W5FD Pi and Pi-L network programs included are:
PI7-CMIN.EXE a pi-network program
PI-LCMIN.EXE a pi-L network program
R2STEP7A.EXE a pi-network program for cathode drive tuned
input circuits
The network formulas and procedures are from reference (a).
Circuit element designation symbols follow those in reference
(a). There are some additions to the reference (a) material to
handle the new Cmin feature.
PI7-CMIN and PI 11211e411l -LCMIN calculate tables of values for pi and pi-L
networks for use as RF amplifier output tank circuits.
The user enters the desired R1 starting value, the R1 ending
value, the R1 step rate and the desired Qo value. In addition
the user enters the value of Cmin in pF where Cmin is the best
estimate of the total capacitance, plate to ground, that will be
present when the plate tuning capacitor is adjusted to its ap-
proximate setting on the highest band (10 meters). This Cmin
value includes the tube output capacitance, stray circuit capaci-
ty, the minimum capacitance of the variable tuning condenser, and
a reasonable amount of capacitance for tuning adjustment. The R2
load or antenna end resistance is fixed at 50 ohms. The RM,
intermediate value in the Pi-L network, is fixed at 300 ohms.
Additional discussion of the Cmin feature is given below.
R2STEP7A is for generating tables of values for pi-networks
intended for use as grounded-grid (or cathode driven) tuned-input
circuits. The user enters the R2 starting value, the R2 ending
value, the step rate, and the Qo value. R2 is equivalent to Rk,
the cathode driving impedance. R1 is fixed at 50 ohms corre-
sponding to the customary designed 50 ohm load for the RF excit-
er. The user should also enter the C-input value in pF where C-
input is the tube cathode input capacitance including cathode to
grid capacitance plus circuit stray capacitance including that
from a coax feed, if used, from the bandswitch to the tube cath-
ode. The program simply subtracts this C-input value from the
calculated C2 value and prints out the proper value of C2 to add.
This C-input value is termed Ck on the printed table of values.
Program operation details:
These programs are intended for generating tables of values for
pi and pi-L networks and program output is to the printer only.
No provision is made for output to the CRT (screen). If the user
desires the program output may be redirected to a file by using
the PC Magazine program PRN2FILE.COM and this file viewed on the
CRT by a word processing or editor program, by the PC Magazine
file BROWSE.COM or by the widely used LIST.COM, a Shareware
program. Else the file may be viewed by the DOS TYPE command:
C:>TYPE Filename.ext | MORE <cr>. Or the program output may be
sent to the printer directly (default).
When the PI7-CMIN or PI-LCMIN program is run to generate a table
of values the main program will calculate a value for C1 and if
this calculated value is less than the entered value for Cmin the
program switches to an alternate procedure using the Cmin value
as the C1 value. This is done on 10, 15, and 20 meters only
since it is unlikely that the main program calculated value for
C1 would ever be less than Cmin on 40 meters or lower frequency
bands. This Cmin addition feature is intended to provide for
more practical and useful Pi and Pi-L networks by taking aware-
ness of the fact of the always present tube output capacitance
plus stray circuit capacitance plus the minimum (completely
unmeshed) output tuning capacitance plus a reasonable allowance
for tuning adjustment. The currently published table of values
for pi and pi-L networks contain C1 plate tuning capacitance
values as low as 8 pF and this is completely impractical since
the typical RF output tube plate output capacitance for RF ampli-
fiers with 1500 watt PEP output capability is about 10 pF. To
this must be added approximately 20 pF for the minimum capacity
of the RF output variable air tuning capacitor, or 10 pF for a
vacuum variable. To this must be added the circuit stray capaci-
tance plus a reasonable amount for tuning adjustment. The intent
of this Cmin addition is to provide the amplifier builder with
practical pi and pi-L networks that will require a minimum, or no
cut and try, and the builder will also know the circuit parame-
ters.
All programs print out a header and 7 R1 (or R2 for R2STEP7A)
table values then issue a formfeed and continue to print out as
many pages as required.
When the calculated value for C1 would be less than the Cmin
entered the program uses Cmin as the value of C1 and calculates
the pi or pi-L network based on this Cmin value. The resulting Qo
value will exceed the entered Qo value and this Qo value is
printed out on the right side of the table line involved. This
increase in the operating Qo value is unavoidable. It is not
caused by the program but simply reflects the conditions that
exist in practical amplifiers and makes this known to the design-
er/builder.
It is desirable to keep Qo in the range of 12-20 for proper tank
circuit operation while maintaining good tank circuit efficiency.
This is not always possible on the higher frequency 10, 15, and
20 meter bands because the minimum capacity plate to ground is
too high to permit it. This requires a tank circuit design that
takes this minimum plate to ground capacity into account and this
is done in the W5FD Pi-CMIN and Pi-LCMIN programs. The partial Q
value in a Pi or Pi-L network, Q1=2*pi*F*C1*RL is the major
contributor to the total tank circuit Qo value. Cmin is the
minimum value of C1, RL is the required plate load resistance for
the tube for the selected operating conditions.
A suggested average value for Cmin for a tube or combination of
the popular Eimac tubes used by radio amateurs at an output power
level of 1200-1500 watts PEP is 35 pF where the plate tuning
capacitor is a vacuum variable such as the Jennings 10-300 pF and
a Cmin of 45 to 50 pF if a standard transmitting air variable
tuning capacitor is used. The minimum capacitance of transmit-
ting air variables varies depending upon the construction. In
general larger, higher voltage air variables have higher minimum
capacitance values. The usual average sized transmitting air
variable will have a minimum of about 20 pF and this can go up to
30 pF or more for physically larger higher voltage air variables.
In any event it is up to the user to determine the total Cmin
value and enter it into the program, the suggested typical values
given above are considered to be suitable values for the Eimac
3CX-1500A7 and 3CX-1200A7 tube RF amplifiers. The remarks given
above concerning transmitting air variables apply for those with
maximum of 250-350 pF and 2500-4000 volt ratings.
The C-input feature in the cathode drive circuits provided by
R2STEP7A.EXE is entirely different than the Cmin feature in PI7-
CMIN and PI-LCMIN. The C-input value entered by the user is
simply the total cathode (filament) to ground capacitance plus
stray and wiring capacitance and this value is just subtracted by
the program from the C2 calculated value and the result is print-
ed out as the C2 value. The actual network in-use value for C2 is
the program value C2 plus the C-input value. The program output-
ted C2 value is the value to use in building the network.
These programs run on IBM-PC and PC compatibles with DOS 2.1 -
5.0 Versions.
I will include the PC Magazine program PRN2FILE.COM on this disk.
This is a free program for non-commercial use, copyright by PC
Magazine. It will redirect the output of the W5FD Pi, Pi-L, and
R2STEP7A programs to a file:
Usage: C:>PRN2FILE CAPTURE.TXT /B24 <cr> (see PRN2FILE.DOC for
info). Note: " <cr> " is same as < ENTER >.
The /B24 is optional. Substitute your desired filename for
CAPTURE.TXT. This will redirect any output to the printer to
CAPTURE.TXT including the formatted output of the W5FD Pi, Pi-L,
and R2STEP7A programs. The data may afterwards be sent to the
printer thus: C:>COPY CAPTURE.TXT PRN, or by the DOS PRINT com-
mand, etc. For proper printing do not add any printer control
commands such as skipperf. A added Left Margin of up to 1/2 or 1
inch is acceptable and desirable for filing the Tables in a
3-ring notebook.
I also will include the PC Magazine program BROWSE.COM, a free
program for non-commercial use, copyright by PC Magazine. It is
for viewing files and is more versatile than using the DOS TYPE
command. Usage:C:>BROWSE FN.EXT<cr>. Uses HOME and END,PgUp,PgDn,
and arrow keys, /W option for Wordstar files. To return to DOS
press ESC key or Ctrl+Break.
CTRL+C or CTRL+BREAK will end any of the W5FD programs in case of
an entry error or lock up. In using PI7-CMIN.EXE answer the
printer on-line question with Y (yes) if the printer is actually
on-line or if PRN2FILE has been run. Entry of a N (no) will end
the program and return to DOS.
Any of the programs may be single-stepped by entering the same
value at the "R1 step from ?" and the "R1 step to ?", and <cr>
(ENTER) at "step rate". Enter the desired Qo value.
The Cmin entry in PI7-CMIN and PI 11211e411l -LCMIN or the C-input entry in
R2STEP7A may be entered as 0 (zero) and the programs will provide
regular Qo based network solution values without the Cmin or C-
input features. You will note impracticably low values of C1 for
the Pi and Pi-L networks as R1 goes to higher values. A run of
similar R1 and Qo values in PI7-CMIN or PI-LCMIN with a proper
Cmin entry will provide an instructive comparison. The C2 value
in the R2STEP7A programs will be higher than the proper value
for C2 by the amount of cathode to ground circuit and tube capac-
ity, C-input.
The PC Magazine utility programs included on this disk are copy-
right by PC Magazine. They are free programs for non-commercial
use and may be freely distributed but no charge can be made. The
conditions for use of the W5FD programs is given in the Copyright
Notice displayed when the program is run.
73, W5FD
6-92
Ref(a): "New and Improved Formulas for Pi and Pi-L Networks",
Elmer A. Wingfield, W5FD, QST August 1983, page 23.
See also QST January 1984, Feedback.
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