Here's a little BASIC program that will give one an approximation
for the length of 450 ohm ladder-line needed to get close to a
current maximum (low resistance/no reactance). It works for the
average HF wire dipole. The supporting data was obtained from
a 102 ft dipole at 37 ft. It works best for 80m-30m, probably
the bands of choice for qrp.
10 CLS
20 INPUT "Enter Frequency in MHz ", freq
30 INPUT "Enter Dipole Length in Feet ", diplenft
40 dipwl = diplenft / (936 / freq)
50 IF dipwl < .5 THEN dipwl = dipwl + 1
60 IF dipwl < 1.5 THEN GOTO 90
70 IF dipwl > 1.5 THEN dipwl = dipwl - 1
80 GOTO 60
90 fedlinwl = .25 - (TAN(2.5 * (dipwl - 1))) / 12.02
100 fedlinft(0) = (886 / freq) * fedlinwl
110 FOR i = 1 TO 7: fedlinft(i) = fedlinft(0) + i * (443 / freq): NEXT i
120 PRINT "Imax points at"
130 FOR i = 0 TO 7: PRINT fedlinft(i), : NEXT i
140 PRINT : PRINT : GOTO 20
150 END
One can use this program to test to see if one's present 450 ohm
ladder-line fed dipole is being fed close to a current maximum.
73, Cecil, W6RCA, OOTC
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