Hi Again, all of this *assumes* a number of things. One of may is that you do have 1 uV sensitivity. If the radio is “broke” in some way and you are at 10 uV or 100 uV then that needs to be fixed.
With something like a Grundig RF-50U, you may well be using a very different antenna than your R-390. Unless the antenna is same / same between two radios, comparing them is not really fair. Bob > On Oct 9, 2024, at 8:12 PM, Barry Scott <[email protected]> wrote: > > One of the reasons I'm looking at the balanced input is how it might > help with weak signals and have I done myself a disservice by aligning > the radio using the unbalanced input. I think that downstream from > T20*, it shouldn't matter but that may be incorrect. If correct, then > I will do the alignment again using the balanced input. > > What got me started is that there's a radio station about 100 miles > north of my location (WSM outside of Nashville) that I sometimes like > to tune in; however, on this R-390/URR, it has been quite weak. I was > blaming propagation but then I tried tuning that station in with my > vintage Grundig RF-80U and I was surprised that I could hear it as > good if not a little better than on my R-390/URR. That was > disappointing and makes me think I have a sensitivity issue - at least > on the 0.5-1.0MC band. > > Thanks, > Barry - N4BUQ > > On Wed, Oct 9, 2024 at 3:10 PM Bob Camp <[email protected]> wrote: >> >> Hi >> >> Does any of this matter in a real setup? >> >> If you make some assumptions to get the math going (yes, change the >> assumptions and the details change …): >> >> 1) You are running an 8 KHz “IF bandwidth”. >> >> 2) Your 8 KHz filter has a 10 KHz “noise bandwidth” (noise bandwidth will >> always be larger by some amount … ). >> >> 3) Your antenna input is looking at a 50 ohm resistor in a nice warm room. >> The radio does not load things down. The input is at 50 ohms. >> >> (like I said … assumptions …). >> >> You get roughly -133 dbm of RF power at the antenna input in that bandwidth >> simply from thermal noise. More or less, that’s 0.05 uv (ALL of these >> numbers rounded a bit to keep things simple). >> >> You now go to measure sensitivity and are looking for a 10 db signal to >> noise ratio. >> >> 1) That’s about a 3:1 ratio >> >> 2) If thermal noise was all that mattered we would have a 0 db noise figure >> front end >> >> 3) With this ideal front end, our signal would be at 3 x 0.05 uV. That gets >> us to 0.15 uV. >> >> If our radio comes in at 1 uV sensitivity, it’s noise figure is 20 log ( 1 / >> 0.15 ). That would be 16 db. Measure something else and you have a different >> noise figure. Change that noise bandwidth or open a window to cool down that >> warm room, it changes as well. >> >> There are lots and lots of folks who have looked at the RF floor in various >> locations and frequencies. As more and more folks generate RFI those numbers >> just get worse and worse. >> >> Next up, what sort of antenna are you using? >> >> Turns out that longer antennas likely give you more signal. If you have a 6” >> piece of wire, it’s not going to do quite as well as a 60’ piece of wire. >> (all other things being equal). Most of us are not going to be using a 6” >> piece of wire *and* expecting to get super duper performance. >> >> With a typical antenna and at most of the frequencies an R390(a) covers, the >> noise on the antenna is *way* more than that 16 db noise figure. Have a >> mismatch loss? It’s still way more. Have a weird input connection? still way >> more. >> >> One of *many* articles on HF noise floor: >> >> http://rsgb.org/main/files/2017/12/221216-Noise-leaflet-issue-2.pdf >> >> Ok, you don’t trust the math. Really simple way to test this: >> >> Fire up the radio with the input shorted (or hooked to a dummy load). See >> what the output is (ideally with AGC turned off and the RF gain turned all >> the way up). >> >> Hook up the antenna. >> >> Does the noise coming out of the speaker go up? (it should ….) >> >> Does it go up by more than 3 db? (I’d bet it does). >> >> If so, the antenna “noise floor” is what rules. No need or value in getting >> any crazier about this. What you have is good enough. >> >> Ok, so the noise goes up a lot. Isn’t that the best thing to have happen? >> >> Well, not so much. Your poor radio also has to deal with issues like >> overload and other problems from strong signals at the antenna. In some >> cases a few db increase in those signals means quite a few db increase in >> the amount of trouble they cause. Adding gain you don’t need is simply >> creating issues you could avoid. >> >> Fun !!! >> >> Bob >> >> >> >>> On Oct 9, 2024, at 3:09 PM, Barry <[email protected]> wrote: >>> >>> That may very well be what I'm thinking of. This is what I remember doing >>> for one of my R-390As. I think winding a toroidal transformer may work >>> better. I used to have a box that connected to the balanced jack and had a >>> BNC connector for the unbalanced coax line to the antenna. I think it >>> wasn't an exact match - something like 65 ohms - but it worked. I don't >>> think I'd be able to use it, though, with this jack. I'm still unsure what >>> it is. >>> >>> Thanks, >>> Barry - N4BUQ >>> >>>> I am aware of a cap/resistor combo Chuck recommended for outputting the >>>> diode >>>> load to an outboard preamp/amp combination. Perhaps that was what you’re >>>> thinking of? >>>> >>>> >>>> >>>> Dan - WB4GRA >>>> >>>> >>>> >>>> >>>> Sent from my iPhone >>>> >>>>> On Oct 9, 2024, at 2:25 PM, Barry Scott <[email protected]> wrote: >>>>> >>>>> What is the correct designation for the two-pin antenna connector >>>>> (balanced) in the R-390/URR (likely the same for the R-390A/URR but I >>>>> want to be specific just in case). >>>>> >>>>> Also, I think Chuck Rippel had a connection contraption that used a >>>>> resistor (or two) so that 50-ohm unbalanced would match just a bit >>>>> better using the balanced connection. He had a website that depicted >>>>> that but I don't know if that still exists. Anyone know? >>>>> >>>>> Thanks, >>>>> Barry - N4BUQ >>>>> ______________________________________________________________ >>>>> R-390 mailing list >>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>>>> 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 >>>> ______________________________________________________________ >>>> R-390 mailing list >>>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>>> 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 >>> ______________________________________________________________ >>> R-390 mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>> 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 >> >> ______________________________________________________________ >> R-390 mailing list >> Home: http://mailman.qth.net/mailman/listinfo/r-390 >> 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 ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 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
