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My radio manual says its current draw is 22A. But in my naivety I would have calculated as follows:

If the radio 'put out' 100W and I have a 13.8V supply then (using the formula power = VxI) 100W/13.8V = 7.2A.

So, it looks that I'm way out (comparing 22A with 7.2A). Is my transceiver terribly inefficient or is this normal?

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  • $\begingroup$ The rule of thumb for current drawn by a transceiver working on a 12V= supply is 1A per 5W RF output. $\endgroup$
    – vu2nan
    Commented Mar 14, 2020 at 8:23
  • $\begingroup$ @Nandu [citation needed]. I would like to see a source for that $\endgroup$
    – Scott Earle ♦
    Commented Mar 14, 2020 at 10:52
  • $\begingroup$ Hi OM Scott, There is no specific source for this information. It has been arrived at from data furnished by reputed brands in their user manuals. Some brands specify current drawn at 100W RF output and others at 200W DC input, though the rated RF output is 100W. Thus it may be inferred that the efficiency of the RF power amplifiers in these transceivers is only 50%. The rule of thumb was found to be applicable even to transceivers with power outputs of 5W, 10W, 15W and 50W. $\endgroup$
    – vu2nan
    Commented Jun 27, 2020 at 11:05

2 Answers 2

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The rest (of the current) is why your transmitter requires heat sinks (and/or airflow for thermal dissipation). Linear amplifiers are usually not anywhere close to 100% efficient. A high SWR might reduce the efficiency even further.

If it's a transceiver, then the receive circuitry, front panel, and transmit idle bias also takes some wattage to run.

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Some of both!

  1. It's normal for a linear power amplifier for HF to be something like 50% - 60% efficient. So to make 100W, the radio probably needs 170 - 200 W.
  2. There are other power draws in the radio, let's say about 20W if it's a fancy modern rig with lots of electronics and a backlit LCD.
  3. Your radio is 13.8V nominal, but it's probably rated for 13.8V +/- 15% (11.73 - 15.87 V) and it probably makes rated power at 12V (people tend to be unhappy otherwise). Combining the previous two points, assuming it pulls 220W at 12V, that could be more than 18A of steady-state draw for key-down 100W out.
  4. Add a little bit for transients (maybe inrush current for fans starting up, or relays switching), accessories (if supported, e.g. the Icom tuner port), and a little bit of fudge factor just to be safe, and 22A doesn't sound outlandish as a maximum rating.

All that said, you should expect the average to be quite a bit less. You're not transmitting 100% of the time; if you're doing SSB phone or any other non-constant-power mode your average power output will be significantly less than 100W; transients aren't happening all the time by definition, etc. But when choosing power supply, wire, etc. you should still plan for 22A at least.

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    $\begingroup$ An excellent and clear answer to my question - thank you. I had already selected a previous answer - but again thank you. $\endgroup$
    – KHWP
    Commented Feb 15, 2020 at 10:09

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