Tutorial FLEX

Tutorial: FLEX

What it is. FLEX is a faster pager protocol from Motorola (1600, 3200 and 6400 bit/s) that runs alongside POCSAG. It is used by emergency services and paging companies in several countries. OnAir decodes the pages on one 25 kHz channel and lists address, type and text.

Read this first. Pager messages can contain personal or medical details. Laws on listening to and keeping them differ by country; some forbid it. Check your local law, and do not publish what you receive. See Legal and Privacy.

What you need

• Any radio that reaches the pager frequency. The mode asks for 2 Msps and works from 1 Msps, so an RTL-SDR is enough. The mode table covers 25 to 1000 MHz.
• A VHF/UHF antenna.
• A frequency. The mode starts on 929.6625 MHz. The frequency database lists 929.6625 MHz and 931.4375 MHz (US 929/931 MHz bands, FLEX and POCSAG) and 169.650 MHz (P2000 FLEX, Netherlands). Which are in use near you varies.

Before you start

Pager transmitters are strong. Start with a moderate gain and raise it only if pages come in damaged. The signal should sit on the channel centre.

Step by step

1. In the mode list open the group Pagers and choose Pagers, 1600-6400.
2. Pick your radio under Source and start the receiver.
3. Enter the pager frequency.
4. Open the Messages tab and make sure FLEX is ticked. Tick POCSAG too if the channel carries both.
5. Choose the channel width in Channel (25 kHz or 12.5 kHz).
6. Wait for pages. FLEX channels can be quiet for long periods.

What you should see

Rows appear in Messages with Time UTC, Protocol, Address, Fn, Type and Message. The Statistics tab counts Good, Repaired and Lost code words for each speed, with Frames (instead of Batches) for the FLEX speeds.

Reading the screen

The message table is the same as in Tutorial-POCSAG. Hover a row to see how the text would look read as alphanumeric or numeric. Rows marked incomplete had damaged words.

Options and settings

• Filter and Copy work as for POCSAG.
• Decoded only hides tone-only pages and pages whose text could not be decoded.
• Channel: 25 kHz is more forgiving; 12.5 kHz cuts a neighbouring channel better but needs the signal exactly centred.

Troubleshooting

• Nothing decoded. Check the frequency and look for short bursts in the spectrum. The channel may use POCSAG instead; tick POCSAG as well.
• Few pages, many Lost words. Lower the gain if the signal is very strong, otherwise improve the antenna.
• Only tones. Tone-only pages carry no text; with Decoded only they are hidden.
• Wrong frequency by a few kHz. Retune slightly until the Repaired and Lost counts fall.

Tips

FLEX sends in frames that are synchronised to the clock, so a channel can look empty between frames. Give it a few minutes.

Legal and privacy notes

OnAir only receives. Pages are other people's messages: keep them private and obey local law.

Under the hood

FLEX uses 2-level FSK at 1600 bit/s, and 4-level FSK at 3200 and 6400 bit/s, sent in 1.875 second frames of 128 frames per 4-minute cycle. Code words are protected by BCH(32,21). Check the panel and Mode-FLEX for the figures the decoder reports.

Related

Mode-FLEX, Tutorial-POCSAG, Tutorial-Multitone-Paging, Tutorial-Pagers-POCSAG-and-FLEX, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.