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[AMPS] 2000 ARRL Handbook question

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Subject: [AMPS] 2000 ARRL Handbook question
From: W8JI@contesting.com (Tom Rauch)
Date: Thu, 3 Aug 2000 08:44:56 -0400
From:                   Peter Chadwick <Peter_Chadwick@mitel.com>

> Rich says:
> >>There was no great difference between them at VHF, but at HF the 
> >>nichrome had much lower Q (as would be expected).
> >>
> >EUR  Tom --  your theory that the resistance of nickel-chromium alloys
> >Increases with a decrease in frequency is not supported by any published
> >measurement.  

I'm sure Rich knows that was never what I said.

What I've always said is Nichrome lowers HF Q, and decreases the 
slope of Q change with frequency. A nichrome suppressor is a low-
Q HF suppressor, it does not de-Q the system at VHF.

The VHF Q reduction is primarily controlled by the amount of 
inductance in the suppressor, and the value of the parallel resistor. 

That's why nichrome suppressors run hotter at HF, and why we 
don't use nichrome tank coils and transmission lines.
 
> Rich, you've lost me here. To me Q is (in this context) XL/r. Let's have r
> constant with frequency; XL increases with frequency. Therefore the Q of
> an inductor rises as the frequency goes up, doesn't it? And falls as the
> frequency (i.e. XL) goes down.
> 
> Now if the material has a permeability other than 1, resistance will rise
> as the frequency goes up, but Q will still drop as frequency goes down.
> Isn't this what Tom said?

Yes, but the slope of the impedance of the suppressor decreases 
when nichrome is used. The suppressor becomes more resistive at 
HF. Wes' measurements prove that, although anyone looking at 
the system would expect that to be true.

> If you put a resistor across the nichrome inductor, then the value that Q
> can reach is limited, and the parallel impedance is, in the limit, the
> value of parallel resistance.
 
Yes indeed, the lines cross at VHF. The VHF Q is identical, or 
easily made identical. The lower the frequency, the more loss the 
nichrome adds. 

Nichrome is excellent for suppressing lower frequency oscillations, 
if used properly. It offers the largest loss increase at DC, all other 
things equal, when compared to a copper inductor and parallel 
resistor combination.







73, Tom W8JI
w8ji@contesting.com

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