I seem to recall that several companies make "bias T" devices that allow RF to pass but block DC in one direction. Thus these are suitable for feeding DC to a antenna mounted preamp. Then the challenge of switching the preamp out of line for transmit is next.
For my EME applications I have made my own bias T. I apply DC to the bias T that switches the preamp in line and supplies power to same. For transmit I remove the DC from the bias T thus turning off the preamp and switching it out of line. Personally, two feedlines, one for RX and one for TX and a separate DC control cable is the more reliable system, specially when running high "legal limit" power. I've vaporized more than one preamp PCB and other components in the past not to mention the more likely probability of a relay failure on the top side. My rule is "the less stuff in the RF path, they better the reliability". Low loss feedline is the best approach. 73 Bob, K4TAX ----- Original Message ----- From: "DM4iM" <hamra...@vr-web.de> To: "Reflector Elecraft" <Elecraft@mailman.qth.net> Sent: Wednesday, April 28, 2010 6:18 AM Subject: Re: [Elecraft] DC-supply for a preamp? > Hello all, > thank you for all the info about losses in a coax cable, > but the original question was if this is a good or a bad idea. > Lots of qsb, so it disappeared in the noise. :-) > > I've been thinking about it myself, my conclusion is: no good idea. > Even if the K3 passes the supply voltage to it's antenna jack, bad > things can happen if you forget to switch the supply off, when you > change antennas, switch to your dummyload, you name it. > So it is a big NONO. > > Martin > > -- > > 73 , DM4iM > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
