A Very Simplex Question - LAPD

[HopperD]

Sometimes when I'm listening to LAPD, an officer or air unit will request to switch frequencies to simplex. What does that mean and how can I easily follow it? I can find the frequency they switched to by scanning but is there a way to easily follow?

As far as I know, there is a simplex 1 on 473.7875 in the database but I'm sure there's more.

[mdulrich]

Excellent resource for LAPD stuff. Los Angeles Police Department Radio Frequencies - LAPD Scanner Frequencies

Mike

[markclark]

HopperD:

The units talk around the repeater and communicate directly on the base frequency; if you listen to the base frequency, and are within range, you will hear the units.

[n5ims]

Simplex is a very local communication path, it's radio to radio, so the range is very small, often a couple of miles at most. While your scanner could pick up the transmissions, you'd need to be very close to them to do so. In an area the size of LA, it's quite possible for several of these simplex conversations to take place at the same time without causing interference but they would need to be spread out around the city for that to happen.

[HopperD]

Thanks mdulrich, that's exactly what I was looking for.

Other Responders: I can hear them most of the time but there were a few instances where I couldn't pick them up so I guess I wasn't in range. I thought they were on a frequency that wasn't programmed into my radio.

Thanks for your responses, that clears things up a bit.

[KMA367]

They informally and incorrectly but invariably use the name "Simplex" when referring to the divisional tactical channels, which are officially named "Enhanced/Fallback" frequencies, channels 101-125. When using them, the officers can select either direct car-to-car (true "simplex") mode or "duplex" repeater mode. It's not at all uncommon for one unit to be TXing on simplex and the other on duplex - they're both still hearing the lower "downlink" side of the pair. These Enhanced/Fallback repeaters are located at each division station, so they don't have the range of the bureau and citywide tactical frequencies.

The eight Bureau and seven Citywide tac frequencies, channels 36-49 and 177, are also officer-selectable for either simplex or duplex mode and those repeaters are located primarily on mountaintops around the city for much greater range.

In the UHF analog years, 1981-2001, they routinely used their divisional "Base" (dispatch) frequency in simplex mode to talk car-to-car, but when digital came along in June 2001 they very quickly discovered that digital's capture-or-kill effect caused too much interference to the dispatchers' transmissions. So the dispatch Base frequencies (channels 1-25) are now capable of duplex mode only, and officers very seldom talk to each other on them except during emergent situations until they are given a tactical frequency to switch to.

[PJaxx]

Originally Posted by KMA367

They informally and incorrectly but invariably use the name "Simplex" when referring to the divisional tactical channels, which are officially named "Enhanced/Fallback" frequencies, channels 101-125.

The eight Bureau and seven Citywide tac frequencies, channels 36-49 and 177, are also officer-selectable for either simplex or duplex mode and those repeaters are located primarily on mountaintops around the city for much greater range.

Just wondering why Citywide Tac 7 is in Channel 177 while all the others are grouped together in 36 to 49. I understand 101 being the simplex for Division 1 Central, 102 for Division 2 Rampart, and so on, but 177 is so far out of sequence.

[KMA367]

Originally Posted by PJaxx

Just wondering why Citywide Tac 7 is in Channel 177 while all the others are grouped together in 36 to 49. I understand 101 being the simplex for Division 1 Central, 102 for Division 2 Rampart, and so on, but 177 is so far out of sequence.

When the Astro Saber radios were being programmed in the mid 1990s, the City Housing Authority Police were using 484.9875 and they ended up in LAPD's channel 177 among the transit, hospitals, and Airport Police frequencies.

Then in 2004 when they did a major reconfiguration of the radios, the Housing Authority Police had just been disbanded so they gave that frequency to LAPD as a 7th citywide tac but left it in its original channel slot.


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