A Home Brew 224 MHz Repeater Project. May 2009 Part 4 The RF Amplifier First Post May 2, Second Post May 6 - Third Post May 8, This Post May 12, 2009
Moving right along... This text is Part 4 of a description of a recently completed 224 MHz Repeater Project. Pictures of the completed repeater project reside in the group photos section. http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list The RF Amplifier choice was based on the Hamtronics T301-6 exciter (transmitter) RF Power (output) drive level, which is only about 1.75 watts real world. In reality I would expect the 224 MHz Hamtronics Exciter to output minimum 1.5 watts to a maximum of just under 2.0 watts of RF. I'm happy to report the T301-6 Exciter makes the manufactures rated power level. For "lock to talk" 100% duty cycle repeater operation I turned down the exciter RF output to about 1.75 watt output per the manufactures recommendation. I'm guilty of using an "HT RF Amplifier" in this project. An HT Amplifier is normally used to boost the power level of portable "HT" (aka hand held) radios. The classic "entry level" HT Amplifier is rated for input drive levels of 1 to 5 watts for a corresponding output of 8 to 25 watts of course depending on a number of situation variables. And of course the HT Amplifier has antenna relay switching based on a detected input RF drive signal. One could easily write an entire article about the pros and cons of using an amplifier with switching relays installed and used but at this time I'm going to avoid doing so... HT Amplifiers are/were made by MFJ, RF Concepts, KLM, Henry Radio and a number of other manufactures. The most common 1-5 in, 8-25 out amplifier models are so similar in most all respects so they easily interchange. In this example I've used an RF Concepts brand but could have easily used an almost exact MFJ Amplifier for pretty much the same results. A few blanket statements for you technical hounds. The TR Relay is rated in number of operations before estimated failure and I would not expect the normal use of the machine to achieve that number for quite a time (years/decades?). In a solar power only situations the RF Amplifier is easily disabled to conserve site energy. One could hard switch the TR Relay with external logic, which would avoid "hot switching" the exciter. Hot switching is the load or lack there of presented to the driving device (the exciter) while the TR Relay changed the RF Path to the Amplifier input. Hot switching is not an every day in your face problem for most people but you should be aware of it. Another option would be to wire directly to the Amplifiers RF Section(s) bypassing the internal TR Relay. With "reasonable operation" no blower or fan was used in regards to heat sink temperature... however, a small blower (not fan) would be advisable. Decent blower time on/off management is possible with many of the more flexible repeater controllers. One could also construct and interface a relatively simple blower timer on/off circuit. Please keep in mind the blower doesn't have to operate with every transmission. As a personal preference I choose to toggle the blower on after x-amount of transmit time has occurred within x-minutes of operation. Blower on-time management keeps me from having to listen to blowers winding up every transmission, saves energy and helps to extend the blowers usable life span. I'm not a fan of leaving blowers (and fans) running 24/7. If you plan to use the repeater for linking, IRLP, Echo-link or similar a blower is smart money well spent. A 30 second time out timer also trains the lock to talk type people to listen more often (a good thing). The TR Relay doesn't chatter on and off with loose RF coming back down the Transmitter feed (coax) line because a circulator is installed. So the Hamtronics Exciter RF Output is routed direct to the amplifiers input via some decent mini Teflon type coax. In a previous post I mentioned how surprised I was in regards to the power (signal) lost in even very short sections of regular RG-58 coax so don't cheat yourself by using the wrong type of coax (trying to save a buck). The supply voltage is provided via a fuse block assembly, which will be detailed in the final Part 5 article (text). Just for trivial I'll say I leave the amplifiers front panel control switch set for SSB operation, which only delays the relay UN-key hang time (of the amplifier, not the repeater). No actual "mode change is done internal to the amplifier via the FM/SSB front panel switch. The Amplifier output is about 20.5 watts with 1.75 watts drive, which is just fine for this project. The repeater site antenna combiner systems eats almost half of the output power so we see about 12 watts heading to the site master transmit antenna. Part 5 will be the wrap up text. Cheers, skipp skipp025 at yahoo.com
