BUILDING A DIPOLE THE EASIER
WAY! This short bit of information hopefully
will save you much time when building a dipole.
It concerns hams that want to make a dipole, (any frequency),
so they usually begin by using the standard formulas below :
234 /
design frequency = each dipole side length in feet. or 468 / design
frequency = total length in feet
The only
problem with this, when you put it in the air, it isn't even close to 1.0
SWR.
In fact the antenna is resonant way low in frequency
or just the opposite, too high. So......up and down, up and down,
up and down.... you start chopping off or adding pieces of wire until the
antenna comes up to your design frequency. This can take many, many
trips to the rig, back to the antenna, back to the rig, etc, etc and can
be very time consuming to get the dipole tuned where you want
it.
The solution to
fewer trips up and down guessing at tuning the antenna:
Frequency scaling using a bit of simple
math!
Example: Lets say you want a dipole
for 18.130mhz. You just used the standard formula 234/freq = length of
each half in feet or 468/ freq = both lengths total in feet...you then cut
each wire to 12.9 feet from the formula.....(234 / 18.130) You
put the antenna in the air and to your horror the antenna is resonant at
17.80! (You rotten no good sacafragamatza punka
mocha fritzalitz !!!!!!!!!) (Choice words edited out due
to family rated website)
What to do?
Here comes that bit of math with a
formula to save the day!
The
formula = Old Frequency (17.80) / New Frequency (18.130) X Original length
from the formula, (12.9 feet) = New dipole length (12.7 feet). Now
since it's much easier to use inches rather than 10th's of a foot, doing
the math to find out how many inches 12.9 feet is:
12
feet + .9 feet = how many feet and inches?
12" X .9
= 10.9 inches. Add this to 12 feet.....12 feet + 10.9 inches rounded off
=
12 feet 11
inches!
Doing the
same with the new dipole length.....gives us, 12 feet + 8.4 inches = 12
feet 9 inches (rounded off) for the new length!
So there is a
difference of about 2 inches between the old length and the new length.
Now since the new length is shorter than the old length...you cut of
2 inches from the original length that you arrived at using the formula
the first time.
This should get you very very close to the exact frequency
using this formula!
If by some
quirk of Murphey's law it still is off a bit, just repeat the procedure
again with no guesswork involved!!!!!
Here is
another example using the same frequency, 18.130mhz and you find the best
swr is 18.500mhz. Now the antenna is toooo short....but how
much?
18.500 /
18.130 X 12.9 feet = 13.16 feet so........,
the
difference between the new length and the old length is 13.16 feet - 12.9
feet = .26
feet which is what would have to add to the original length to
make it longer and much closer to the correct length. So we get .26 feet
added to 12.9 feet = 13.16 feet for 1/2 of the dipole!
Just in case
your need to know, a foot divided into 10th's of a foot = 1.2 inches per
10th of a foot. Don't let this formula confuse you. It is
not a formula for designing a 1/2 wave dipole......not even
close...it only helps you tune the antenna much quicker than the cut or
add and try method! Math comes to the rescue from lessons in school, years
ago, that you thought you would never use! Thanks to the teachers, math
made your antenna building experience much easier!
Editor's note:
Here is another way of stating the formula in real life
words:
Unwanted lowest swr frequency divided by Wanted frequency
multiplied by results of standard formula equals new length for half of
the dipole.
Don't forget to add length when the
dipole is too short and to take away length when it is too
long!
Practice the
formula in your spare time using various design frequencies and
results. This formula works very well and has been around for a long
time. Just thought I would send it your way. I use this technique all the
time, I'm just too old (smart)..... to make all those trips anymore. I am
interested in getting it on the air, not on the ground! Have
fun! 73
