N4XE, Yes, a 4 dB pad in line with the antenna would be bad for receive. I'm no good at making ascii diagrams and didn't have time to make a real circuit drawing, so my word picture wasn't clear enough.
I suggested the extra coax be placed between the T/R relay and the input of the amplifier. That way, the receive path is isolated from the extra coax. The amplifier most likely employs a DPDT coax relay (or equivalent pair of SPDTs) to put it in the circuit only during transmit. The amplifier has to be bypassed during receive or one would have to "listen backwards" through it. The extra coax goes between the VU5K side of the DPDT transmit path and the amplifier input stage. There is already a piece of coax in the circuit between these points that is only active during transmit. One simply needs to replace the short piece of coax already in the circuit with a piece that is "4 dB longer". I hope the above is a little bit more descriptive. Cheers, Mike - W8MM On Aug 13, 2012, at 5:22 PM, <bearcreekcatt...@juno.com> <bearcreekcatt...@juno.com> wrote: > What about increased noise? I was educated that loss equates to > increased noise on receive. > > de nx4e > On Mon, 13 Aug 2012 20:09:32 +0000 Mike Valentine <m...@valentine1.com> > writes: >> Patrick, >> >> Yes, of course. That's why I cautioned to make sure the power >> handling of the coax was up to snuff. There is an absolute power >> limit at each operating frequency unique to any particular coax. >> It's hotter closer to the transmitting end. >> >> All we have to do is select a coax that can handle 50 watts @ 144 >> MHz and we're good to go. It's on the data sheet for the coax in >> question. >> >> For instance, the 150 MHz (close enough) loss for Times LMR-100 is >> 8.9 dB for every 100 feet and it can handle 100 watts average (long >> time constant - 10 minutes?) power. So, there is plenty of power >> (heat) margin. If one wants 4.0 dB loss then simply roll out >> (4.0/8.9 x 100) = 45 feet and put connectors on it. >> >> 45 feet of LMR-100 is safe and effective, if a little too long for >> convenience. >> >> Choosing somewhat lossier coax would lead to needing less of it. >> RG-178 is a Teflon dielectric coax that's very thin and has about >> twice the loss of LMR-100 and can handle even more power. One would >> only need 22-25 feet of it for 4.0 dB @ 2 meters. >> >> A little web browsing can be marvelous. >> >> Cheers, >> >> Mike - W8MM >> >> BTW, we used to make precision terminations for 450 MHz testing by >> using a 100 ft roll of rg-174 (with nothing at all connected to the >> far end). The return loss was twice the attenuation, so the >> reflection from the open or short at the far end was much weaker >> than the imperfections of the coax characteristics and connector. >> Easier, cheaper, and more robust than a precision load. >> >> >> -----Original Message----- >> From: flexradio-boun...@flex-radio.biz >> [mailto:flexradio-boun...@flex-radio.biz] On Behalf Of Patrick >> Greenlee >> Sent: Monday, August 13, 2012 9:34 AM >> To: flexradio@flex-radio.biz >> Subject: Re: [Flexradio] Flex 5000 VU module >> >> Wouldn't you want to be careful regarding how lossy the coax is? Too >> lossy >> and you have to shed all the watts of energy in a short length of >> coax so >> heating might be a problem where a less lossy but longer >> coax/attenuator >> would distribute the heat over a larger surface area and not get so >> hot. >> >> Patrick >> >> -------------------------------------------------- >> From: "Mike Valentine" <m...@valentine1.com> >> Sent: Sunday, August 12, 2012 9:15 PM >> To: <flexradio@flex-radio.biz> >> Subject: Re: [Flexradio] Flex 5000 VU module >> >>> Why not work around the problem by making an attenuator to knock >> the 52 >>> watts down to the 20 watts you need? >>> >>> A cheap piece of RG-58 or smaller (lossier) coax of the right >> length >>> attached to the input of your amp is a guaranteed way to make it >> work >>> right. >>> >>> You want a 50/20 = 2.5:1 attenuator which is darn near exactly 4 >> dB. Wind >>> off 4-dB-loss of coax and put it between the T/R relay and the >> input of >>> your amplifier. You can use your watt meter to prove you've done >> it >>> right and test the power output before you hook it up. >>> >>> One caution, make sure the coax can handle 50 watts @ the >> frequency of >>> interest. >>> >>> Bonus advantage: The 4 dB pad will make the amplifier think it's >> in a >>> pure 50 ohm system instead of looking askance at the output >> impedance of >>> the VU5K. Potential instability problems from things popping >> outside the >>> Smith Chart circle are magically pulled back inside ;-) >>> >>> I just love pads! >>> >>> 73, >>> >>> Mike - W8MM >>> >>> >> >> >> >> _______________________________________________ >> FlexRadio Systems Mailing List >> FlexRadio@flex-radio.biz >> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >> Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >> Knowledge Base: http://kc.flexradio.com/ Homepage: >> http://www.flexradio.com/ >> > > > Jim Broaddus > Bear Creek Feline Center (BCFC) > Panama City, Florida USA > 850 722-9927 > ____________________________________________________________ > 53 Year Old Mom Looks 33 > The Stunning Results of Her Wrinkle Trick Has Botox Doctors Worried > http://thirdpartyoffers.juno.com/TGL3141/5029707563ab270754d73st02vuc > _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
