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Is it true that high SWR reflections don't fry finals?

Feb 22nd 2021, 06:59

N1AUP

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Is this true?

"So, why do people think they can blow up their rigs or linear amplifiers when there is a high SWR on the antenna?

Because that can happen, but it is not due to the reflected power!
There is a totally different reason.
A high SWR on an antenna probably means that the antenna is not tuned to the frequency that is being used. This, in turn, means that the antenna has some inductive or capacitive reactance that is de-tuning the final amplifier. De-tuned final amplifiers draw far too much current and can burn up. The rig or linear amplifier will have to be re-tuned to avoid creating too much heat.
Many linears and nearly all tube amplifiers have some tuning knobs that allow you to "dip the plate current" or adjust the SWR by adjusting something on the front of the device.
Transistor rigs usually do not have any tuning adjustments. To avoid the extra heat created when running a de-tuned amplifier, there is a protection circuit that will significantly reduce the output power if the SWR is high. "

Thanks
Feb 22nd 2021, 09:16

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Based on the questions you have been asking it may be useful to study the topic of amplifier stability or what is the difference between and amplifier and an oscillator. Many hams have unintentionally built oscillators when intending to build amplifiers.

https://www.w8ji.com/neutralizing__amplifier.htm
Some tube amplifiers will oscillate into certain loads when not properly neutralized.

http://home.sandiego.edu/~ekim/e194rfs01/lec23ek.pdf
A rigorous academic treatment of stabilty.

Zak Lau W1VT
ARRL Senior Lab Engineer


Feb 22nd 2021, 10:05

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The ideal situation would be able to perform destructive testing like they do with automobiles and to record what burns up with video using IR cameras to measure the temperatures of the parts on the boards. But, it takes a lot of money to be able to do that.

My ham radio budget doesn't have the funds for that.
Feb 22nd 2021, 10:53

N1AUP

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I was always taught that SWR mismatches blow up finals because too much RF is reflected back at the transistors. The quote from a ham's web site seems to say that this is not correct.

So I'm still not sure. Which is correct?

I am reading those two articles.
Feb 22nd 2021, 10:56

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
https://www.g8jnj.net/cometcha250b.htm
This web page may be of help. It shows that transformers can have more loss when not matched optimally.

When I first became a ham excess voltage was an issue with transistors as it was difficult to make transistors with adequate voltage ratings. But, this is less of an issue with improved transistor designs.

A bigger issue these days is getting rid of the heat, now that hams run higher duty cycle modes like FT8. SSB is very low duty cycle much easier on equipment. It is hard to convince hams to run a conservative amount of power for the equipment they are using.

Another modern issue is the need to cover all frequencies. In the old days there were fewer ham bands and one could safely hide unwanted resonances outside the ham bands. It is a lot harder to design something that is reliable on 1.8 to 54MHz continuous compared to 80/40/20/15/10 meters. Folks will perform a "MARS mod" to open up a transmitter and expect it to work on all frequencies.

Typically excess voltage will short out a transistor, while excess heat will create an open circuit. A protection circuit may allow the short to remain for an autopsy of the dead circuit.

It is a ton of work but it should now be possible to take open source NanoVNA technology and characterize active devices like RF power transistors. Then build a complete circuit model to look for weak points in circuit designs.

In the October 1978 QST page 34 Roy W7EL discovered that an inductive input matching network would generate high voltage ringing that blew out PA transistors.

There are many ways an amplifier may fail. One can't automatically rule out SWR reflections as a high SWR may impact the stability of an amplifier and cause it to oscillate in a manner unexpected by the designer.

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