At 06:47 AM 02/05/07, you wrote: >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.
Might want to ask the knowledge base here for recommendations and/or warnings... This group has over 3,000 members and they can tell you what works or doesn't work. >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. OK, so the surrounding equipment can handle it. But can the receive side? Might want to mention what repeater you are using, and if you have any RF neighbors at the same site. This can be significant... for example if you have a 375w base on a UHF paging channel in the same building then our recommendations might be a bit different. >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. Good. Your system isn't an alligator (all mouth and no ears) or an elephant. >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." Big question: Do you want/need the increased activity? The increased power will raise the signal strength out in the fringe areas, and make it easier to hear, but can the users out there get back in? Yes, 3db can be difference between the repeater being heard or not in a fringe area... but it makes ZERO difference if the user can't get into the repeater. That's where the receive side of your system comes into play. If it can't hear them, they all you've done is raise the frustration level because the users expect that if they can hear it then they can get back into it (and if they have supersensitive flea-powered HTs then the ball game is different than if they have 50w mobiles....) If you don't meet those expectations then you will have a different set of complaints to handle. Years ago I worked with a midwest group that had a similar coverage problem on 2m. Their solution was unique: they were on a low-end 147MHz channel with a plus offset and simply added a minus offset receiver for the other end of town. They told the users that if they were on the east side to use plus offset and if on the west side to use minus offset. If you boost the transmit power and find yourself up against the physics in repeater receiver performance the only way to get back to a balanced system is to add outlying receivers and linking them back in and voting them. Note that going to a voted environment is a HUGE can of technical worms. See these two articles: <http://www.repeater-builder.com/tech-info/remotereceivers.html> <http://www.repeater-builder.com/tech-info/votingcomparators.html> >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? Is there another system at the same site (or nearby) that runs higher power? Can you use them as a test? For example, is there a UHF commercial repeater(even a police/fire one) that has the same or similar ERP of your 100w system? If so, do a coverage check on that channel. If you are friends with the tech people of that system they may already have done a plot. Maybe download a copy of Radio Mobile and do a set of plots for yourself. You will find that boosting the antenna system performance makes more of a change. Remember that 3db more gain or 3db less loss in the antenna system will help the receive side and the transmit side and do it as much as the transmit side gets by doubling the power, and without changing out the transmitter. Note that in many areas coverage can depend on your geography more than the power level. Locally one popular system is limited by the fact that it's in a valley. Twenty watts into 6db does a great job of covering it, but they use 40w only because that's what was on hand. You could boost it from the current 40 to 1,000 watts and all you would do is raise the s-meters (and get further into the underground parking garages). In that environment once you've covered the valley floor, all you end up doing is spinning the AC mains power meter faster. If you have a flat-as-a-pancake service area then increasing the power level makes your footprint larger. But picture this: if you double the area of the footprint (which the 3db won't do, but for simple math, lets say it does), how much larger is the radius? So the answer is, "it depends, but usually not as much as you'd like". BTW, this question was asked on this group back in 2004 and answered in 2005... Check out <http://www.repeater-builder.com/antenna/3db.html> >2) >Will we risk generating additional receiver noise by doubling our >output power, thus losing coverage in the process? You will have to answer that yourself. Or maybe the collective technical knowledge here can help - you can start by describing your repeater hardware environment. If the receiver (or the receiver+preamp combination) has enough isolation "headroom" then probably. The text (more than the diagrams) in this article may help: <http://www.repeater-builder.com/antenna/separation.html> And these three articles may also help. <http://www.repeater-builder.com/tech-info/receiversensitivity.html> <http://www.repeater-builder.com/measuring-sensitivity/measuring-sensitivity.html> <http://www.repeater-builder.com/tech-info/effectivesens.html> In fact, the last one may help answer your question. Just raise the signal level from the generator by an additional 3db (or even 4db) and see what your effective sensitivity is. >3) Will using a >higher power level shorten the life of other system components over >time (e.g., power supply)? Not if they components are still within their ratings. But if you have an on-site battery backup then the carryover time will be shortened appropriately. >By the way, our frequency coordination would be valid even if we >doubled our output power. Good. That's one less concern. BTW one idea on coverage increase: have you considered another repeater on a different frequency further out and linked in? The linking would make it part of the same user community but solve the whiners coverage complaints.
