A digipeater, most simply put, repeats digital packets. However the how it repeats and what packets it repeats and why it repeats them are very important. Perhaps though we should get some terminology out of the way for the uninitiated.
Generally speaking, we are talking about licensed amateur radio operations, involving transmitting a burst of Radio Frequency (RF) energy on a specific frequency with the intent to receive it in another location, generally with another radio transmitter, for some purpose.
Licensed amateur radio operations: any person that has studied, tested, and received an amateur license, and the activities they use it for.
Burst of Radio Frequency (RF) energy: pushing the “push to talk”, commonly called the PTT (each letter spoken naturally) button of the radio causing electricity to radiate from the antenna.
Specific frequency: depending on the license class an amateur holds, we are limited to specific ranges of frequencies we are allowed to use without further cost or licensure. Each range of frequencies is referred to as a “band”, and the specific bands of frequencies are often referred to by the average wave length of the RF waves of its included frequencies. Such as the two meter band, including the frequencies from one hundred and forty four megahertz to one hundred and forty eight megahertz.
Other frequencies, not allocated to amateurs are used for commercial purposes and cost extremely high amounts of money, paid to the FCC for their task of allocation. Such as, commercial radio (a standard car radio) occurs between eighty eight and one hundred and eight megahertz. Air traffic uses from one hundred and eight to one hundred and thirty eight megahertz.
Another location: depending on many factors RF energy can be received and transmitted very small or very large distances.
Radio transmitter: could be a handheld walkie talkie, a mobile radio in a vehicle, or even a nine volt battery, twenty feet of wire, a few odd electrical parts an amateur stuffed into a tuna can.
Digital packets: a digital packet on a VHF frequency is usually a five to eight second burst. It can be heard on any analog radio tuned to the appropriate frequency, however it will just sound like an AOL dialup connection noise. The packet may contain up to fifty six characters of information, digitally (ones and zeros.)
Some purpose: The fifty six characters of data could be anything, most commonly they are the foundation of the APRS system in use across the globe. In the US, this activity occurs on the one forty four point three nine megahertz frequency.
APRS: Automatic Position Reporting System
Okay, those are some definitions I suppose, but what does that really mean? Well, let me explain how I use digipeaters. On my waist, I carry a small HT (Handie Talkie.) The non-amateur world would call it a walkie talkie, yet.. amateurs prefer the term HT (each letter pronounced naturally.)
My dual band HT (a Yeasu FT-3DR) also has a GPS chip and integrated antenna for it, allowing it to tell me where I am. That’s great and helpful information. However if you’re lost or incapacitated, you knowing where you are, may not be very helpful. Hence, the reason the GPS is coupled inside my radio transmitter. According to the settings I use, my HT sends out a digital packet every thirty seconds to thirty minutes.
Anyone that is able to decode and receive those packets, would know my callsign (AD8GN,) my location, and even what voice frequency I have my HT tuned on. (An advantage of most “dual band” HT’s, is that they actually have two radios inside and each can use the two meter or seventy centimeter bands.). This allows me a single small radio on my waist to allow voice and automatic digital packets to be transmitted.
However, if a bear shits in the woods and no one smells it, does it even matter?
And that my friends is the point of a digipeater. My HT only has five watts of power, my vehicle radio has fifty watts, and my digipeater radio has eighty watts – and most importantly – a much taller antenna then I stand holding my HT or even the taller antenna on my car.
So, as long as I’m within a mile or two with my HT sending a packet or if I’m within ten to fifteen miles away in my vehicle, my digipeater radio will receive that RF energy on it’s antenna, and if that signal is strong and clear enough (hence the previous limitations,) a computer connected to that radio can then decode the information. Step one. My digipeater, itself, really doesn’t care much about the data, it’s just a middle man to read, and appropriately deliver the packet.
Although I generally only send out position packets, and even those have ‘destinations’, messages to specific other amateurs can also be sent and will be (attempted) to be delivered through the APRS system. Although I use a digipeater, I suppose I don’t have to. I could also configure my computer to only display the packet information on itself, and not retransmit them. Someone at my base camp could see the information on a map and know my location.
By digipeating (my computer having decoded the packet, assembles a new nearly identical packet and transmits it with its height and power. Those packets can travel much farther to other tall antennas allowing more people to know my location. Many digipeaters are also connected to the Internet, which allows it to transmit the packets, via the Internet to an APRS server. There is a global network of these servers and I also run a private one for myself.
Okay, I could write more of course, yet as a one oh one article, I think I’ll end it right here. Please feel free to comment and ask any questions.