This will be my last comment on this thread, which seems to be digressing from the original inquiry. However, when I see information like the one below, I feel it needs to be addressed, so inexperienced readers do not get a wrong impression and experience equipment failure as a result.
The biggest reason I can think of for running the preamp voltage up the receive cable is to AVOID what K4TAX recommends below. Running a separate unshielded power line up a tower to a preamp is a sure way to generate voltage spikes which will destroy a sensitive preamplifier. Of course such spikes can be caused by induction of nearby lightning but the most common problem is simply switching transients from the relay control lines also run up the tower. Using bias T's to send the voltage to the preamp via a shielded coaxial cable is the safest way to provide a stable controlled DC voltage to a preamp. The bottom line for K3 users is to keep the coaxial loss low and keep the preamp attached to the K3. If you are operating higher frequencies where your feed line loss will degrade front end performance and you MUST mount a preamp and relays at the antenna, use shielded cable to provide voltage to the preamp. GL and VY 73, Lance On 4/28/2010 11:42 AM, Bob McGraw - K4TAX wrote: > 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 >> -- Lance Collister, W7GJ (ex: WN3GPL, WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8, E51SIX) P.O. Box 73 Frenchtown, MT 59834 USA QTH: DN27UB TEL: (406) 626-5728 URL: http://www.bigskyspaces.com/w7gj LIVE MESSENGER CHAT: w...@hotmail.com 2m DXCC #11, 6m DXCC #815 Interested in 6m EME? Ask me about subscribing to the Magic Band EME email! http://6meme.com/mailman/listinfo/magic_6meme.com ______________________________________________________________ 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
