Foundations of Amateur Radio

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 this experiment.

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 interesting discoveries.

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 right.

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 you expect.

There are percentage correction factors. 5% seems to be a favourite number, but no indication as to what the origin of that number is.

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 isn't average.

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 one?

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 calculators.

Dipole calculators, wonderful invention, shame about the implementation.

I'm Onno VK6FLAB