Foundations of Amateur Radio

How far can I talk on radio?

A question that regularly hits the enquiring minds of people who are not (yet) radio amateurs is one about distance. For both amateurs and those who are not ye) inducted into our community the concept of distance speaks in ways that other parts of our hobby don't. It's a simple concept, between these two points, how far can you talk?

The interesting thing to me about this phenomenon is that distance isn't a metric that we as amateurs use for anything other than calculating repeater coverage and then only for frequencies that are line-of-sight. If you're not an amateur then this might be unexpected or even illogical.

Let me give you two questions:

How far can you talk in amateur radio? - and - How far does light shine?

If you're an amateur you'll know that those two questions are pretty similar, if not identical for certain frequencies, but if you're not, then these two questions appear completely unrelated to each other.

Let me start with something that you might not realise. If you tune to a local AM radio station, let's say ABC 720 in Perth. It's located in the AM broadcast band and the number of the station, 720, is the frequency at which it's transmitting. 720 kHz, or 720 thousand Hz. If you had a radio capable, you could turn the dial to the right, and after passing 810 Radio National, eventually, if you kept turning to the right, you'd find ABC Classic FM at 97.7fm in the FM broadcast band. The station indicator, 97.7 is again the frequency, 97.7 MHz, or 97.7 million Hz. So, 720 and 97.7 are both on the same dial, just at different ends.

Now if your radio was capable, you'd be able to keep winding it to the right, and after passing by Wi-Fi, at 2.4 GHz, or 2.4 billion Hz, you'd eventually come across light. Green light for example is about 560 THz, or 560 trillion Hz. You could keep going and end up with even more exotic stuff, like X-rays and Gamma-rays, in the exahertz range, a 1 with 18 zeros, but you get the point. Radio and light are the same thing. If fact, there are experiments around that are using light for Wi-Fi communications.

So, How far does light shine is the same thing as How far can you talk in amateur radio?

Before you start complaining about when it's different, let me point out that the only difference between these two is the frequencies at which we're comparing, with the characteristics that come with that. I'll get to that in a moment.

Look at light.

If you have a light bulb that's bright enough, you can see it in full daylight. If it's dark outside then you'll need less of a light bulb to see it. If it's raining, or if there is smoke in the air, you'll need more. If there's a wall between you and the bulb, you'd need a pretty bright light to shine through the wall, but you already know this. Covering up a torch with your palm shows the bones in your hand. Light gets through different parts of your hand in different ways.

Another thing you've seen is when you put a straw into a glass and it looks like it's broken. That too is related to how light travels through different materials. You may even have been underwater in a pool and looked up to see a reflection. That too is a phenomenon familiar in amateur radio.

Something that you might not realise is that something like an X-ray is identical to shining a light of a torch through your palm. Only X-ray's are used for diagnostic purposes, we shine an X-ray light at your body and some gets through and some doesn't. We take a photo of that and use it to figure out what's under your skin.

Back to radio.

The same phenomena happen in radio. Buildings are good at stopping certain radio frequencies, in much the same way as they block light, but other frequencies barely get noticed, they shine right through. Similarly, the ionosphere around the earth can act as a reflection like the surface of a swimming pool for some frequencies, but not for other frequencies. Interestingly this changes throughout the day, depending on the sun and a whole range of other factors which I'm not getting into today.

Finally, just like with light, you can turn up the brightness for different effects, you'll get further, but only if the conditions allow for it.

To answer the original question about how far you can talk on amateur radio becomes much harder and now you know why.

I'm Onno VK6FLAB