Ed (K7WIA): OK, so it looks like we have a very similar set ups. Your numbers let me think about this a bit more.
Your ambient noise is maybe 1-2 s-units higher than mine: P50VDG: you=S7, mine=S4.5-5 PRE (K3): you=S3, mine=S3 [interesting - I'll come back to this] K3 with all off: you=S2, mine=0.5-1 All I can think of is that you are reducing noise input when you lower your RFG to match the internal noise produced by your radio plus preamps (if on). And that explains the drop in received noise to a point where the gain drop faster. Receiving sensitivity on HF/MW/LF is mainly determined by receiver gain, bandwidth, and detector threshold. If you have enough gain you maximize sensitivity. Noise Figure (and its counterpart noise temperature) are not significant compared to the huge level of sky noise present <30-MHz. Sky noise temperature is something like 15-20,000 Kelvin (K) OK, at 1296-MHz sky temperature is in the range of 5-10K, so obtaining a very low receiver noise temp (noise figure) is extremely important for good receiver sensitivity. Current Gas-Fet technology provides us with 18-30K low-noise preamps. At 50-MHz we are half like HF and half like VHF/UHF as sky noise runs about 2000K. The ARR P50VDG and the K3 PR6 are about 0.5 dB noise figure (or 35.4K). If we add Tr=35.4K to Tsky=2000K and maybe add Tant=100K the result is 2135K. As you can see lowering receive noise temp (noise figure) will not make much improvement in sensitivity due to the sky noise background. In urban environments you add local noise, Tn to Tsky and that might be several additional hundred's K of noise temperature. So at 6m a receiver's signal sensitivity is mainly a function of having sufficient gain to drive the detector in the radio properly. It looks like this is reached somewhere between the gain of the PRE and either P50VDG and PR6. I will hazard a guess this is about 18-dB gain. Both preamps produce about 24 dB gain so that is why the s-meter reads higher than with the PRE. The PRE is set in gain just about at the level for best reception thus we see little change when RFG is lowered. In K7WIA's case he lowers the gain to reach this point. In my case I have less external noise so lowering gain does not appear to have any effect on received noise. What would be a better test is with a weak signal looking at detected SNR. Also being sure both radio parameters like bandwidth and mode are the same. One of the reasons to narrow bandwidth at VHF is to reduce the amount of noise power being received. You reach optimum at about the point the bandwidth matches the width of the signal being received. For CW this about 5-15 Hz (depending on speed) filters ring when that narrow so the human mind is the final CW filter with 50-Hz about the practical minimum bandwidth that is usable. Digital modes like WSJT (JT-65) are 4.7 Hz wide so DSP filters of that width are used to dig out the signal way below the level of a signal that can be heard with human ears (about 10-dB weaker than CW). Anyway I digressed from the main topic. I would be interested if other K3 owners looked at their 6m noise indication when RFG is backed off slightly. 73, Ed - KLUW ------------------------------ Message: 12 Date: Thu, 21 Jul 2011 13:00:27 -0700 (PDT) From: K7WIA <emoss98...@msn.com> Subject: Re: [Elecraft] RF gain sweet spot To: elecraft@mailman.qth.net Message-ID: <1311278427206-6608306.p...@n2.nabble.com> Content-Type: text/plain; charset=us-ascii Yes I am using the K3 I was using the preamp that is in the K3, I also have the ARR P50VD preamp on the back of the K3 but using it just raises the background noise to S7. Remember I live in the city. I can turn off the preamp that is in the K3, disconnect the antenna listen to the hiss of the K3, and then hook the antenna back and hear the noise increase.. i am going to try it with out using any preamp at all...I can still detect the weak beacon that are in my area. and still hear a increase in noise when the antenna is connected. but just to reduce the background noise has helped my 73 yo ears make the weak ones stand out better. even without the preamp on I still get the 2nd bar on the S meter to flicker, and i can reduce the Rf gain just slightly ( not much) to dip the S meter. just the joys of living in a big city, powerline, and wireless headphones on 6 meters Ed K7WIA 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-? DUBUS Magazine USA Rep dubus...@gmail.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
