EMRFD Message Archive 10410

Message Date From Subject
10410 2014-11-03 19:02:43 mmeyer504 SWR Protection

Is it necessary to provide SWR protection for a QRP RF power amp using power FETs like the RD16HHF device?  I have built an amp similar to the DK1HE design, but have wondered if I should add protection in the form of Zener diodes like what is often done with bi-polar transistors where a Zener is added from collector to ground for over voltage protection.




10411 2014-11-04 06:07:18 kb1gmx Re: SWR Protection
For linear operation the zener is not a best choice.  They hehp when the devices are 
voltage marginal or when bipolars are used to avoid certainbreakdown conditions they are
prone to.  The added capacitance can seriously upset a design not calling for it.

Also SWR protection is meaningless if the SWR is good, Its there to protect the 
user and vendor if the antenna and feed is less than good.  

That said I've run multiple amps built without SWR protection as the devices used were 
either sufficiently robust or plain cheap and have yet to replace any of them due to broken 
antennas, shorted connectors, no connections, and even high SWR. Those amps are both 
HF and VHF designs.  The only fail to date  was from none of the above, due to a switch 
error a small 30W QRP amp was got in line with my 20W radio.  Most MOSFET amps are 
far more sensitive to excessive input power than output conditions. An amp that was 
designed to take 2-3W does not fair well with 20W at the input!  

My thoughts is if there is risk of bad SWR during operation include it.  If there is 
uncertainty over the initial SWR then at least do SWR metering to correct it before 
running the amp.  There are only two things that change SWR at higher power, 
arcing in the antenna/cable or tuner or excessive harmonic production (overdrive!).

When you consider the part is under 5$ each the cost and effort to include 
SWR protection may be out of scale.  If you run the device at less than max power
(two push pull are good for 20W) then you gain robustness as the voltages and 
currents are lower.

I've built a four  RD16HHF push pull amp for the 40-50W range and its doing well
in testing.  Its basically a copy of the Veatch design published in QST[aug-2010] 
with a different board of my own layout. He chose to not use SWR protection either. 
Testing includes loading to 4:1 SWR (12.5 and 200 ohm)  loads for stability at 
power when warm.   So far the 4 spares have not been needed.  Seems like a 
fairly robust device and stable.  At those power levels a good large finned  
heat sink and maybe even a small quiet fan is your friend.

10413 2014-11-04 17:03:12 augustinetez Re: SWR Protection
The RD16HHF1 and others of it's range are designed to withstand 20:1 mismatch, so no protection needed.

(All information in the datasheet)

Terry VK5TM