# J-pole equations, program, & info

From: KE3FL@delphi.com
Date: Sat Apr 27 1996 - 21:18:58 EDT

To ALL at QRP-L:

The J-pole equations I use in my program are a combination of those in
QST September 1994, pg. 61 & 62 in an article by Jim Reynante/KD6GLF
and those in an article by John S. Belrose/VE2CV also published in QST
in April of 1982.

I programmed these equations into the present program which allows for
different wire velocity factors, and for display of the lengths in either
English or Metric. The calculation results can also be sent to a printer
by setting the printer toggle switch to the ON position (Tools Menu).

I have also included a change calculation that computes the expected
change in frequency due to a length change. Remember: cutting the
antenna increases the frequency (decreasing the wavelength) and adding
length to the antenna decreases the frequency increasing the
wavelength). The equations I use are:

factor = 11811 (English, inches) or 30000 for (Metric, cm)
delta = 0.25, .5, 1.0, 10, 20", or .5, 1, 2, 25, or 50 cm
The size of the recommended cut, dependent on frequency.

lambda = (factor/freq)
stub = vf * lambda / 4 (vf = .81 - .85 for twin-lead)
radiator = vf * lambda / 2
tap = lambda * .0155

gap = .25

changes = freq - [(vf * factor/4)/(stub+delta)] MHz per delta cuts
(My program is available from Circle Software, PO Box 74, Mt Airy MD
21771 for \$12, + md state tax + \$3 shipping.)

One last thought. If you already know the size of the antenna for 52 MHz
simply scale it for 50.125 MHz as in:

(SIZE * 52)/50.125 = New size

Example: SIZE = 141.4 ft then New size = 146.7 ft.
(The info from my program below is no better than this!)

Using a velocity factor of 0.83 my program gives:

Freq Stub Radiator Total Tap Gap
MHz in in in in in
52 47.1 94.3 141.4 3.5 .25
50.1 48.9 97.8 146.7 3.7 .25

I hope this helps. 72 & 73 de KE3FL/Phil
:-)

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