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
