Cheap Repeater Woos
Building repeaters is an art. I built my first repeater 10 years ago. I learned a lot about how radios work and even more on what it takes to build a repeater. The Maxtrac radios make good repeaters that see small amounts of use. The Desktrac is just 2 Maxtracs in a cabinet with a mobile duplexer. First issue is duty cycle. This is how long you can transmit without over heating the TX radio. If a UHF radio puts out 45 watts of power, which is for a 25% duty cycle. The power level for 100% duty cycle would be 15 to max 20 watts. Which is not a bad output for a GMRS repeater. Maxtrac radios have a built in circuit that lowers the power down to 1 or 2 watts if it gets hot. I found this out the hard way. Fans help with this problem, but it will always be a problem if you carryon a conversation for several minutes. Next is the antenna and to use a duplexer or two antennas. All duplexers need coax cable that has very little loss of signal. The coax that runs from the radio to the antenna also acts as an antenna unless it has a good insulator around the center conductor. Hardline is the best but the most expensive. If you use RG-58 like the coax in a mobile radio installation, most of your signal at the UHF frequencies used will be lost traveling down the coax to the antenna. To use two antennas we are talking about a separation of 50 feet vertical and 200 feet horizontally. That’s alot! So your coax is going to cost you a bunch of money or you will lose a lot of your signal using cheep coax. Best bet is a duplexer, which is one of the most expensive parts of a repeater system. However the good news is since you are limited to 50 watts you can use a mobile duplexer. IMHO the best bet is to buy one of the China models advertised on eBay. The come pretuned to your frequency and usually only run you about $125. I have used them for GMRS and portable repeaters without any problems. (I did get one that was tuned to the wrong frequency, but the seller replaced it and I got to keep the wrong one too) You also need a good antenna, since you will be turning down the power output of the radio. My suggestion is an Omni directional fiberglass antenna of at least 3 to 5 db gain. These antennas are generally 5 to 11 feet long. Coax that will work and not cost you an arm and a leg (which you have already given away) would be Belden 9913 or similar. Hardline would be better but cost much more. Sometimes you can find good deals on hardline on eBay, just make sure both ends have a connector that you can use. I've seen 40 to 50 feet of hardline at a very good price. This would be enough to go from the repeater to the antenna and meet the regulations required by the FCC. Make sure you spend time reading all the regs, they can be found on the internet. The repeater-builders web site is a very good place to start. There is a huge amount of information there. Also shoot me an email if you have any specific questions: k9rod at k9rod dot net Good Luck, hope this helps some, Rod K9ROD
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