Eric, Obviously we could probably get by with less power, but I still want to reduce the RF power when the AC is out to maximize the battery life.
We tried the trick of lowering the power to 10 watts without telling anyone and a few people in the fringe area that are regular users could not use the repeater. So we do need a little more power. We also really need more than 200 AH as this is a primary repeater for our area for emergency communications. Unfortunatly, the generators are not shared with us (we have our own building and power at the site). The last hurricane/tropical storm that came through here knocked out two of the three repeaters, so it really needs to last a good amount of time with high usage. It looks like the seperate small amp will be the best solution in terms of maximum battery life. I checked another repeater on the bench tonight and found pretty much the same thing as Ron mentioned. You can lower the RF power, but the efficency gets worse. Thanks for all the input, Dwayne Kincaid WD8OYG > > Dwayne, > > You raise an excellent point! The first question I must ask is, do you > really need 100 watts? Since a repeater's range is limited primarily by how > well it can receive distant stations (primarily handhelds), you may have far > more transmitter power than necessary. Maybe 20 watts is all you need, all > the time. > > Try this test: Without telling anyone, disconnect and disable the 100 watt > PA and connect the exciter directly to the TX input on the duplexer. With > perhaps 1/2 watt, you may find that few people notice the difference. In > reality, of course, the exciter won't be enough to cover the area you can > receive, and something in the 10 to 25 watt range will suffice. > > I wonder why you think two 100 AH batteries will only last a few hours. One > of my UHF repeaters with a 50 watt PA ran for three days on one 125 AH > battery. I'll admit it wasn't keyed 100% of the time, but it was well-used > during the power failure. The controller has a voice announcement that > reveals when it is on battery power, and the users naturally avoid using the > repeater for rag-chewing when they know the situation. > > But back to your original question- if you can alter the PA power output > when running on batteries, that is the best way to go. It is easy to do > with a Motorola Micor PA, dunno what is needed with the Mastr II PA. Just > be aware that reducing the RF power output on a given PA does not always > result in a proportional reduction of DC input power. In fact, I once had > to replace the PA module in a FT-2500M radio which burned up after running > for a year in packet service at 5 watts. It seems that reducing the RF > output from 45 watts to 5 watts only reduces the input DC current about 10%. > The DC power that is not used winds up heating the PA module! This is one > area where "conventional wisdom" falls short... > > 73, Eric Lemmon WB6FLY > > > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of Ed Yoho > Sent: Tuesday, October 16, 2007 6:37 PM > To: [email protected] > Subject: [Norton AntiSpam] Re: [Repeater-Builder] Reducing power out when on > battery backup. > > ldgelectronics wrote: > > >Hello All, > > > >I have a requirement to install a battery backup system at a local > >ham repeater. It's a GE Mastr II running about 100 watts. With that > >much RF power, a couple of 100 AH batteries is only going to last a > >few hours. > > > >My first thought was to add a second lower power RF amp (something > >like 20 watts or so) and use coax switches tied to the AC mains to > >switch to the smaller amp when the AC power was out. This should give > >me a factor of 4 or 5 more amount of time on the backup batteries. > > > >The second thought (and here is where I need input), was to bring the > >variable resistor (R8 on the VHF version) from the 10 watt driver > >board to a smaller external board. Then add a second variable > >resistor and a relay to switch between the two. This should give me > >two independent amp settings that can be controlled by a single > >control. > > > >Is there any reason why this should not work? It would save the cost > >of the second smaller amp and two fairly expensive coaxial relays. > > > >The relay could be controlled from the repeater controller or > >automatically with just a 12v DC wall wart. > > > >Dwayne Kincaid > >WD8OYG > > > > > > Dwayne, > > A (hopefully) better method would be to use two RF relays to switch in > or out the final stage of the stock amplifier. This will allow the > sections to operate with their normal / nominal power settings and give > a larger current reduction when in battery mode. > > relay 1 common to driver output > relay 1 normally closed to final input > relay 1 normally open to relay 2 normally open > relay 2 normally closed to final output > relay 2 common to antenna filter assembly > > This assumes the relays would be energized when in battery mode. A > little more drain on the battery, but 99% of the time the coils would > not need to be energized. > > Ed Yoho > WA6RQD >
