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Section 2 10 transmit

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madmedicnl edited this page Sep 30, 2026 · 3 revisions

2.10 Transmit

On a TX-capable rig the Transmit module appears:

  • PTT — key the transmitter.

  • TUNE — send a carrier at the tune-drive level for tuning an ATU.

  • Drive — transmit drive (0–100%). One number for every band; if your amplifier makes a different power on each, calibrate it once in Transmit drive by band (Settings → Radio, 6.2) rather than resetting this on every band change.

  • Tune — the (lower) drive level used by TUNE.

  • Mic — microphone gain.

  • TX audio — how loud a digital mode is handed to a radio that modulates it itself, in dB below full scale. It stands in the Mic rail's place, and only in the modes where the microphone is not what goes on the air: select FT8, RTTY, PSK or CW-as-MCW on a CAT rig and the rail changes from Mic to a dB reading; go back to SSB and Mic returns. The two can never both apply — in a digital mode the microphone is drained and discarded, so a Mic slider there would move nothing — and the strip has room for one rail, so it shows the one that works. (FreeDV RADE is the exception on both counts: the microphone is the payload there, so RADE keeps Mic and reaches this level from the TX menu.)

    This level is kept per mode. FT8, RTTY, PSK and MCW do not load a transmitter the same way, and the figure you are setting is where your waveform sits against the radio's ALC — so each mode remembers its own, saved as you set it. A mode you have never touched starts from the level for the carrier it goes out on, which is also what a mode added in a later release inherits, so nothing ever comes up at full scale behind your back.

    The adjustment on sideband is: bring it down until the rig's ALC is barely moving, then set the power at the radio. ALC riding on a constant-envelope digital mode is what splatters. On FM — VHF packet, APRS, RIFP — the same control is the deviation instead: an FM transmitter turns audio level into frequency swing and has no ALC to catch it, so full scale into a data input set for voice over-deviates, which sounds completely normal to anyone listening and decodes for nobody. Turn it down until other stations report you, or set the level at the radio.

    The same control is in each mode's setup window, where there is one, and the radio's own input level is the other half of it either way.

    On a radio sdroxide modulates itself — a Pluto, an HPSDR board, an SDR — none of this applies and the rail does not appear: there the modulator and Drive own the level.

Transmit EQ (Settings → Radio, above the per-interface section, since it applies the same way whichever radio interface is selected) is a 3-band parametric equalizer on the microphone audio, ahead of the modulator: Low shelf (cuts/boosts rumble and handling noise), Mid peak (presence, a narrow bump or dip somewhere in the voice band), and High shelf (brightness/de-ess). Each band has its own frequency and gain (dB), and the mid band also has a Q (how narrow the bump is) while the two shelves have a Slope instead (how sharply they roll off). Off by default, and flat (0 dB every band) the first time you turn it on, so enabling it changes nothing until you actually move a slider. Voice modes only (SSB/AM/FM); digital modes and CW carry synthesized or keyed audio that never reaches it. Applies immediately, like the IARU region setting: there is no Apply step — including while you are transmitting, so you can set it by ear against the transmit monitor.

It sits after Mic gain and before whatever modulates the audio, on every interface: a radio sdroxide modulates itself, and a rig that modulates its own audio (a CAT rig over its sound card, TCI, Icom LAN, SmartSDR) alike. Boosting a band therefore raises the level going into the transmitter, and there is nothing behind it but a limiter at full scale — on a transmitter, clipping is splatter on your neighbours' frequencies. After boosting anything, watch the ALC/TX meter and take the same amount back off Mic gain. Cutting is free; a cut band and a little more mic gain gets the same tone with none of the risk.

Transmit safety: by default sdroxide refuses to transmit outside the amateur bands (tx_ham_only), and stops the transmission if the radio reports a high SWR (swr_guard, below). Transmit hardware gains start at minimum and the tune drive defaults low. Raise drive deliberately. The one exception built in is the 11 m citizens' band — a transmitting service, but not an amateur one — which opens when the station switches on Allow transmit on 11 m (CB) (confirmed once); it opens 11 m and nothing else. The general band lockout can otherwise only be lifted from the command line, one run at a time, with --oob-tx (12).

The SWR guard stops the over when the antenna system is not what it should be — a feeder that has come off, a coax switch left on the wrong port, a dipole leg that has parted in the wind. It is armed by default at 2.5:1 and is set in Settings → General (6.1).

  • It needs a rig that measures SWR and reports it over CAT or TCI. On an IQ radio there is no such reading, and the guard is inert rather than approximate.
  • It does not wait for high power. A rig with SWR foldback answers a bad match by dropping to a few watts, so a power threshold would gate out exactly the emergency; the first fifth of a second of each over is ignored instead, on the clock, to ride out the key-up transient.
  • When it fires the transmitter stops and transmit stays locked out until you press Acknowledge on the warning bar. That is deliberate: a latch you can clear by pressing PTT again is not a latch. Turning the guard off also clears a standing trip.
  • The banner names the SWR that fired it, so you know whether you are looking for a bad connector or a completely disconnected antenna.

TUNE is treated differently, because feeding a mismatch is the whole point of it. An antenna tuner has nothing to work on until the rig keys into the very mismatch the tuner exists to remove, and a sweep that begins at 10:1 is what tuning looks like. So a tune gets double the limit — 5:1 with the default 2.5:1 setting, shown next to the figure in Settings so you never see a number you cannot find — and about five seconds before the guard applies at all, instead of a fifth of one. What survives is the case worth keeping: a feeder that is simply not connected still reads at the top of the scale once the tuner has had its five seconds, and still stops the carrier.

A manual tuner can easily take longer than five seconds to find a match. That is the one case the guard gets wrong, and the answer is to turn it off for the session rather than to raise the limit — you are deliberately sitting at a high SWR for as long as it takes, which is the situation the guard exists to end.

On a rig with its own power control — a TCI rig, or a CAT rig on any of the three dialects — Drive and Tune command the rig's output power directly rather than scaling anything on this side, and the slider adopts the rig's own setting when sdroxide connects: a level set in ExpertSDR3, or on the radio's front panel, carries over instead of being overwritten. The level that applies is asserted before every over, including a CW message handed to the rig's own keyer — which is the one place where the rig's power is the only transmit control there is, the sound card having no part in CW at all. After a TUNE the operating level is put back, so the radio is not left at the tune level for the next call.

The level is asserted behind the mode, not only when you move the slider, because a rig keeps its output power per mode: a radio set to 100 W in USB and 25 W in USB-DATA transmits at whichever of the two the mode command last left it in, and one put into AM is held to a quarter of its rated power on top of that. So a mode command and a power command go out in that order at every key-down, and the level lands in the register the over will actually transmit on. This matters most on a quadrature rig, where sdroxide demodulates the I/Q itself and the radio only learns which mode to transmit in when the transmitter comes up: there every over changes the rig's mode, and so the register its power comes out of.

On a CAT rig the power is a fraction of what the radio can do. Icom carries it that way natively; Yaesu and Kenwood take a number of watts and have no way to say how many they have, so their sliders are read against 100 W — right for nearly every rig those dialects cover, and low (never high) for the few that go above it, such as an FTDX101MP or a TS-480HX. Elecraft takes watts too but can be asked: the option-module query at connect says whether there is a KPA3 or a KXPA100 on the end of the cable, which is the difference between a slider that spans 110 W and one that spans a KX2's 12.

Drive and the hardware's TX gain are not the same control. On an IQ radio — a Pluto, an HPSDR board, an SDRplay or a SoapySDR device — Drive is digital: it scales the modulated baseband on its way to the converter, with a hard limiter at digital full scale so it cannot overflow. The radio's own output level is a separate slider on the Radio tab (TX gain, or whatever that device calls it), and the two multiply. Set TX gain once for the antenna and the amplifier you are feeding, and use Drive as the level you actually work with; the sensible starting point is Drive near the top and TX gain low, not the other way around, because backing off in the digital domain throws away converter resolution and Drive is the one with the limiter behind it.

What Mic gain does depends on the mode. In SSB it sets how hard the microphone drives the modulator, so Drive and Mic between them decide both level and how much the limiter is working. In FM it sets deviation — sdroxide's FM modulator is constant-envelope, so Drive changes the power and does nothing to the audio, while Mic gain alone decides how wide the signal is. 50% is unity gain, which puts a full-scale recording at the ±5 kHz peak deviation NFM wants; higher over-deviates and distorts however low Drive is. In FreeDV RADE it sets how hard the microphone is fed to the vocoder, the same 50% being unity. In every other digital mode it does nothing at all — the burst is synthesized and the microphone is discarded — which is why the rail becomes TX audio there.

CESSB — more average power for the same peak. The CESSB rail is controlled-envelope single sideband, David Hershberger W9GR's 2014 technique. It is the second vertical rail in the Transmit module, standing beside Mic on a desktop window, and a rail in the TX menu on a tablet or a phone. The thing an amplifier runs out of is the envelope — the magnitude of the sideband signal — and that is not the audio waveform, so a limiter on the microphone does not limit what the amplifier sees. The envelope of clipped speech overshoots by a long way, an SSB transmitter has to be backed off for peaks nobody hears, and the average power reaching the far end is a fraction of the transmitter's rating.

CESSB works on the envelope directly: it clips it, filters the resulting splatter away, and then removes the peaks the filter put back by subtracting a band-limited copy of the excess — a subtraction rather than a gain, which is what keeps the transmission inside your own filter instead of spreading it to twice the width. The measured result here is about 3.8 dB more average power at the same peak at 9 dB of compression, with out-of-band energy at the arithmetic's own floor.

The control is one number: how many decibels the voice is driven into the processor, 0 being off, which is where it starts. 6 dB is a sensible first try and 9 dB is about as far as most voices want to go — past that it starts to sound like a processor rather than like you. It is set by ear, remembered per radio, and it does not touch Drive: what leaves the processor is still held at full scale, so your power setting means what it did before.

It applies where sdroxide makes the sideband itself — an HPSDR board or Hermes Lite 2, a Pluto, a LimeSDR, a SoapySDR device. A transceiver that modulates the audio from its own sound card is making the envelope in its own DSP, where this cannot reach it. The rail is drawn where it can do something and nowhere else, so on such a radio — or in any mode but voice USB and LSB, whose payload is the envelope this flattens — there is no CESSB rail to find.

From the User Manual. Home.

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