Foundations of Amateur Radio #175:
October 12, 2018
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Everything you know about dipole
(calculators) is wrong ...
The other day I did an experiment. I searched
for "dipole calculator" and using the first
20 results I calculated the length of a
dipole suitable for 7.130 MHz. I chose the
frequency for no other reason that there is a
7130 DX net every Monday, Wednesday and
Friday and for the longest time I've been
unable to participate due to the lack of a HF
antenna in my new shack.
So here's some things I learnt from doing
Depending on which calculator I use, the
length of my dipole can vary by over a meter
from longest to shortest result.
Depending on my desire to use metric or
imperial measurements, my dipole will be a
different length, because of course electrons
move at a different speed if you're not using
the metric system.
In case you're wondering, 1 inch is defined
as being exactly 2.54 cm, so there's plenty
of opportunity to vary that.
Speaking of standards, we all agree that the
speed of light is a constant, right? Turns
out that for some calculators, you can change
the speed of light.
I'll skip over the notion that none of the
calculators actually show what they're using
as the speed of light and move on to other
Apparently you can determine the length of a
dipole down to the sub-atomic length, with
one calculator going down to the size of an
electron to indicate how much wire you should
cut from a spool.
There are forms that make doing the
calculation really easy, single box to type
in the frequency, so the answer must be
There are some that use random standard
numbers, even a text book example that uses
some number, but no indication where it comes
from. For example, the number 486 features
regularly, but so does 150 and 5905.
There are forms that provide you with several
boxes, but no indication which box needs what
value, so your answer may or may not indicate
the number of eggs per chicken per parsec.
One dipole calculator result is actually for
a vertical, so your search engine helping you
might not actually give you the calculator
There are percentage correction factors. 5%
seems to be a favourite number, but no
indication as to what the origin of that
There's a calculator that allows you to
specify the feed point impedance, not sure
how that works, but it's a nice feature to
have when you're calculating the length of
your dipole. Not.
One regular instruction is to cut long, that
is, measure your wire and cut it longer than
the calculator states. How much longer is
left as an exercise to the reader. Should it
be 1 mm longer, 1 cm longer, or should it be
1 m longer and how much should that change if
the frequency changes?
Let's move on. The word ground features
heavily in these calculators. The phrase
"average ground" does too. No indication as
to what makes an average ground, or how to go
about determining what changes if your ground
We all agree that the dipole should be half a
wave-length above the ground, right?
How much is that?
The same wave length as the length of the
dipole we've just calculated, or a different
How does the length of the dipole vary if the
height varies? While we're looking at
variation, how much variation is there
depending on how thick the wire you're using
is and what about insulation? None of those
things are even mentioned in any of these
Dipole calculators, wonderful invention,
shame about the implementation.
I'm Onno VK6FLAB
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