[AMPS] Re: Ferrite Rod for 6M Amp

[Rich Measures]

hello, Tom
>
>While I agree an air core alone is sufficient to make a reasonable 
>filament choke on 50 MHz, Rich has a total misunderstanding of 
>what creates "distortion" that we can hear in a linear PA.
>
It was not a SSB signal.  It was an AØ signal of 100w.  The type-61 
material performed well at 10MHz.  The waveform looked good.  I slowly 
increased freq.  At 14MHz, the waveform did not look good. .   However, 
with a ferrite material rated for 50MHz max. instead of 10MHz max., no 
distortion was observed at 14MHz.  

>Not only that, something must have been wrong in Rich's test 
>setup. Operation at higher frequencies does not suddenly create 
>waveform distortion in a ferrite material. 
>
Sweeping from 10 to 14 MHz did not produce any sudden change.  The change 
was pretty gradual.  

>If a certain material did create harmonic distortion as Rich wrongly 
>claims, the manufacturer would NOT recommend it for EMI 
>suppression!! That's because it would create harmonics and create 
>EMI through generation of harmonics.
>
Perhaps this is why we often see a bypass cap on the outgoing side of a 
ferrite RF attenuator bead?

>The claim operating 61 material at 14 MHz creates harmonics is  
>nonsense. Clipping is a flux density problem, not a frequency 
>problem. 

The observed waveform was not clipped at 14MHz. .  Between 80-deg. and 
100-deg., the waveform looked pretty much like a sine wave. //  Permag 
Pacific clearly indicates that the permitted flux density varies with 
frequency.   I suspect that if I had decreased P, the waveform at 14MHz 
might have improved.  
>>
>> €   In the test I performed with type-61 material operating at 14MHz, the
>> slope of the waveform was noticably different from that of a sinewave --
>> esp. between 60-degrees to  80-degrees and 100-degrees to 130-degrees.   .
>>  My guess is that the distortion was >15%.  
>
>Great "guess". What scope did you use? Maybe it needs repair.
>
€  A Hewlett Packard 1807.  The RF from my 2m HT produces a sinewave on 
the screen.  
 
>> >Quoting from Fair-rite Corp, an actual manufacturer of soft iron 
>> >materials, 61 material is suitable in broadband transformers and 
>> >EMI/RFI suppression devices up to 200 MHz, and tuned 
>> >applications up to 10 MHz. Q at ten MHz is 80.
>> >
>> € The cathode of a g-g amplifier is akin to a tuned circuit. 
>
>Nonsense. The choke is intentionally designed so its impedance is 
>several times the cathode impedance. That means the choke has 
>little RF current, low magnetization, and is NOT a part of any 
>resonant circuit.
>
>Besides that, the eddy currents (the "thing" that limits upper 
>frequency Q) simply add linear loss resistance to the system.
>
>> >> I would consider a filament choke to be in the latter category and if
>> >> its inductive reactance is the correct value, its impedance should be
>> >> far greater than that of the cathode and therefore not affect the
>> >> waveshape a significant amount  That effect has NOTHING to do with 
waveshape 
>> >> distortion.
>> >
>> €  Harmonic distortion has everything to do with sine waveshape 
>> distortion.  
>
>That's right. But harmonic distortion has no ill effects in a narrow 
>band PA. 
>
It does when your neighbors are watching the NBA playoffs.  

>People who aren't totally familiar with PA operation sometimes  
>confuse harmonic distortion with the type of distortion that causes 
>splatter. Splatter is NOT caused by harmonic distortion, splatter is 
>caused by amplitude non-linearity in the envelope shape.   
>
Is this a plea-bargain?

>That's the reason a single-ended class B PA is just as linear and 
>can produce just as low distortion in a SSB signal as a push-pull 
>PA.
>
¿say what?

>If 61 material did cause harmonic distortion (which it doesn't), it 
>could STILL be successfully used in a resonant PA. 

Agreed - provided the flux-density was sufficiently reduced above 10MHz.  
However, a material with a Mu of 40 and a max. freq. rating of 50MHz is 
available at the same cost, so why not use it?

>It would be useless in broadband applications, and EMI suppression.
>
>There is no other explanation for your results and conclusions.  

One explanation is that someone made a somewhat faulty assumption. 
>
>"The test" you made must have been flawed, and your end-conclusions 
>make absolutely no sense at all when compared to well published 
>data.

The results of the "The test" seemed to be in general agreement with  
Permag Pacific's little orange book.  
>
-  cheers, Tom


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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