Spring, my little dearies, has sprung. And with spring come bunnies. Oh so adorable bunnies with their STUPID bunny nests on the ground – also known as my dogs’ yard. Harmless little killer bunnies, run away, run away, run away…

Wait… I was going somewhere with all this.

Oh yeah.

Spring has sprung. With spring comes (stupid, stupid bunnies and) bike rides. And with bike rides comes talk of sagging. And with sagging comes talk of… Yep. APRS and digipeaters, of course. (Gah, where did you think I was going?)

Anyway, last year I was a fairly new ham when I started hearing the terms APRS and digipeating being batted about with great fervor. At the time, my club was actively involved in a very, very large bike ride, so I figured out the APRS thing pretty quickly. (Well, basically anyway.)

But I never really did grasp digipeating and digipeaters. This year, I swore I was going to be a much savvier ham, so I’ll share what I found with you.

Digipeater is short for digital repeater; a repeater for packet data rather than voice. A packet is a unit of data that is routed between an origin station and a destination station. The means by which it travels are many and mostly depend on the device involved. (Computer, phone, etc.) In the ham world, packet data is transmitted via radio frequency.

With the word “peater” on the end, it’s tempting to think of a digipeater as just a plain ol’ repeater. But not so, Dainelson.

The digipeater receives a packet of data, stores it in internal memory and then retransmits it. While voice repeaters hear and speak on different frequencies, digipeaters hear and speak on the same frequency. Additionally, while repeaters are assigned their very own frequency pair, digipeaters can operate on any frequency the operator chooses, though there is an ARRL suggested band plan. The most typical frequency for packet APRS is 144.39. Packets not associated with APRS are usually transmitted on 145.01, .03, .05, .07 and .09. There are also HF data frequencies as well.

Since digipeaters send and receive on the same frequency, each transmission is time and frequency intensive. There’s the time it took for the original sender to send plus the time it took for the digipeater to retransmit the packet. In addition, since digipeating is sequential and not simultaneous, there may be additional retransmissions, or hops, depending on how the originator’s station is configured. Requesting three or four hops indiscriminately can reduce the channel capacity by 75% or more.

Digipeaters have become synonymous with APRS, however digipeating is actually a stand-alone process. It can send any kind of packet data. However, as it’s been pointed out in this blog before, the trends change in ham radio as fast as they do in computer technology. And currently, 99% of digipeating is associated with APRS.

Briefly, APRS stands for Automatic Packet Reporting system. (Huh, there’s that “packet” word again.) There’s a lot involved in those four words, but the simplified version is, it’s like GPS for hams -- it combines satellite for position placement, radio frequency and, hey, packet(!) transmission.

There are a lot of issues involved with ARPS and digipeaters.

First there’s the receive, transmit, and retransmit time involved. There is the added issue of sending signals, or “beaconing”, from a moving vehicle. The minimum beaconing interval is 20 seconds, so positioning is definitely not real time.

Also, packet data transmission is all or nothing. The entire packet has to be received perfectly or it’s not received at all. The things that bother voice transmission, the white noise, pops and intermod, will be fatal to a packet transmission. This means signal levels that are perfectly fine for voice, ain’t going to cut it for data. Packet data must be run lean, clean and green.

A station that receives an incomplete packet transmission from another fixed station will automatically send a request for a retransmission. But with APRS packet data, there is no send/receive error checking method. The beaconing station hopes the transmission arrives error-free. The receiving station ignores any packet that is defective.

And there lies the issue. With a 15-20dB possible fluctuation in signal strength to and from a mobile station, you could see how this could get tricky. The bottom line is, if you are the beaconing station, be sure that you have perfect signal strength over the entire intended path. Bump the power up, the S-Meter should read full scale so that even if you have a 20dB drop, the packet will arrive error free.

I’m seeing a future topic on APRS. While the two are intertwined, there’s a lot more to the APRS side of things than what we’ve touched on here. However, I hope this explains, in plain language, what the functions of a digipeater.

But right now I’ve got bunnies to scare off.

~73
Allison
KG5BHY