>
>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.
>
Rich knows that this was what you said, Mr. Rauch. This was one of the
major rubs in the grate parasitics debate.
>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,
At HF, L-supp has little effect. 40 - 120 nH or so is all we are talking
about in a circuit whose RL is typically several k-ohms. .
>
> it does not de-Q the system at VHF.
At 100MHz, in Wes' tests, the resistance-wire suppressor exhibited a
lower VHF Q.
>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.
>
[chortle]
>> 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?
no
>> 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.
>
Smoke. . His measurements are not of a system, they are measurements of
a R/L suppressor device.
>> 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.
>
According to Wes' measurements, at 10MHz, the nichrome suppressor had a
measured Q of 7.6. At 200MHz, Q was 0.6. It sort of looks like you
have things backwards, Tom.
>Nichrome is excellent for suppressing lower frequency oscillations,
>if used properly.
more smoke.
>It offers the largest loss increase at DC, all other
>things equal, when compared to a copper inductor and parallel
>resistor combination.
>
wake up, Tom.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|