At 04:10 AM 8/23/05, you wrote: >I haven't seen any replies to this yet so I'll take a stab >at explaining the basics. (Hey guys, this sort of thing is >a FAQ, obviously... maybe we need a good basic "Repeater >Building 101" article for RBTIP? If there is one, I >haven't found it. I am aware of the "What's a Repeater" >article but that seems to be aimed more at repeater USERS >and is a bit too basic for the novice builder, IMHO.
That's exactly the audience it was intended for. >Just curious / asking....) Hey Paul, if you want to do a follow-up article to the "What's a Repeater" article go right ahead... Kevin won't mind. Aim it at the novice repeater builder, and don't pull any punches. Repeaters are fun, but they are cash sinks and divorce generators. >Scott, I gather you are thinking of using both radios... one >as a receiver and one as a transmitter. That will work, >but with those radios I think another option is to use just >one of them. Take out the receiver and transmitter, mount >them in two separate metal boxes (for shielding). >Preferably all wires coming out of each box would by via >feedthrough capacitors to filter any stray RF (except the >coax of course). I will leave it at that since I have not >built a repeater from those rigs myself. If I remember correctly the Cobra 200 was a 13-509 with a face lift and lipstick. Splitting a Midland was pretty easy - the receiver crystal sockets and trimmers were on the exciter board, but other than that the split consisted of taking apart the radio - there was the front panel, the receiver board, the exciter board, the PA deck, and a combination PTT relay and VSWR sensor board. Midlands and Cobras are what a lot of guys started out with in the early to mid 80s, but that's all that there was then unless you wanted to convert a UHF GE tube radio. Why start with a 25 year old radio today? These days I'd go UHF and start with two $100 (or less) Maxtracs. A D04 series will crank back to 3-4w and run continuous duty (think of it as an exciter), and will drive an amplifier to whatever power level you can afford. UHF radios are more common, there are more folks to talk to and get help from, UHF duplexers are smaller, and more available. UHF repeater antennas are smaller and easier to build. >You will probably want to add a tone decoder to the >receiver. I know I'll open a can of worms with this, but I >pretty much subscribe to the belief that most, if not all >repeaters these days should use tone access... not just >carrier squelch. There's just soooooo much RF out >there.... in and out of band. Having a tone decoder available to turn on and not needing it certainly beats the alternative. >You will need a repeater controller. This could be very >basic, providing ID, timeout timer, and of course keying >the transmitter whenever the receiver goes active. Or it >could have many other features like fancy courtesy beeps, >voice ID, DTMF control of repeater functions, etc. etc. I >would suggest you check out the repeater controller >suppliers listed on this page to see what they have to >offer. That may give you a better idea what you want... > >http://www.repeater-builder.com/rbtip/supplyindex.html The NHRC-2 comes as a kit and is about as low as you can go and still get something useful. >The controller, at its most basic level, takes audio and >carrier / tone detect from the receiver and supplies audio >and PTT to the transmitter as needed. > >If you want to run on a single antenna (repeater transmits >and receives on the same antenna) you will need a duplexer. >This is an expensive component. Its function is to block >the transmitter carrier from entering the receiver and >either damaging it or overloading it so that it can't hear >what it is supposed to be hearing... and also to keep noise >generated by the transmitter (which spreads out around the >carrier and some of it will fall on the repeater receive >frequency) from degrading receive performance. The Midland with a good pass cavity added to the front end is pretty good. I've also seen converted RCA 1000, Motrac and Micor receivers on unsplit Midlands. >If you have enough space you can run on two antennas without >a duplexer. Since I haven't built a 222 repeater this is a >guess, but I'll take a stab in the dark and say perhaps 50 >feet of vertical separation between antennas or 200 feet >horizontal separation. Perhaps someone with 222 experience >will offer to correct me on this. Vertical separation is >more effective because most repeater / base station >antennas are designed to radiate best toward the horizon >and have nulls in the up / down directions. Vertically >spaced antennas are mounted in each other's null, thus >increasing isolation between them... whereas horizontally >spaced antennas are in each other's maximum radiation lobe. >One difficulty with two antennas is trying to match receive >and transmit coverage... especially if one antenna is >mounted above the tower and one is side mounted. Visit this page and use the 150mhz curves and add 10-20db. <http://www.repeater-builder.com/antenna/separation.html> Realize that the curves are from the days of 4-cavity duplexers, tube-type (i.e. less noisy) transmitters and tight front ends, and which the Midland has neither. >Antennas can be chosen to suit your requirements (where do >you have to mount them and what coverage do you need?) but >there are a few things to watch out for. It is desirable >to use an antenna that is rugged enough to survive >conditions at your repeater site. Wind and ice factors >vary with location, tower height, height of hill / >mountain, etc. Also be aware that some "ham grade" >antennas may be noisy in duplex service (single antenna >repeater). Read the theory articles at <http://www.repeater-builder.com/rbtip/ant-sys-index.html> Noisy antennas / duplex noise is covered at <http://www.repeater-builder.com/antenna/cracking.html> Stick with grounded dipoles and home-brew them. You will learn something and end up with as good as an antenna as you chose to build. Read the 440 dipole antenna construction article at <http://www.repeater-builder.com/antenna/440fdipl.pdf> and scale all of the dimensions except the diameter of the rod stock. Figure the elements will be about twice as wide and long, and spaced twice as far apart. Use GOOD coax for the antenna harness, and be generous with the non-acidic RTV as the sealer. >All interconnecting cables on the repeater should be good >quality double shielded cable. I like RG-214 but RG-400 is >suitable for short runs to interconnect receiver, >transmitter, duplexer, etc. The feedline run to the >antenna(s) can be RG-214 if it is very short, otherwise >hardline. Most radio sites will not allow 214 as a feedline...heliax-type only. Here's one site's rules, expect other sites to be somewhat similar: <http://www.repeater-builder.com/rbtip/radiositerules.html> >It is important (especially for the single antenna repeater) >that all connections and hardware in and around the antenna >/ tower be tight and not able to move... also free of rust. >Loose parts or rusty hardware can cause noise that your >duplexer will not be able to keep out of the receiver. > >Where do you plan to put the repeater? If at a managed site >with other repeaters / communications, there may be >specific technical requirements you have to meet in order >to be allowed at the site. Some of them may require >additional equipment not mentioned here (for example an >isolator). And don't make the mistake of trying to tune an isolator or mount it on a steel rack panel. See the notes in the "Isolator" section on the Antennas page. >What did I forget to mention? Of course there are many fine >points I didn't attempt to cover here. > >Paul, N1BUG Budget? Repeaters always cost 5x to 10x what you plan on. And then the users complain that it does not work better than it does, and refuse to help make it better. Mike WA6ILQ >On Sunday 21 August 2005 02:46 am, n7any_1 wrote: > > I'm new to building repeaters, I have had a cobra 200 > > since they came out and used it as mobil rig, I have > > aquired a midland 13-509 and would like to build a 222 > > mhz repeater out of them. Since I know almost nothing of > > the details of building a repeater any information would > > be welcome. I am especially interested in a description > > of the main parts of a simple repeater ie. radios > > controllers, antennas, duplexers, whatever I will need to > > put this together. I am also interested any experiences > > others have had building a repeater from these rigs? > > Thanks > > Scott Frazier > > N7ANY Yahoo! 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