[!WARNING] P25 TX transmits a simulated trunked radio control channel. Transmitting on frequencies you are not licensed for is illegal and may interfere with public safety communications. Use only on licensed frequencies, in RF-shielded environments, or with a dummy load.

P25 TX generates a continuous P25 Phase 1 trunked control channel (TSCC). It cycles through TSBK (Trunked System Block) packets that identify the network and assign voice channels to talkgroups, allowing P25 radios and decoders to lock onto and follow the simulated system. Frames are transmitted continuously in a loop until stopped.

Transmission Parameters

Parameter	Value
Default frequency	155.000 MHz
Bandwidth	12,500 Hz (12.5 kHz)
Sample rate	2,400,000 Hz
Modulation	C4FM, 9600 baud
Frame type	TSBK (DUID 0x07)

Controls

All fields are hexadecimal.

Field	Digits	Description	Default
NAC	3 hex	Network Access Code	293
SYS	3 hex	System ID	001
WACN	5 hex	Wide Area Communication Network ID	BEEF0
RFSS	2 hex	RF Subsystem ID	01
SITE	2 hex	Site ID	01
TG	4 hex	Talkgroup ID included in voice channel grant messages	0001
VCH	3 hex	Voice Channel number assigned in group voice grant messages	001

• Frequency / Gain / AMP: Standard TX controls. The configured frequency is also embedded into the IDEN_UP TSBK packet as the channel centre frequency.
• Start / Stop: Begins or halts continuous TSBK transmission. Status shows TX TSCC while running.

TSBK Sequence

The app transmits a repeating 5-packet cycle:

1. IDEN_UP (0x3D) — Channel identifier with frequency
2. NET_STATUS (0x3B) — WACN and System ID
3. RFSS_STATUS (0x3A) — RFSS and Site identification
4. GRP_V_GRANT (0x00) — Group voice channel grant for the configured TG / VCH
5. NET_STATUS (0x3B) — repeated

Each packet is BCH(63,16)-encoded, 1/2-rate trellis-encoded, interleaved, and status-symbol-inserted per the P25 Phase 1 standard before transmission.

Settings Persistence

Settings are saved to tx_p25_tx.ini.