Hi Don, That is why I mentioned the case of: >I still don't understand the mechanism of how a preselector improves >S/N unless its BW is better than the BW of the receiver- and I can't imagine >that situation either. Now, if you are saying the wideband noise may be desensing or causing IM- I understand. That is one of the reasons we have passive filters up front in a rig. But I still don't understand how a preselector with say 50KHz -3dB BW can improve S/N in a modern communications RX- Dale W4OP
> Dale, > > The bandwidth can make a difference - the noise is broadband while the > signal is narrowband. > If the bandwidth of the receiver input is wide, then a preselector will > make a difference because it decreases the total noise that the receiver > is handling. > > Note that many(most) tuners will not produce that pre-selector effect. > Only those with a resonant circuit will do that. Most common T-network > of L-network tuners create either a high pass or a low pass filter. > > 73, > Don W3FPR > > On 3/29/2011 9:55 AM, Dale Parfitt wrote: >> I believe I understood all of your comment Ron. >> But as our noise floor at HF is limited by external noise (atmospheric, >> manmade and galactic) I don't understand how efficiency, within reason, >> comes into play for a receive antenna. Certainly, for signals above the >> noise floor, adding in 3 or 10 of attenuation makes no difference in the >> S/N >> ratio, and since the preselector cannot discriminate between signal and >> noise, I still don't understand the mechanism of how a preselector >> improves >> S/N unless its BW is better than the BW of the receiver- and I can't >> imagine >> that situation either. >> One of my best low band antennas is a rotatable half sized Flag whose >> gain >> runs around 30dB below my sloper. >> >> Dale W4OP >> >>> The antenna tuner acts as a pre-selector so to speak, by making the >>> sytem >>> of >>> the antenna, the transmission line, and the radio, resonant, and matched >>> impedence. >>> >>> All of this adds up to minimize losses in a couple of ways. The >>> impedence >>> matching means that the xmission line and antenna are as close to the >>> input >>> impedence of the radio (nominally 50 Ohms these days). If it is not at >>> 50 >>> Ohms then you do not get maximum power in to the rig based on simple >>> simple >>> Ohms law power. >>> >>> A highly simplified way of explaining resonance part of "a resonant >>> antenna" >>> is the act of making sure all of the current is in phase with the >>> voltage >>> so >>> that when the power does get to the reciever, that it is in a useable >>> form >>> to drive things. It is somewhat the reverse or inverse of a good spring >>> and >>> shock absorber system on a car....if you have things wrong, and drive >>> along >>> a rough road with bumps spaced jsut right relative to how fast, then all >>> heck breaks loose; maximim power transferred, and that is a resonant >>> system. >>> If things are not resonant, well, you get a smooth ride in your car, or >>> in >>> the case of your radio, poor reception (and transmission). >>> >>> -ron WT5RZ >>> >>> -- >>> View this message in context: >>> http://elecraft.365791.n2.nabble.com/A-comment-about-Receiving-tp6216105p6217679.html >>> Sent from the Elecraft mailing list archive at Nabble.com. >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:[email protected] >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >> >> -------------------------------------------------------------------------------- >> >> >> >> No virus found in this incoming message. >> Checked by AVG - www.avg.com >> Version: 9.0.894 / Virus Database: 271.1.1/3536 - Release Date: 03/28/11 >> 14:34:00 >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> 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:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG - www.avg.com Version: 9.0.894 / Virus Database: 271.1.1/3537 - Release Date: 03/29/11 02:34:00 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
