Tony, As you can tell from the number of replies, this is a topic that generates a great deal of interest! I agree with the majority of posters who state that a power increase will have relatively minor positive effect, and MAY have significant negative effects. In fact, one fellow posted a comment a year or two ago, that his 50 watt PA died, and he connected the exciter directly to the duplexer in place of the PA for several weeks- a 100:1 change in power output- and none of the regular users noticed the difference! In this particular case, the repeater was far from being "balanced." Nevertheless, it does lend credence to the oft-repeated statement that "increased power does not equate to increased coverage."
It has been stated many times on this and other sites that the primary limitation of repeater coverage is the ability of the repeater to hear the radios in the field. If the pipsqueak-power handheld portable radios in the field cannot reach the repeater's receiver, no amount of power increase will make any difference. Quite the contrary, as many have pointed out, more power will possibly increase receiver noise and/or desense, intermodulation, and other ills. These MAY have the effect of reducing the coverage area! I strongly suggest that a thorough analysis be performed on the entire system, to ensure that the feedline loss is as low as it can possibly be, the antenna gain and pattern is appropriate for its location, and the receive chain is as efficient and noise-free as possible. If your site noise floor is low, you MAY be able to profoundly improve your coverage with a couple of bandpass cavities followed by a very good preamp, between the duplexer output and the receiver input. A preamp is never the "final answer" but when appropriately filtered CAN result in a significant improvement. Please note that I have emphasized the words MAY and CAN. I have learned from experience that effective site engineering comes from a proper consideration of all factors. Moreover, the "knee-jerk" suggestion that an increase in power output will solve repeater coverage issues is laughably misguided. I sincerely hope that you can convince your repeater users that a careful analysis of the repeater operation may reveal other, more effective means to improve its coverage. 73, Eric Lemmon WB6FLY >From: "Tony L." <[EMAIL PROTECTED] <mailto:railtrailbiker%40yahoo.com> > >Reply-To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: [Repeater-Builder] Effects of doubling RF output on UHF repeater? >Date: Mon, 05 Feb 2007 14:47:34 -0000 > >One of our 70cm Amateur Radio repeaters is currently outputting 50 >watts into the duplexer. We're considering replacing the existing RF >power amp with a 100 watt model. > >Current draw on the 50 watt unit is 8 amps. The 100 watt unit will >draw 20 amps. Our power supply is rated at 36 amps continuous, and >the duplexer is rated at 250 watts. > >Half of our users believe that the repeater's output power is >perfectly matched to its receiver. That is, users of high powered >mobile radios generally lose repeater reception at about the same >time the repeater's receiver loses them. > >However, the other half of our users believe doubling the repeater's >power output would generate increased activity since the repeater >could be heard more "comfortably." > >We could upgrade without changing any of our other infrastructure. >However, these questions arise: 1) Will the hundreds we pay to >upgrade actually translate into significantly increased range? 2) >Will we risk generating additional receiver noise by doubling our >output power, thus losing coverage in the process? 3) Will using a >higher power level shorten the life of other system components over >time (e.g., power supply)? > >By the way, our frequency coordination would be valid even if we >doubled our output power.
