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Power stage backoff

  • Ian
  • 3 days ago
  • 1 min read

I'm sure you have heard that modern radios will automatically backoff their power output stage if the SWR gets "too high", due to a poor antenna match. I have occasionally wondered what a "too high" SWR actually is and by how much would the radio drop its output power?


In this video I share my accidental discovery that at 2:1 SWR the Icom 7300 drops to less than half its maximum power! This is a much greater backoff than I imagined, so it caught me by surprise.




 
 
 

3 Comments

colrrr
2 days ago
•

Yes good work Ian.

Interesting to see the analog meter still correctly showing the swr of 2 while the radio was happy with swr of 1

and sending full power to the antenna.


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Craig vk3ncr
Craig vk3ncr
2 days ago
•

Great work Ian.. very interesting.. Thanks for adding to the blog.


Ian, so if using the onboard Tuner tricking the radio to see a close 1:1, can I assume the power then remains at the requested output, even though the Antenna SWR is still high, but the radios sees a matched antenna because of the Tuner? Hope that makes sense.


Cheers.


Craig.


Edited
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Ian
2 days ago
•
Replying to

That’s right Craig. The radio tuner just presents 50 Ohms to the radio, so the radio is happy to push out full power. That’s all it does.


The antenna hasn’t changed, so the antenna feedpoint impedance remains wherever it was, but the reflected power now bounces between the feedpoint and the tuner, rather than the feedpoint and the power amplifier.


This is why some hams get picky about calling it an ATU. It doesn’t ‘tune’ the antenna at all. Some prefer to call them Antenna Matching Units.

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