Author Topic: damage finals  (Read 1624 times)

August 31, 2016, 06:02:59 AM
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buzz-ki4dei

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damage finals
« on: August 31, 2016, 06:02:59 AM »
Just a basic question:  most know that "keying" the mic with no antenna connected will (or might) trash final drive transistors.  So I ponder what do the finals consider a "sufficient" antenna?  And what of defective RF amplifier in the transmission line.  Could an inoperative "linear" cause finals to fail? 

September 12, 2016, 07:24:20 AM
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jon-kd4amp

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Re: damage finals
« Reply #1 on: September 12, 2016, 07:24:20 AM »
  Good morning buzz----i know they will or could be hurt without an antenna but cant answer other questions . :o

September 12, 2016, 09:35:01 AM
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ron-kk4hgy

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Re: Damaged finals
« Reply #2 on: September 12, 2016, 09:35:01 AM »
I think most of the newer radios have circuitry that checks the SWR and reduces the output or stops transmitting entirely if it detects a high SWR.  Then again I imagine quite a few hams don't have the newer radios.

Here's an article that might shed a little more light on the subject. 

http://www.qsl.net/zl3dw/Files/SWR%20myths%20and%20mysteries.pdf

September 13, 2016, 08:27:41 AM
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buzz-ki4dei

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Re: damage finals
« Reply #3 on: September 13, 2016, 08:27:41 AM »
Jon KD4AMP:   Kinda like when your mom tells you “don’t touch the stove”…. She may never tell you why, just don’t touch the stove.  Good enough to invoke caution.  And the question of “good enough”  (as in ‘sufficient’ antenna) is always a variable. 

September 13, 2016, 08:28:50 AM
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buzz-ki4dei

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Re: damage finals
« Reply #4 on: September 13, 2016, 08:28:50 AM »
Ron-kk4hgy,  I agree.  Many "late model" units are well protected. Manufacturers strive to make their stuff  "idiot proof."  Less "fatalities" means greater customer satisfaction. Consider:   how many clutches are burnt out by those learning to drive.  Those with funds easily buy an antenna tuner and skip the whole chapter.  People buy cars with automatic transmissions and skip the clutch chapter.   NOW... I wonder how many drivers cannot operate a stick shift.   Ponder:  a transmission line consisting of   TX,  short coax jumper to RF linear amplifier and then coax to antenna.  Where would one install a SWR meter? Between the TX and the linear?… Perhaps between the linear and antenna?  Where and Why?  Any wavelength considerations for the jumper between the TX and the amp?

September 20, 2016, 05:05:58 PM
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dave-K4VLF

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Re: damage finals
« Reply #5 on: September 20, 2016, 05:05:58 PM »
"Ponder:  a transmission line consisting of   TX,  short coax jumper to RF linear amplifier and then coax to antenna.  Where would one install a SWR meter? Between the TX and the linear?… Perhaps between the linear and antenna?  Where and Why?  Any wavelength considerations for the jumper between the TX and the amp?"

I would first put the SWR mtr on the output of the TX. Linears have adjustable/buried input matching pads to ensure full output of the TX device.
Then of course, move the SWR mtr to the output of the linear and tweak it as required for minimal SWR.
Of course, many radios and amplifiers have built in SWR meters, so moving one araound may not be required.