Foundations of Amateur Radio
Today I'm going to talk about dipoles. You know the tried and true antenna, the go-to design for getting on air, the simple first antenna you ever make, the one you learn from, you know the one. It's the mainstay of every amateur, of any field-day, of all things Amateur Radio.
It's a simple thing. Using metric, rather than imperial measurements, but the point stands, you use the speed of light in vacuum divided by the required operating frequency and you get the overall wave length for that frequency. In absolute terms, roughly 300 m/s divided by 50 MHz, gives you 6 meters. Surprise, that's the band name for 50 MHz.
Now the dipole is a half-wave contraption, so, 6 meters divided by 2 gives you 3 meters for your total half-wave dipole. Each leg is half that, a quarter wave length, so you have 3 meters divided by 2 again and you end up with two bits of wire, a meter and a half long each.
If you're following along, that's 300 divided by 50 divided by 2 divided by 2, or using the same numbers in a slightly different order, 300 divided by 2 divided by 2 divided by 50 MHz, or 75 divided by 50 MHz. Still one and a half meters per leg of your shiny new dipole.
So, the basic formula for a metric dipole can be stated as 75 divided by the frequency in MHz per leg. If you're playing with feet and inches, 300 m/s becomes 984 ft/s, half that is 492, half that is 246 feet, so the imperial version is 246 divided by the frequency in MHz, gives you length in feet for each leg.
So, you've cut your wires, tied them to some magical feed point contraption, plug it into your radio and you're good to go, right?
Now, anyone who's actually done this knows that this is not what will actually happen. It's never that simple, and frankly if it were, we wouldn't be Amateurs, we'd be, unlicensed or something.
So, what affects the actual length of this magical antenna? Lots and lots of things. Here are a few that come to mind:
- The thickness of the wire you're using.
- The thickness of the insulation on the wire.
- A thing called the end-effect.
- The height of the contraption above the ground.
- The kind of ground.
- The price of the copper you're using.
Sorry that last one isn't right. I have been told, time and time again that there are only two kinds of wire, cheap wire and free wire. The preference is for the latter. So, price of the copper doesn't matter. Another thing that does matter is that some wire can stretch while in the air, making the antenna longer, so keep that in mind.
Fine and well I hear you say. But how does this really matter?
If you increase the thickness of the wire, the resonant frequency goes down, that is, the antenna is "too long". If you increase the thickness of the insulation, the resonant frequency goes down as well. The end-effect is like adding a capacitor to the end of the wire, making it "longer" as well. The height effect is different for each height. Generally the effect is that the antenna resonates at a lower frequency. Each type of ground has a different amount of effect. Water vs rock vs sand vs clay.
I can hear you groan at this point.
First comment to make is that all of these effects make the antenna resonate at a lower frequency. This means that the suggestion to cut your antenna longer than the calculated number doesn't make much sense. If you start with the basic calculation, 75 divided by the frequency in MHz, you'll end up with an antenna that's extremely likely to be too long.
I can hear you screaming at me right about now. Hold your tar and feathers. I gave you the First comment. Here's a Second one.
If you don't have space for a straight dipole, say, you need to go around a corner, or put a bend in the wire, all of what I just said goes out the window. If you put this above a metal roof, poof, also out the window. If you cannot terminate the wire to a rope without bending the end of the wire, poof.
A Third comment. You cannot cut wire longer. You can only cut it shorter. Cutting it longer is called soldering and that's a whole 'nother thing.
Let me finish with some tips for new players.
Don't ever cut your wire. It won't work like you expect and it will only give you more work. Always, always, always, fold your wire back over itself and wind it back over the end. If you leave it dangling you're adding capacitance and doing all manner of other weird stuff to your antenna.
Also, if you need extra length because the 80m dipole just won't fit into the back yard, you can use the end capacitance to good effect, hang it down towards the ground and you'll end up with an antenna more to your liking.
Final comment and then you can start sending me emails about how I'm wrong.
The humble dipole antenna is a magnificent contraption. It has more variables than you can poke a stick at and anyone who tells you that it's just a case of calculating this by doing a simple division hasn't got the faintest idea of what's actually going on. There are times when simplification is helpful. This is not one of those times.
So, have fun, play with your dipole, try different things and observe the differences when you try different things. I've spoken with Amateurs who've been doing this longer than I've been alive and they still can't calculate the actual length of a dipole, put it in the air and have it work first time.
Sometimes you get lucky. More practice, more luck.
I'm Onno VK6FLAB