What is a repeater offset and how does it work?

Foundations of Amateur Radio Every week I run a net for new and returning amateurs. A variety of people join in with varying degrees of skill, knowledge and number of birthdays. One of the regular things I say during that net is that if I'm not acknowledging you, it's because I cannot hear you. I then start a spiel about repeater offsets and give some examples, but what is it really and how does it work? As you might recall, a repeater is a radio, generally located somewhere useful, like on a hill or tall building, that offers the ability to talk to other amateurs who are not within range of your radio. For bands like 2m, 70cm and 23cm, generally speaking, contacts are line-of-sight. If you're standing on a hill, you can talk to more people because your line of sight is further away. This is also why you can talk to the International Space Station with a hand-held, since it's in your line-of-sight, at least some of the time. A repeater acts as a line-of-sight extender. If it can see both you and another station, it can act as a bridge between you. How it does this is pretty simple. A repeater listens to your signal and transmits that to the other station. It uses two separate frequencies to make this happen. A receive and a transmit frequency, or more precisely an input and an output frequency. To remember which is which, you can think of a repeater as a giant megaphone, you talk into it and sound comes out. Said differently, think of a repeater as a device that takes an input from one station and makes an output for everyone to listen to. To actually use a repeater, your radio needs to be setup to transmit on the repeater input and it needs to receive on the repeater output. This means that when you transmit, the repeater can hear you and when you're listening, you can hear the repeater. To achieve this, you can set your radio up using repeater mode. It uses a thing called an offset to set the difference between the input and output frequencies. To find out what the offset is, you take the repeater input frequency and subtract the repeater output. If you've set-up your radio correctly you're tuned and listening to the repeater output. When you hit the Push to Talk or PTT, you'll transmit on the input frequency and when you let go, you're back to receiving on the output frequency. One final roadblock might be that your local repeater has a tone lock. If it does, the repeater will ignore you even if you have all the frequencies correct. This tone is generally published by the repeater owner or your local regulator. You can also check a website called repeaterbook.com to see many of the world's repeaters and their specific settings. Now, I should point out that while repeater offsets are standardised, they're not the same across bands, across the world, or even within a country or city. Depending on where you are, what the density of repeaters is and what band you're on, the offset number and direction will change. It's even possible that you have a variety of offsets on the same band in the same city. This means that you cannot just pick a standard offset for your radio but most modern radios will have a method to deal with this. It's easy to get this wrong. Setting up your radio for using a repeater is deciptively simple. Three things to look out for when it's not working. You have the input and output reversed, the offset is wrong, or there's a tone blocking your transmission. I'm Onno VK6FLAB