w5fd-pi.doc

[ARRL.无线电通讯手册].The.Arrl.Handbook.For.Radio.Communications.-.2007

        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-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-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.