PRINT  "A program for interdigital filter design"
 PRINT "Author Prof.C.J.Kikkert Amiga version Oct 1988"
 PRINT "Based on Dishal M. IEEE MTT 13 no 5 Sep 65"
 pi=3.141592654# : pi2=pi/2!
 c=3E+11
 Flog10=LOG(10)
start:
 INPUT"   Centre frequency in MHz =";fc
 INPUT"          Bandwidth in MHz =";bw
 w=bw/fc
 IF w>.1 THEN 
   PRINT "Warning Dishal's equations are only accurate for narrow bandwidths" 
   PRINT "the bandwidth selected is too big to give accurate results"
   PRINT "To go back to the start of the program enter a negative number of resonators"
 END IF
 fc=fc*1000000! :bw=bw*1000000!
 resonlength=c/(fc*4!) :resonlp=.9*resonlength :cavhgt=resonlength
 PRINT USING "    Ideal resonator length = ######.# mm";resonlength
 PRINT USING "Suggested resonator length = ######.# mm";resonlp
 PRINT USING "Suggested    Cavity height = ######.# mm";cavhgt
 INPUT"      Number of resonators =";nres
 IF nres < 0 THEN start
 nres1=nres+1
 nres2=nres-1
enttype:
 PRINT "Do you wish to use 1 Normalised Low Pass Filter tables"
 PRINT "                   2 K and Q tables"
 INPUT "  Enter selection";selec
 IF selec=1 THEN lowpass
 IF selec=2 THEN kandq
 PRINT "Incorrect selection" : GOTO enttype
lowpass:
 PRINT "Enter normalised components for equilalent low pass filter from"
 PRINT "filter tables, Zverev pp312-340 or Matthaei Young & Jones Ch4"
 FOR i=1 TO nres1/2
   il=2*i-1 :ilm=nres1-il
   PRINT USING "C## or g## =";il;ilm;
   INPUT z(il)
   il=il+1 
   IF il<nres THEN
     ilm=nres-il
     PRINT USING"L## or g## =";il;ilm;
     INPUT z(il)
   END IF  
 NEXT i
 PRINT USING " Rs or g## =";nres1;
 INPUT z(0)
 z(nres1)=1
 q1=z(0)*z(1)
 qn=z(nres)*z(nres1)
 FOR i=1 TO nres-1
   k(i)=1/SQR(z(i)*z(i+1))
 NEXT i
 GOTO spacing
kandq:
 PRINT "Enter k and q values from filter tables, Zverev pp 341-379"
 INPUT "   Normalised input Q (q1) =";q1
 INPUT "  Normalised output Q (qn) =";qn
 FOR i = 1 TO nres2
   PRINT USING "Coupling (k) rod ## and ## =";i;i+1;
   INPUT k(i)
 NEXT i
spacing:
 q1=q1*fc/bw
 qn=qn*fc/bw
 piw2=pi*w/2!
 INPUT "          Rod diameter mm  =";d
 hrec=2.8*d 
 PRINT USING "Suggested side wall spacing ####.## mm";hrec
 INPUT "      Side wall spacing mm =";sidewall
 hrec=sidewall*.7
 PRINT USING "Suggested end wall spacing to 1st rod = ####.## mm";hrec
 INPUT "       End wall spacing mm =";endwall
 z0=138*LOG(4*sidewall/(pi*d))/Flog10
 peh=pi*endwall/sidewall :eppeh=EXP(peh) :enpeh=EXP(-peh)
 taneh=(eppeh-enpeh)/(eppeh+enpeh)
 z01=138*LOG(4*sidewall*taneh/(pi*d))/Flog10
 FOR i = 1 TO nres2
   k(i)=k(i)*w
 NEXT i  
 ke=SQR(z0/z01) 
 k(1)=k(1)*ke
 k(nres2)=k(nres2)*ke
 sum=0
 PRINT "rod number to number  spacing in mm"
 FOR i=1 TO nres-1
   ch(i)=((-LOG(k(i))/Flog10)+(.91*d/sidewall)-.048)/1.37
   ch(i)=ch(i)*sidewall :sum=sum+ch(i)
   PRINT USING "    ##       ##         ####.##";i;i+1;ch(i)
 NEXT i
 sum=sum+2*endwall
 PRINT USING "       Total filter length = ####.## mm";sum
 INPUT "           Input impedance =";rin
 INPUT "          Output impedance =";rout
 zin=q1*4!*z01/pi :zout=qn*4!*z01/pi
 dsin=SQR(rin/zin) : dsq=SQR(1-dsin*dsin) :dsin=ATN(dsin/dsq)*180/pi
 dout=SQR(rout/zout) : dsq=SQR(1-dout*dout) :dout=ATN(dout/dsq)*180/pi
 intap=dsin*resonlength/90! :outap=dout*resonlength/90!
 PRINT USING "       Input tapping point = ####.## mm";intap
 PRINT USING "      Output tapping point = ####.## mm";outap
 END