HF Transceiver Overview

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HF transceivers have a large selection of controls that affect their operation. Below is a brief practical overview of the controls in an analog, solid-state HF transceiver, including how to operate each control to achieve optimal transmission and reception performance. These controls may be buttons or knobs on an older transceiver, menu options on a newer transceiver, or configuration variables on SDR transceivers or computer-controlled transceivers. The controls listed here are derived from those in the IC-751 and FT-100D transceivers, but the descriptions apply to nearly all HF transceivers designed in the past 40 years. Because of this variation, it is critical to consult the manual of a transceiver to operate it properly; this page is intended to supplement these manuals and not to supplant them.

Vacuum tube or hybrid (i.e. using vacuum tubes for output amplification but transistors everywhere else) transceivers have other metrics and controls related to the performance and tuning of vacuum tubes. These are not covered here.

Marked as a stub while waiting for citations and sources.

Power On/Off

This switch switches the transceiver on and off. If the power switch is a momentary button, it often must be pressed for around 0.5 seconds to turn the power on or off; this is so that accidental presses do not power off the transceiver.


The meter (or meters) measures several important parameters associated with transmitting and receiving; an associated switch or button selects which one is being measured currently. When receiving, the meter most often measures signal strength in S-units. When transmitting, the meter most often can measure SWR (standing wave ratio, a metric of impedance mismatch), output power, and automatic level control output.

While receiving, the S-meter provides an at-a-glance view of the received signal and the noise floor. When transmitting, different metrics are useful at different times, but the SWR reading is useful for tuning antennas and preventing transceiver damage, so it is most often used.


The S-meter measures signal strength in terms of S-units; the weakest signal is S0 and the strongest on the S-unit scale is S9; stronger signals are measured in decibels over S9 (e.g. S9+40, read as "40 over S9"), often in 20 dB increments. The highest S-meter reading is typically S9+60. While S-meter calibration varies wildly between radio manufacturers, the standard defines S9 as -73 dBm at the input terminal, with 6 dB between S-units. The S-meter is critical to proper setting of the RF gain (see below), and forms an integral part of the RSx exchange used in HF contacts and certain contests.


Standing wave ratio (SWR)âalso called VSWR (Voltage Standing Wave Ratio)âis a measure of impedence mismatch. Often quoted as x:1 or just x, the SWR represents . Lower SWR values are better; transmitting with an SWR of 3:1 or greater for extended periods of time could damage your transceiver (citation needed), while an SWR of 1.5:1 or below is often seen as perfectly adequate. It's important to glance at the SWR reading after powering on your transceiver, switching bands or antennas, or making antenna adjustments. When manually tuning an antenna, adjust the controls on the tuner to reduce the SWR below 1.5:1.

Power output

This display indicates the PEP power output by the transceiver. For modes like SSB, this will vary with the volume of the audio being transmitted and the action of the ALC (automatic level control) system in the transceiver. For modes like FM, CW, or certain digital modes, the power meter is likely to stay constant.


Automatic Level Control (ALC) is used to reduce the transmitted audio level such that speech is transmitted at the highest possible level without distortion. (citation needed) ALC is very similar to AGCâAutomatic Gain Controlâbut it operates while transmitting; AGC operates while receiving. Usually, when the ALC meter goes past a certain level, the transmitted audio is being reduced in volume significantly to prevent the transmitting circuitry from being overdriven and the transmitted audio from being distorted.

The ALC meter is used to properly set the mic gain; for more information, see the section on mic gain below. It is also used when adjusting digital mode audio levels. The ALC system can be adversely affected by reflected power, so it is important to transmit into a dummy load when relying on the ALC meter indication for adjustments.

Mode selection

A button, switch, or knob allows one to switch between different modes. Often, special action is required to use unconventional modes, like SSB using the opposite of the conventional sideband, AM, or CW using lower-sideband demodulation (sometimes listed as CW-R).

When using digital modes, non-obvious mode settings are sometimes required. Sometimes, upper sideband must be used if a dedicated setting for digital modes is either nonexistant or unusable. Other times, a special digital (sometimes called "DIGU" for digital upper-sideband) mode exists and can be used to receive and transmit digital modes. In either case, other settings (such as those relating to transmit bandwidth and gain) must be adjusted to transmit and receive digital modes.

VFO display and controls

The VFO display shows the current frequency that the radio is tuned to. On most radios, a large VFO knob without detents allows the current frequency to be adjusted in granular increments when turned at a low speed and is used to search for signals when turned at a high speed. A "dial lock" button is often provided to mechanically or electronically lock the VFO knob in one position. A discretely-stepping switch is sometimes provided to coarsely adjust the frequency; one detent corresponds to one discrete frequency increment. A "tune step" button or switch is also often provided, allowing the operator to change the rate at which the VFO knob changes the frequency.


Most radios provide a memory function by which frequencies and modes can be stored for future use. The characteristics and operation of the frequency memory vary significantly among radios (so check your manual for details), but generally the radio can be switched into "memory mode" to recall previously-stored memories. In the memory mode, a discretely-stepping switch is used to select individual memory channels, rather than the continuous, linear frequency stepping provided by VFO mode.

Band selection

Often, special "band up" and "band down" controls switch the radio between frequency bands. A "general coverage" switch may also be used to allow the radio to be tuned outside of the ham bands (Only for reception).

Transmit-specific controls

Mic gain

Microphone gain adjusts the amplification of the voice signal going into the HF modulator. It has to be adjusted so that the signal isn't too quiet but also doesn't cause the LF amplifier to clip.

VOX (voice-operated keying)

VOX is a function that allows the operator to transmit without using the PTT. When enabled the transceiver transmits when it detects a audio level above a adjustable threshold. This is useful when you participate in a contest so that you've got your other hand free to, for example, use the logging software. It is sensible to use a headset or a directional mic when using VOX so that background noises don't activate the transmitter.

Compression or speech processing

QSK, full break-in, or semi break-in

Receive-specific controls

Automatic Gain Control (AGC)

RF gain

The RF gain adjusts the gain of the RF amplifier stage of the front-end of the transceiver. Since this amplifier is subject to intermodulation, proper setting of the RF gain control is critical to effective reception. The RF gain control should be set just above the point where the noise floor becomes inaudible. If the RF gain is too low, weak signals cannot be heard; if the RF gain is set too high, noise is amplified, either causing intermodulation or having no effect.

When set low or high, the RF gain control also significantly affects the reading on the S meter.

AF gain

The AF gain control sets the volume of the audio sent through the speaker or headphones. On voice modes, it should be adjusted to a level that allows noisy signals to be heard while being comfortable for the operator and courteous to others in earshot. On digital modes, the AF gain control may or may not set the level of the audio sent to the computer.

Preamplifier and attenuator

Most transceivers contain an attenuator that can be switched in and out of the RF input chain, along with a preamplifier. These act to extend the effective range of the RF gain control. They can be controlled by a menu option, a soft key (a button whose function is determined by a legend on the screen), or a three-position switch. On Yaesu radios, activating the IPO soft key will inhibit the preamplifier while activating the ATT soft key will switch in the attenuator (which seems slightly confusing).

Use the preamp to pick up weak signals when the noise floor is low; use the attenuator to reduce front-end overload and intermodulation when the noise floor is high.