transmitter power levels - efficiency decrease at low power?

[lkish99]

I have an IC-7300, which has "100 watts of output". When i transmit with the RF Power at 100%, it draws about 24 amps at 13.6 volts. ASSuming its putting out 100watts, thats an overall efficiency of around 30%, which i suppose is reasonable for a class AB amp.

But when for instance i set RF power down to 10%, it still draws about 10 amps. ASSuming its only putting out 10 watts now, the efficiency has dropped to less than 14%.

Am i missing something here? (besides an RF wattmeter ).

Either the amplifier efficiency really goes down at low power, or maybe I'm overdriving the input somehow? (I'm doing FT-8 modulation, using the USB input). Thanks for your thoughts.

 

[MTS2000des]

I see similar behavior on my Icom IC-910H. I think it's just the way the multi-stage PA's operate efficiency wise. On VHF at full 100 watt rated output, current draw is right at 22 amps. Dropping the power down to 50, it's around 12, yet at the lowest setting, 5 watts, it's drawing 7-8 amps.
That's what it's rated at per the manufacturer's specifications.

 

[prcguy]

What your seeing is normal and I'm surprised the radio draws 24 amps at 13.6V. I would expect it to be a little less like 18-20 amps. Anyway, a 100w linear amplifier will be biased to draw an amount of quiescent current whether there is RF or not and that can be a couple of amps just for the PA transistors depending on the design. If you look at how much the radio draws in receive, a similar amount of current is required to run the synthesizer, front panel display and other common circuits. All that adds up to using several amps of current even with no RF coming out.

A linear amplifier designed for a lower power level like 10 watts with smaller components would be biased at a much lower level and you might save 1 1/2 amps right there. Portable QRP rigs are also designed for very low overall power consumption to run off batteries and my Elecraft KX2 probably draws less current at full power on transmit than the IC-7300 does on receive. Current draw on receive for the KX2 can be 140ma if you turn the display light off. Different radios made for different needs.

 

[edweirdFL]

That's a good explanation for the reasons that why using a full power radio turned down to lower power might not last as long as a QRP radio using the same battery power source at the same output level.

On the other hand the full power radio might have a larger display, better filtering, and easier to operate controls making it a better choice for how you plan on operating.

 

[prcguy]

Radios are all over the map. Take for example the Elecraft KX3 QRP rig, its got a world class receiver with filtering that competes with some $5k range radios and is also draws less than 200ma on receive and not much on transmit.

That's a good explanation for the reasons that why using a full power radio turned down to lower power might not last as long as a QRP radio using the same battery power source at the same output level.

On the other hand the full power radio might have a larger display, better filtering, and easier to operate controls making it a better choice for how you plan on operating.