[AMPS] Another arc question
Rich Measures
measures@vc.net
Sat, 9 May 98 20:02:16 -0800
>Rich Measures wrote:
>>>Rich Measures wrote:
>>>>>Sample problem: Part I: Ls = 200nH, Rs = 200 ohms, f = 100MHz. What
>>>>>is the Admittance, Y, Mr. White? A running description of your solution
>>>>>would be helpful.
>>>>>
>>>>>No, I won't bite. There is absolutely no need to drag admittance into
>>>>>this discussion.
>>
>>When L is in parallel with R, admittance Is the name of the game. When L
>>is in series with R, impedance Is the name of the game.
>
>Not correct: when L and/or C and/or R are connected in parallel, there
>is NO automatic change into susceptance and conductance. These
>admittance parameters are only an alternative, optional route through
>the math. You can choose to stay in normal units of resistance and
>reactance, all the way.
In a parallel circuit, E is constant, and I is variable. In a series
circuit, I is constant and E is variable. The "normal units" are
different.
>
>>
>>>>Then on to Part II: what is the parallel-equivalent resistance of 200nH
>>>>in parallel with 200 ohms at 100MHz? (presumably using the method
>>>>described below)
>>>>
>>>200 ohms, at ANY frequency, by definition - no calculation is required.
>>>
>>.....not the definition of Rp in Wes' measurements.
>
>Or rather... not in your interpretation of "Rp" in Wes's measurements.
>
>Wes - the guy who made the measurements - emphatically disagreed with
>your interpretation, and he made the measurements.
>
How could Wes' have possibly measured 166 ohms of Rp for a suppressor
that uses a 109 ohm resistor?
>>
>>>Part III - it's my turn: what is the series-equivalent form, ie what are
>>>Rs and Xs at 100MHz?
>>>
>>>XL = 2*pi*100E6*200E-9 = 125.66 ohms
>>>
>>>XL is equal to Xp by definition, because L and R are connected in
>>>parallel.
>>>
>>However, when an L, or a C, and an R are connected in parallel, we have
>>susceptance and conductance in Siemens instead of reactance (X) and
>>resistance in Ohms, which, at a discrete frequency result in an
>>admittance (Y) instead of an impedance (Z).
>
>As I said above, that's not correct. Admittance, susceptance and
>conductance are merely an alternative route through the math. You can
>choose to stay entirely in resistance and reactance, as shown in my
>earlier posting which is completely standard network theory.
Uncomfy with conductance and susceptance?
>>
>>However Y can be converted to Z, and vice-versa.
>
> ...and when you do, you reach exactly the same impedance (R and X)
>results as I gave, only more slowly because of having taken an
>unnecessary detour into admittance.
>
If the classical slow-route allows one to avoid stepping into a Quagmire,
why not?
>>
>>>R = 200 ohms = Rp by definition as we hopefully agreed above.
>>>
>>We do not because we are discussing Wes' measurements, not the *ARRL
>>Handbook* (which used to be called the *Radio Amateur's Handbook*).
>>
>If you don't agree that the parallel-equivalent resistance in a
>parallel-connected circuit is - by definition - just R itself, you're
>marching out of step with every definition "Rp" I've ever seen.
>
We are talking about the Rp is Wes' measurements.
>
>>>What do you make Xs, Rs and Q, Rich?
>>>
>>Q is not 1.59. "Xs & Rs" look ok, however, Wes and I use "Rp" where you
>>use "Rs" to designate the parallel-equivalent R.
>
>That's incorrect on three counts.
>
>1. This "Wes and I" line won't stand up to examination. The record shows
>that Wes emphatically disagreed with Rich's interpretation of his
>measurements, so much so that he actually asked Rich to remove all
>reference to those measurements from his web pages; Rich refused. Wes's
>own web pages (referenced by Gerard a few days ago) tell the full story.
>
The full story is that Messrs. Rauch and Stewart took the same position
during the grate suppressor debate -- i.e., that the VHF-Q of a
VHF-suppressor can be made as low using copper-wire as it can using
resistance-wire. They are right, however, to do so, one needs to
increase R and increase L. The net result is that more L and R increases
the power dissipation in R at 28MHz. Thus, the advantage of using
resistance-wire in a VHF suppressor is that it reduces the dissipative
burden in R. // Wes had already published his measurements on the
Internet, so what purpose could possibly be served by not quoting them?
Wes' Q and Rp measurements undoubtedly are an embarrassment. . To me,
the incredible thing is that Mr. Rauch quickly removed his post of
28-Nov., wherein he proposed using a suppressor L made of resistance-wire
whose ESR was 5 ohms -- and he would not discuss the subject again,
period. However, Wes never seemed to notice.
>2. My terminology and analysis agree exactly with Wes's.
If 109 ohms = 166 ohms.
>It was I who
>originated the log-log plot of suppressor Q versus frequency, and sent
>it to Wes. He agreed with my calculations and published it on his own
>web pages.
You have not explained how lowering R is a parallel L/R suppressor lowers
VHF-Q. By your logic, the lowest Q would result when R = 0-ohms
>
>3. I'm the guy who keeps saying that the "s" in Rs stands for series-
>equivalent; and the "p" in Rp stands for parallel-equivalent.
In some book it may. In Wes' measurements, it undoubtedly does not.
>Please
>read my earlier postings correctly, and don't stand them on their head.
>
One must stand on one's head trying to use resistance and reactance in a
parallel L/R circuit.
>
>Multiple series<->parallel transformations are an area where it's very
>easy to accidentally turn equations and/or terminology upside-down and
>come to exactly the wrong conclusions. I firmly believe that is what
>Rich is doing.
>
>I'm happy to leave Rich and Carl to discuss the hands-on aspects of
>suppressors, but I do feel it necessary to defend the good name of
>network theory when it's being misused.
>
>
>It's nearly 0130 here. Goodnight, America.
>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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cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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