Rich Measures wrote:
>If my series-parallel transformation method is wrong, Mr. White, then:
>
>1. I managed to fool the entire technical review board at *QST*
>Magazine. {March, 1989 issue of *QST*}
>
Not having the whole article to hand, I can't comment on that.
>2. the Hewlett-Packard Model 4191A RF Impedance Analyzer that Wes used
>is wrong.
>
No, just your interpretation, which is not the same as Wes's or almost
everybody else's.
>3. the Third Edition of Herbert W. Jackson's *Introduction to Electric
>Circuits* is wrong. {pp. 376-7, "sect. 13-8, "Impedance and Admittance"}
>
Once again I think the answer is, no, just your interpretation.
>When one calculates the admittance and parallel-equivalent resistance
>(Rp) of the VHF suppressor's Ls/Rs parallel circuit,
It's the bit about the "Ls/Rs parallel circuit" that indicates that
N7WS's measurements are not being interpreted correctly. Ls and Rs are
properties of a SERIES equivalent circuit - that's what the "s" stands
for - so "Ls/Rs parallel" indicates a confusion that is going to lead to
the wrong answers.
one finds that as Rs
>and Ls Increase, the resultant Rp Decreases. Thusly, Rp could be
>decreased if the existing 100 ohm Rs were replaced by 250 ohms.
Same confusion: Rp is the equivalent PARALLEL resistance, which is
pretty close to the value of the physical resistor used in the
paralleled L-R circuit. If you increase that resistance, then by
definition it's R-p-for-parallel that you are increasing. (The
transformed equivalent Rs DEcreases.)
>However,
>if the 100 ohm resistor were paralleled by a 250 ohm resistor, as Mr.
>Stewart suggests, the resultant 71 ohms of Rs Increases Rp, which
>would move the Rp curves farther apart.
>
Same again: the 100 ohm resistor is an Rp because it's in a parallel
circuit. Adding a shunt resistor will DEcrease Rp.
N7WS's own interpretation is correct. If you add 250 ohms of resistance
in parallel with the already paralleled 100 ohms resistor and L, you're
DEcreasing R-p-for-parallel to 71 ohms. This decreases the Q of the
paralleled L-R circuit and shifts the whole Q curve downwards, so that
it practically sits on top of the two nichrome curves.
In summary, what N7WS's measurements actually showed was:
1. At VHF, the Qs of the two different types of suppressors were very
similar. The Qs of the two nichrome suppressors were almsot identical,
but the Q of the conventional suppressor was 40% higher. This difference
could be completely eliminated by shunting 250 ohms of additional
resistance across the coil, to bring the Q down.
2. If this was done, the Qs of the two different types of suppressors
would track closely across the low VHF range, increasing towards lower
frequencies. This is what parasitic suppressors are supposed to do, but
the conventional paralleled L-R approach does it better because its Q
increases faster as the frequency is reduced into the HF region.
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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