ICAS = Intermittent Commercial and Amateur Service, as compared to CCS (Continuous Commercial Service). 

These two service types were first defined in the power-tube industry. The classical definitions are as follows: 

Continuous Commercial Service (CCS) is defined as that type of service in which long life and reliability of performance under continuous operating conditions are the prime considerations. 

Intermittent Commercial and Amateur Service (ICAS) is defined to include the many applications where the transmitter design factors of minimum size, light weight and considerably increased power output are more important than long tube life. In this service, life expectancy may be one-half that obtained in Continuous Commercial Service. 

Under the ICAS classification are such applications as the use of tubes in amateur transmitters, and the use of tubes in equipment where transmissions are of intermittent nature. Intermittent operation implies that no operating or 'on' period exceeds 5 minutes, and every 'on' period is followed by an 'off' or standby period of at least the same or longer duration. 

As applied to solid-state transmitters, ICAS does not imply reduced device life, as unlike vacuum tubes, the RF power devices are not regarded as consumables. Rather, ICAS in a solid-state environment implies de-rating to keep the device junction and case temperatures within their maximum ratings. Thus, ICAS suggests a limited operating or 'on' period followed by an 'off' or standby period of at least the same or longer duration, whilst CCS assumes continuous, key-down operation at reduced output (assuming the same heatsink area and/or fan air-flow rating for both applications). 

In this context, "100% ICAS duty cycle" can be a little misleading, since the very term ICAS implies some de-rating by time (duty cycle < 100%) for rated power output, compared to CCS for the same PA/cooling assembly. If we use the tube-industry definition given above, we arrive at "safe operation at rated power output for 5 minutes transmitting, followed by at least 5 minutes standby". This implies "key-down" CW transmitting (e.g. RTTY, SSTV). For SSB, the allowable transmit time will be longer. 

Typically, a PA cooling system should be designed to keep the device case temperature in the range 70 ~ 80ºC (158 ~ 176ºF) at 25ºC (77ºF) ambient, for a 30-minute SSB voice transmission or a 5- to 10-minute “key-down” CW transmission at rated output. These are minimal duty cycle values for an amplifier operating in average amateur service. Longer “key-down” intervals, necessitating larger heat-dissipating surfaces and greater air circulation, are required for contest operation or at high ambient temperatures.

A third service type, IMS (Intermittent Mobile Service), is also discussed in the references. IMS tube ratings are based on a 15 sec. transmit period followed by a 60 sec. standby period. A 5 min. ON/5 min. OFF cycle is allowed for testing, with the proviso that total ON time < 10 hours during the llife of the tube. The duty cycle for a solid-state transmitter is specified in terms of device junction and case temperatures, device dissipation ratings and ambient temperature.


  1. RCA Transmitting Tubes Handbook, 1962 Edition, p. 84.
  2. ARRL Handbook, 2023 Centenary Edition, p. 13.4.
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