Rich Measures wrote:
>>Rich Measures wrote:
>>........
>>>In Wes'
>>>measurements, he uses the term "Ls". What is "Ls", Mr. White?
>>
>>Copied and pasted directly from Wes's own page: "the measured effective
>>(series) inductance".
>>
>Mr. White: You, sir, are correct. Sorry for the confusion.
OK!
>Henceforth, I propose that "Lsu" designate the vhf parasitic
>suppressor's inductor, and that "Rsu". designate the suppressor's
>resistor.
>
Rather than use "s" at all (which could still be confusing), could I
propose simply "R" and "L", AKA "the resistor" and "the inductor".
>As you see it, at 100MHz:
>What was the inductance of the copper-wire Lsu?
>What was the inductance of the resistance-wire Lsu?
>
Remember that the Impedance Meter can only measure the *effective*
values that it sees between its binding posts. In network theory jargon,
what it measures are always *equivalent* values. Internally the
instrument measures something like magnitude and phase, and then it
calculates either Rs and Xs (or Ls), or Rp and Xp (or Lp). It's entirely
your choice, and all the answers are equally valid at the measurement
frequency.
There isn't much point in looking for the "true" values of R and L in
the suppressor, because the amplifier circuit sees the same *effective*
values that the Impedance Meter measures.
Anyhow, to pick a few more bones out of Wes's tables...
For a perfect inductor with no losses and no self-capacitance, Rs=0,
Rp=infinity, Ls=Lp.
You can see this in Wes's tables, where Ls and Lp are identical for the
copper-wire inductor because the losses in the inductor itself are very
low. The value down at 10MHz is 105.7nH, and that's as close as you'll
come to a "true" inductance measurement. The *effective* inductance
increases at higher frequencies because of self-capacitance, which Wes
estimated to be about 2pF.
The nichrome inductor has built-in resistive losses, so Ls and Lp are
different, even at low frequencies. The skin effect makes Ls and Lp move
apart quite quickly at higher frequencies, but they are both valid
quantities.
>At 100MHz, the measured Q of the copper-wire suppressor was 2.2, and the
>measured Q of the resistance-wire suppressor was 1.5. How do you account
>for VHF-Q being c. 46% higher in the copper-wire suppressor?
Because of the resistive losses in the nichrome wire. When you think of
the lossy inductor as its parallel equivalent, the losses appear as an
equivalent resistance in parallel with the physical resistor. This makes
Rp lower and so Q is lower too.
>What change
>would you make to the copper-wire suppressor to decrease its VHF-Q to 1.5?
>
Reduce Rp by adding physical resistance in parallel with the existing
resistor (or decreasing the resistor value).
However, one thing this discussion has taught me is that the Q of the
suppressor is *not* a magic number; it doesn't tell you everything you
need to know about suppressor performance. To get the optimum
performance in a real-life amplifier, you have to optimize several
different things at the same time... and that's where the hands-on
experience comes in.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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