When you are to get ready to key up for the first time, there are three things in particular, that need to be on the money: input frequency, tone, and offset. As in, if you don’t have these things set correctly, you will not be able to reach the repeater so they are some pretty important things.
The input frequency is the repeater frequency you key into your radio. This number reflects the frequency the repeater “hears” on, or the receiving frequency. There are a lot of aps for your phone that will tell you the repeaters closest to you, but I like repeaterbook.com. There you can download a .csv format of repeaters by county, by state, by band, or other attributes you’re interested in – like ARES, RACES, Linked Systems, highway route coverage, who operates the repeater, if it’s open or closed, etc
The next thing you will need is a tone. Now I’ve been accused of having a tone every now and then, but that’s really not the one I’m referring to.
See, most repeaters will not pass a signal on for retransmission unless it contains a tone. You set the required tone number into your radio and that number corresponds to a very specific sub-audible tone that is then transmitted along with your audio signal to the repeater. Each repeater has its own required tone, which indicates to the repeater that your signal is intentional and should be retransmitted.
There are at least two reasons for tones. Usually mine has something to do with my dogs or my man. But back to radio. The first reason for a tone is that most repeaters are close to other repeaters, especially here in the metroplex. The repeaters that are closest together in input frequency sometimes mix signals and create false tones called intermod or intermodulation. Without a tone, the intermodulation would be retransmitted. The second reason is actually the reason why tones were invented in the first place. Repeater access tones were actually created by Motorola to allow different commercial users to share a repeater without having to listen to each other’s conversations. This is why they are sometimes called PL Tones or Private Line Tones, which are a Motorola trademark. You may also hear the referred to as continuous tones, but don’t get this confused with Continuous Tone Coded Squelch Systems, or CTCSS, which I will explain in just a second.
If a repeater requires a tone -- setting one is not optional – you must have it set correctly in order to access the repeater.
CTCSS tones are optional and the key word to remembering the difference is the term “squelch”. Focus in on that one word if you are confused. If you squelch something -- you remove it. In addition to setting the PL Tone, you *may* also set a CTCSS Tone.
The concept is the same – you are still dealing with tones. The difference is, while PL tones work to get you into a repeater, CTCSS tones – or squelch tones – are used to filter out erroneously *received* signals from your radio.
Here’s an example for you. This is a linked repeater system, meaning there are two frequencies that are linked together – repeater A (442.325, the KM5R repeater) and repeater B (442.225, the K5HIT repeater). When a system is linked, whatever is transmit on one, transmits on the other. Here’s the rub. Repeater A and Repeater B have different PL tones. If you set your CTCSS tone on your radio to only receive PL Tones from Repeater A, you will not hear anyone coming in from the linked repeater B, because you have effectively squelched out the other tone.
Does your head hurt yet? Mine does! It took me forever to wrap my head around the difference. The way I finally remembered it was zoning in on the word “Squelch”. And after that, please know that this will be put into a blog post so you can go back and read them.
Alright, let’s let the tones rest for a bit.
The last thing you will need to get on the air is offset. Offset is the difference between the repeater’s input or receiving frequency (meaning what you say) and its output or transmitting frequency (meaning what the other guy hears). Two meter offsets can be either positive or negative and there is a .6 megahertz difference between the input frequency and the output frequency. The 440 or 70 cm band offsets are always positive and 5.0 megahertz. Most of the time, modern radios have an automatic offset function and you rarely have to set the offset.
If you want to know whether a repeater has a positive or negative offset, hit the PTT and throw your callsign out there. Watch the number on your screen. If the number went up when you pushed the PTT, you're offset is positive. If it went down, you have a negative offset. By default, if you have the offset correctly set, it allows you to determine the input frequency if, say, you were scanning the band and heard a transmission on the output frequency.
It's also possible to listen and transmit on the repeater's offset frequency and that's called simplex, but that's beyond the scope of this particular topic.
So that's a wrap. The three things you need to get on the air are : frequency, tone, and offset. I've hope I've been clear, it's a confusing subject and one it's taken me awhile to wrap my head around. So if you don't understand, don’t worry about it if you don’t grasp it immediately.