Amplifier thought: antenna tuner seldom needed with LDMOS amp

[Elecraft mailing list]

As was posted on the Amps reflector a few months back:
 

> The LDMOS devices are designed to tolerate 65:1 SWR, 3:1 probably

> wouldn't be a problem so where is the need for a tuner?

 
Most ham antennas don't exceed 3 to 1 SWR even on the band  edges.  New
amps that are constructed with LDMOS finals should be  able to handle nearly
all antennas without a tuner.  
 
Operating a legal limit amp into an antenna with an SWR greater  than 3 to
1 should be avoided for many reasons, such  as: poor radiation efficiency,
harmonics, RF in the shack, RFI,  etc.  
 
73,
Dick- K9OM      
 
 
 
In a message dated 4/1/2017 8:48:28 P.M. Eastern Daylight Time,  
[hidden email] writes:

Date:  Sat, 1 Apr 2017 12:03:01 -0500
From: K9MA  <[hidden email]>
To: [hidden email]
Subject: Re:  [Elecraft] Amplifier Thoughts
Message-ID:  <[hidden email]>
Content-Type:  text/plain; charset=windows-1252; format=flowed

On 4/1/2017 09:22, John  Perlick wrote:
> In the case of a solid state amp, the output is matched  to 50 Ohms or
so.  Period.  So any mismatch means that RF is  reflected back into the
amp....where it is converted into heat.    Depending on your SWR, that could be
substantial!  So let's say 3:1. Then  25% of the power is reflected.   Note:
at 10:1 you reflect 67% of  the power so that means 500 W reaches your
antenna and 1000W is dissipated in  the Drain-Source junctions of your expensive
RF transistors!
Clearly, a  practical solid state amplifier capable of tolerating, say, a
3:1 SWR  needs an autotuner.  (Very few of us have the luxury of
perfectly  matched antennas across all bands.)  That said, the autotuner
should  permit maximum flexibility, including multiple settings for the
same  band.  There should also be an easy way to change settings, so the  
amplifier can be easily integrated into the station control  system.

73,

Scott   K9MA

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