I've been working through some Pi-network design for an input tuning
network recently and have tried out several online calculators as
well as traditional formulas and common software. Using the same
input variables for each tool, I generated three very different
results.
For Rin = 50 ohms, Rout = 135 ohms, f=28.03 MHz, Q=2.7
Group 1: Tonne's PI-EL software, ARRL Handbook formulas, and Jim
Hawkins online Java Radio Calculator.
Cin = 93pF, L=457nH, Cout=79pF
Group 2: VK1OD online calculator, electronicdesign.com calculator,
and raltron.com calculator.
Cin = 307pF, L=251nH, Cout=194pF
Group 3: EEweb.com calculator, VK2ZAY online calculator, and
Impedance Matching Network Designer online calculator (sandiego.edu).
Cin=163pF, L=383nH, Cout=113pF
So then, you can see my dilemma here. Why are these so different?
And why the common groupings?
Steve, KK7UV
#### The above was from way back in Nov, 2013.
Using the GM3SEK PI software, it's easily explained. In Group 1, an an
OVERALL network Q of 2.7 is used. IE: input Q of .8 and an output
Q of 1.9 (.8 + 1.9 = 2.7)
In Group 2, an INPUT Q of 2.7 is used ! (and an output Q of 4.5)
Overall network Q = (2.7 + 4.5) = 7.2
In Group 3, an OUTPUT Q of 2.7 was used. (input Q is 1.4)
Overall network Q = (1.4 + 2.7) = 4.1
In EACH case, when the OVERALL network Q is inputted into the GM3SEK
PI software, it spits out IDENTICAL results, as steve quoted.
for each of the 3 groups.
There is no real discrepancy per se. The discrepancy lies in whether
one specifies OVERALL network Q, OR INPUT Q... OR OUTPUT Q.
Myself, I use an overall network Q of 4. When using tubes with a
50 ohm input Z, both the input and output Q = 2.
Note GM3SEK's PI net software will not handle 2 identical Z's.
To fool the software, the output / cathode /tube end is inputted
as 50.00001 ohms. Input Z is entered on Line 35. Output Z is on
line 37. Enter 0 in all other fields EXCEPT the plate choke.
Enter 9999 for the plate choke.
Note his group 2 has way too high an overall Q for use as a
tuned input. It would be extremely narrow banded.
Ok, the mystery is finally solved after just 9 years !
Later.... Jim VE7RF
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