However there are times when this is bypassed by going to direct mode.

What occurs when direct mode is used?

 

[DMS11B]

What do you mean what occurs?

In direct mode the radio operator transmitts on the output frequency of the repeated channel, typically at low power 5watts-ish which is received on any receiver that is within reception on that particular frequency and tone.

 

[brendan3]

Taking cal fire as an example most of the radios are setup to receive a specific frequency and tone. So when a user enters simplex mode, how is that the other radios still receive the transmission?

 

[wtp]

because they are using the same frequency and tone.

 

[f40ph]

The radios (in direct mode) transmit the PL tone that is normally used for rx protection along with the repeater output freq.

 

[brendan3]

And the repeater will still receive the transmission? If so, what is the point of having tones and this direct mode?

 

[f40ph]

And the repeater will still receive the transmission?

Nope. Pressing direct mode is like saying "I don't need the repeater, I'm transmitting as-if I'm the repeater"

If so, what is the point of having tones and this direct mode?

If a radio is programmed as "tone protected" on a repeater channel, you wouldn't hear anyone transmitting right next to you unless their radio acts like the repeater. The direct radio MUST transmit the PL tone in order to break through everyone's squelch listening on the channel locally.

 

[mmckenna]

And the repeater will still receive the transmission? If so, what is the point of having tones and this direct mode?

Direct mode transmits on the repeater OUTPUT frequency. NOT the repeater input frequency. The repeater never hears it.

 

[wtp]

 

[wtp]

repeater output on 159.xxx tone 123 input on 151.xxx tone 123
handheld regular mode RX 159.xxx TX 151.xxx tone 123
handheld direct mode RX 151.xxx TX 151.xxx tone 123
so the local handhelds 'think' it is the tower and opens up.

 

[brendan3]

Ok so when this mode is used the communications center has no way of hearing the transmission? If this is the case then what is the benefit of using direct mode on the fire ground?

 

[MiCon]

Ok so when this mode is used the communications center has no way of hearing the transmission? If this is the case then what is the benefit of using direct mode on the fire ground?

The comm center MIGHT hear the direct mode transmissions if the units are close enough to a comm center receiving antenna, which are usually at a high elevation (hilltop or high tower). However, direct mode is not intended for comm center conversations, it's meant for local comms (on-scene unit-to-unit) that won't tie up the repeater.
Why use it? Because the units can talk to each other locally without tying up the repeater, but because they're using (and receiving on) the repeater output freq and PL, they can still monitor the repeater traffic while in direct mode.

 

[mmckenna]

Ok so when this mode is used the communications center has no way of hearing the transmission?

Right. In most cases.

If this is the case then what is the benefit of using direct mode on the fire ground?

It's not intended for communicating with the comm center.
It's intended to be a simplex (radio to radio) resource on the fire ground for fire fighters/command to talk to each other. It does not rely on any infrastructure, just the hand held or mobile radios.

-Maybe the repeater doesn't cover inside the structure, so direct radio to radio communications can be useful.
-Maybe they are outside the coverage footprint of the repeater and still need to talk on the fire ground.
-Maybe the repeater is out of service, it can still be used for simplex communications.

 

[Eng74]

Ok so when this mode is used the communications center has no way of hearing the transmission? If this is the case then what is the benefit of using direct mode on the fire ground?

Say you get dispatched to a call, the next in engine is in the area, you can go to direct to talk to the other engine without everyone else having to hear the radio traffic and ask them if they want you to take the call or maybe you have ran a call on that address before and want to give them some info, or you can just poke fun at the other crew. So there are lots of things that can be done on direct.

 

[AM909]

repeater output on 159.xxx tone 123 input on 151.xxx tone 123
handheld regular mode RX 159.xxx TX 151.xxx tone 123
handheld direct mode RX 151.xxx TX 151.xxx tone 123
so the local handhelds 'think' it is the tower and opens up.

Usually, "talk-around" direct simplex usage is on the repeater output frequency (i.e. 159.xxx in that example), not the input (though there are reasons for either). Usual usage is just quick chatter between units on-scene that you don't need the whole world to hear or bother the dispatcher with. It's also used on systems like some LASD analog and CHP channels that don't normally repeat the mobiles through the base stations (only the dispatcher hears the mobiles normally). Usually the same tone is used on TX and RX so you don't just blindly "step on" traffic coming through the repeater, though some will use a different tone to make it seem like a separate channel, where only the units that have switched to direct will hear units talking on direct.

A lot of fireground usage is just on dedicated simplex frequencies where there are no repeaters.

 

[nokones]

In the case of CDF a.k.a. CalFire, the mobiles Tx on the 159 Meg freq up to the repeater and Rx on 151 Meg of the freq/channel pair from the repeater. In the direct/simplex mode the mobiles will Tx and Rx the 151 Meg freq which is also known as talkaround of the repeater.

I don't recall if CalFire uses CTCSS on the mobile Rx. They definitely use CTCSS for repeater selection for the area they are in.

 

[mmckenna]

I don't recall if CalFire uses CTCSS on the mobile Rx. They definitely use CTCSS for repeater selection for the area they are in.

Right. On the Command repeaters, they usually use Operator Selected Tones on the mobiles/portables transmitting on the repeater INPUT. Use the tone to select which mountain top you want to hit.

On the simplex TAC channels, some of which are reused repeater OUTPUTS, they often use 192.8Hz on both TX and RX.

 

[kayn1n32008]

repeater output on 159.xxx tone 123 input on 151.xxx tone 123
handheld regular mode RX 159.xxx TX 151.xxx tone 123
handheld direct mode RX 151.xxx TX 151.xxx tone 123
so the local handhelds 'think' it is the tower and opens up.

Direct is on the repeater transmit frequency, not receive frequency.

 

[fireinoc]

If your broadcasting on "repeat" you have to pick an associated tone that is connected to the desired repeater tower in order for the repeat mode to work. Each tower covers a certain geographic area and has a set of specific PL tone numbers assigned to it. The FF's carry a map book for areas they don't know showing the repeater coverage statewide. For CAL FIRE, PL tones are assigned with certain purposes as well. For instance, the PL tone 103.5 is the CAL FIRE designated PL tone for all command frequencies. (A little different for the US Forest Service, they put all of a given National Forest's frequencies on the same tone, so different tones used for the Angeles NF than say the Sierra NF).

Bac to CAL FIRE; If your using "direct", only the engine or crew on the nearby piece of line will hear you since the transmission does not connect with a mountaintop repeater. In both repeat and direct more, the receive (RX) frequency is the same. For instance CAL FIRE Command 1, RX is 151.355 in both the receive and direct mode. The difference is for "repeat" mode the receive frequency (RX) will be paired with a transmit (TX) frequency that is different, that unlocks the desired repeater tower when you push the mic key by broadcasting a sub audible tone. In the case of CAL FIRE Command 1 Repeat: TX 159.300, RX 151.355 Tone 103.5. But In the transmit mode for "direct" both the TX and RX frequency will be the same, example: CAL FIRE 1 Direct: TX 151.355, Receive 151.355...no tone

For scanning purposes, if one was to enter just the RX frequency desired without listing a tone for the frequency and just leave the PL slot open, both repeat and direct mode radio transmissions will be heard since the RX frequency is the same for both. Or, if you live in an area with a lot of interference, you can enter them in your scanner twice, once with a PL tone for repeat, and once without for direct. (The one without the PL tone may be subject to more interference).

 

[norcalscan]

I'm seeing a lot of close but not quite info in these posts. All CalFire transmissions are tone-protected on RX, so you have best results statewide if you program the RX tone in the scanner, especially at higher elevations where you might hear the repurposed frequency as a command net or tac. There are no "no-tone" transmissions anywhere in their VHF system. Direct will transmit on the other resources' RX freq with the RX tone. Otherwise the other resources would never hear them because they are tone-protected.