This week's technical topic was presented by Bobby Hedge, AE5FZ, or just "Hedge" for short. Hedge owns – and by default maintains -- several repeaters in the DFW area. By my books, this qualifies Hedge as a Subject Matter Expert (SME). Repeater ownership and maintenance is a baptism by fire process. You have to be part mad-scientist, part-witch doctor to tweak all the little things that keep those repeaters up and functioning properly so they carry our QSOs with ease. (And aren't we thankful?)
Anywho, Hedge was gracious enough to school us in "Repeaters 101". This topic covered the basics of repeaters: how they function, the equipment involved and the rules that govern repeaters and repeater owners. He was also kind enough to hang around for a nice question and answer session at the end. (Thanks, Hedge.)
I'll do my best to regurgitate the gist of it to you, but I can assure you, I'm no mad-scientist. But I have been called a witch from time to time. Oh wait. That was something else. (crickets) Well anyway … let's talk repeaters.
What is a repeater? Well, it's pretty much what it sounds like. It repeats stuff. When you engage the push-to-talk button on your radio, and if you have the tones set up correctly, (See blog post "Tone, Tone, What's in a tone?" for more info) the repeater receives your transmission and re-transmits it. Repeaters are very similar to radios with one major difference: they transmit and receive at the same time. Your signal travels into the receiver, over to the controller and out to transmitter for rebroadcast, instantaneously. The unverified rumor is that the repeater can do all this while patting its head and rubbing its belly, too. Pretty multi-talented, those repeaters.
So, why do we need them if they work just like radios?
Well, in the olden golden days of radio, repeaters were invented to assist low-powered and mobile stations obtain better coverage. What a coincidence! That's *exactly* why we use them today. Of course, we caught onto their advantages very quickly and began to incorporate them into our emergency communications plans. Today, there are huge networks of linked repeater systems all over the US. Some of them are large enough to cover a mid-size state! (Googlate "North Central Texas Connection".)
Good coverage is the key to a good repeater. To get good coverage, you need towers. Really, really *big* towers. There's no rocket science involved in repeater setup. It looks just like a radio set up, only on a much larger and grander scale.
The actual repeater is located at ground level. Makes sense. Commercial repeater equipment is the size of a two-door file cabinet so this stuff is heavy! The repeater is connected to the antenna via feedline. Not just any feedline, mind you. We're talking a special kind of feedline called hardline. At a minimum, it's 7/8" thick, but 1 5/8" or larger is preferable.
One of Hedge's repeaters is located at a 450' tower. So let's see…. we're talking 475(ish) feet of 1 5/8" hardline to get to the top of the tower and connected to the antenna. Even with low loss feedline, large loss is inevitable due to the magnitude of the run. It is completely possible to be transmitting 45 watts of power at ground level, and outputting 20 watts at the antenna. The gain of the antenna will help some, but still… the length of the run is a killer when it comes to repeater towers.
As for the antenna itself, this ain't your rubber ducky! These antennas are monsters. One of Hedge's repeaters has an aluminum 20' tall antenna that weighs 36 POUNDS. It has sixteen quarter-wave dipoles and gets 11.3 dB of gain. It can withstand 100mph wind, which is handy with Texas Springs.
For UHF / VHF, the input and output frequencies are close together and can cause interference for each other. For this reason, an additional piece of equipment is required called a duplexer. The duplexer separates the signals so there is only one signal coming in and one signal going out. In essence, it keeps the high-powered transmitter from drowning out the receiver. This is called "desensing" and is why duplexers are also referred to as Band Pass Band Receive filters. Also, real estate is precious commodity at the top of a tower. By using a duplexer, only one antenna is required. If two antennas are used -- one for receive and one for transmit – they must be isolated horizontally and / or vertically and that's where things can get a little cramped.
The same FCC Part 97 rules that govern hams also govern repeaters and repeater owners. There are only a few rules, but the ones that exist are crucial.
These rules include repeater identification every ten minutes, monitoring the frequency for misuse, and having control of the station at all times. This means the trustee must be able to turn off the repeater when necessary, either at the station or remotely. If the repeater owner is out of town, an alternate person must have the codes to shut down the repeater.
Repeater owners voluntarily work with the local area Frequency Coordinators. Coordinators maintain a database of local repeater statistics and, based on those statistics, recommend the optimum frequency pairs for new repeater owners.
Theoretically, this cuts down on intermodulation caused by combining of signals that are close in frequency. The problem with this idea is, due to the size of the DFW area and the large quantity of existing repeaters, there are no more frequency pairs to be assigned.
As new repeaters come online (remember, participation is voluntary) interference increases, as well. Because of this, your repeater should only transmit as far as it can receive. An "alligator station" – all mouth and no ears -- causes interference for others.
The unwritten rule is "there must be a 70 mile spread for repeaters that share the same frequency pair". Which would work quite well, if it weren't for early morning and late afternoon signal propagation. At those particular times of day, signals can be heard well beyond 70 miles. For this reason, tone squelch is recommended for the greater DFW area. (Again, see blog post "Tone, Tone, What's in a tone?" for more information on tones.)
If you are considering buying a repeater, be aware that true repeaters are built for government and municipal use, so they are not cost effective. Hedge recommends buying city surplus and converting the high grade equipment down to the ham bands. His favorites are the Motorola and GE brands, but replacement parts for these old chestnuts are getting more and more scarce.
Conversely, commercially available ham repeaters are not nearly as big or as powerful as the ones built for government use. The ICOM DSTAR repeater is expensive, but not very successful commercially. The YAESU FUSION repeater is cheap, but isn't a true repeater. It's actually two side-by-side radios connected by a controller and housed in a slick-looking box. Because it's not a true repeater, the transmitter hears the receiver and a good duplexer is necessary.
Well, that's a wrap. There was a lot of information provided in Repeaters 101 and I'm sure I missed some things in the retelling of the tale. But the next time you hit the PTT, say a quiet "thank you" to all of those the mad scientist / witch doctor types out there. It's a lot of responsibility, this repeater ownership thing. If you're one of "the ones", thank you as well. The entire VHF / UHF side of the hobby owes its success to you.
Thanks again to Hedge for sharing his time and his knowledge with us Monday night.
~73
KG5BHY
Allison