Transmitter Control

This page covers the commands used to control the transmitter on the K3/K3S: keying and unkeying, setting power output, adjusting mic gain and compression, reading SWR and power meters, and configuring VOX. For the complete alphabetical command listing, see the K3/K3S/KX3/KX2 CAT Command Reference.

Commands Used

Command	Description	GET	SET
TX	Enter TX mode	No	Yes
RX	Enter RX mode	No	Yes
TQ	TX query (are we transmitting?)	Yes	No
PC	Power control (watts)	Yes	Yes
MG	Mic gain	Yes	Yes
CP	Speech compression	Yes	Yes
ML	Monitor level	Yes	Yes
VX	VOX on/off	Yes	Yes
SW	SWR reading	Yes	No
BG	Bargraph reading	Yes	No
TM	TX meter mode	Yes	Yes

1. TX/RX State Control

The K3/K3S can be keyed into transmit mode either from the front panel (PTT switch, VOX, CW key) or remotely via the TX and RX commands.

• TX; or TX1; — key the transmitter (equivalent to pressing PTT)
• TX0; — key TX in “test” mode (reduced power, useful for testing without full output)
• RX; — unkey the transmitter (return to receive)
• TQ; → TQ0; (receiving) or TQ1; (transmitting) — query the current TX state

TX;                  Key the transmitter
TQ;                  Query TX state → TQ1; (transmitting)
RX;                  Return to receive
TQ;                  Query TX state → TQ0; (receiving)

Tip

Always check TQ; before sending TX-sensitive commands. Some parameters behave differently or cannot be changed while the radio is transmitting.

Caution

Always ensure you have a proper antenna connected and the correct band/frequency before commanding TX. Transmitting into a mismatched antenna can damage the power amplifier.

2. Power Control

The PC command reads or sets the transmit power in watts. The value is a 3-digit, zero-padded number.

PC;                  Query power → PC100; (100 watts)
PC010;               Set power to 10 watts
PC100;               Set power to 100 watts
PC005;               Set power to 5 watts (useful for tuning or QRP)

The valid range depends on the radio model:

Radio	Range
K3 / K3S (with KPA3)	000–110
KX3 / KX2	000–015

Note

Setting power to a low value (5–10 watts) before commanding TX; is a good practice during development and testing. This limits the risk of damage if something goes wrong with your antenna system.

3. Mic Gain and Compression

These commands control the voice transmit audio chain and affect SSB, AM, and FM voice quality.

Mic Gain (MG)

MG;                  Query mic gain → MG020;
MG030;               Set mic gain to 30

Range: 000–060. Higher values increase the audio level from the microphone or line input.

Speech Compression (CP)

CP;                  Query compression → CP010;
CP020;               Set compression to 20
CP000;               Turn compression off

Range: 000–040 (0 = off). Compression increases average transmit power by reducing the dynamic range of voice audio.

Tip

When adjusting mic gain and compression remotely, monitor the ALC meter (TM1; then read BG;) to ensure the audio level is not driving the transmitter into excessive ALC action. A small amount of ALC activity on voice peaks is normal; constant high ALC indicates the gain is set too high.

4. Monitor Level

The ML command controls the transmit monitor, which lets you hear your own transmit audio through the speaker or headphones.

ML;                  Query monitor level → ML030;
ML040;               Set monitor level to 40
ML000;               Turn monitor off

Range: 000–060. This is useful for checking audio quality and compression settings without needing a separate receiver.

5. VOX Control

VOX On/Off (VX)

VX;                  Query VOX state → VX0; (off)
VX1;                 Turn VOX on
VX0;                 Turn VOX off

When VOX is enabled, audio input on the microphone automatically keys the transmitter. The radio returns to receive after the audio stops, following the configured delay.

QSK/VOX Delay (SD)

SD;                  Query delay → SD050;
SD100;               Set delay to 100

Range: 000–255. The delay value is in approximately 10 ms steps for CW (QSK delay) but uses a different scale for voice VOX. Higher values keep the transmitter keyed longer after audio or keying stops.

Note

VOX is most commonly used in voice modes (SSB, AM, FM) for hands-free operation. In CW mode, the SD command controls the QSK (full break-in) delay instead.

6. TX Metering

The K3/K3S provides several metering commands for reading transmit power, SWR, and ALC activity.

Bargraph Reading (BG)

BG;                  Query bargraph → BG08;

Returns a value from 00 to 10 representing the number of power meter segments lit on the front panel display. The meaning depends on the current TX meter mode set by TM.

SWR Reading (SW)

SW;                  Query SWR → SW0150;

The response is a 4-digit value where 0100 represents a 1:1 SWR. Common readings:

Response	SWR
SW0100;	1.0:1
SW0120;	1.2:1
SW0150;	1.5:1
SW0200;	2.0:1
SW0300;	3.0:1

Caution

SWR readings are only valid while the radio is transmitting. Reading SW; while in receive will return a stale or zero value. Always key the transmitter first and allow approximately 500 ms for the power amplifier to stabilize before reading SWR.

TX Meter Mode (TM)

The TM command selects what the TX meter displays:

Value	Meter Mode
0	SWR
1	ALC activity
2	Power output

TM;                  Query meter mode → TM0; (SWR)
TM1;                 Switch to ALC meter
TM2;                 Switch to power meter

Power Reading via S-Meter (SM)

When the radio is transmitting, the SM; command reads power output instead of the S-meter. This provides a higher-resolution power reading than the bargraph.

7. ALC and Transmit Signal Chain

The diagram below shows how audio flows through the transmit chain, from input to antenna, and where each command acts:

Audio enters through the microphone or line input, passes through mic gain (MG), speech compression (CP), and the TX DSP (equalization and filtering). The resulting signal is scaled by the power control (PC) setting and fed to the power amplifier. At the output, the SWR sensor (SW) and power meter (BG) provide real-time measurements.

8. Practical Patterns

Safe TX Sequence

A typical pattern for testing transmit from software, using low power to minimize risk:

PC005;               Set low power for testing
TQ;                  Check if already transmitting
                     Expect TQ0; (receiving)
TX;                  Key transmitter
                     Wait for operation...
RX;                  Return to receive
TQ;                  Verify TX stopped
                     Expect TQ0;

Tip

Wrapping every transmit operation in a TQ check before and after ensures your application always knows the true TX state, even if the operator keys or unkeys the radio manually.

Reading SWR During Transmit

To measure SWR, key the transmitter and allow the power amplifier time to settle:

TX;                  Key transmitter
                     Wait 500ms for PA to stabilize
SW;                  Read SWR → SW0120; (1.2:1)
BG;                  Read power → BG08; (about 80% of scale)
RX;                  Return to receive

Note

If the SWR exceeds approximately 3:1, the K3/K3S automatically reduces power to protect the amplifier. If you see unexpectedly low BG readings paired with high SW values, the radio is folding back power due to a mismatch.

Next Steps

Continue to Voice, CW & Data to learn about CW keying, stored messages, and data sub-mode operations.