OK Jim, I see why you ended up there. But, none of his allowable choices work in our situation. The da-121 has the shunt resistor before the dropping series resistor, none of the scenarios on k7mem's website account for the format of the da-121. The closest he could get is using the 'T' type format with r1 = 1 ohm, but that is not what we need. We need r1 = 0. By using his 'T' attenuator format and specifying 50 in and 125 out with db = 8.96 attenuation, I was able get his program to set r1 = 0, r2 = 96.83 and r3 = 64.56, so this does not work either, but it is close. I assume it is @ 8.96 db loss, but I'd feel better if r2 = 100 and r3 = 68, but I could not get there.
Regards, Larry On Mon, Oct 14, 2024 at 7:57 AM Jim Whartenby <old_ra...@aol.com> wrote: > Larry > > > I used the T-pad calculator found at: > https://k7mem.com/Res_Attenuator.html > > > I entered the input impedance of 50 ohms, output impedance of 125 ohms and > then solved for various losses. The first loss that gave a solution with > all positive values of resistance was 9 dB. Any loss lower then this > resulted in a negative value for R1. This solution found the exact > resistance values used in the DA-121 except that R1 is deleted since it's > value is only 0.22 ohms. > > > Jim > > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > > On Monday, October 14, 2024 at 04:48:56 AM CDT, Larry Haney < > larry41...@gmail.com> wrote: > > > Hi Jim, I'm not sure how you came up with your loss numbers for the > da-121. As I see it, to calculate the loss due to the da-121, it is due > only to the 100 ohm resistor inside it that is in series with the 125 ohm > load in the 390 input. So the loss is the voltage drop across the 100 ohm > resistor which is 44% (100/(100+125)) of the sig gen output. For 3 uv that > is 1.33 uv loss, so the voltage at the 390 is 1.67 uv, or 55% of the sig > gen output. > > Am I not seeing this correctly? > > Regards, Larry > > On Sun, Oct 13, 2024 at 9:15 PM Jim Whartenby <old_ra...@aol.com> wrote: > > I did a search and did not find any mention of the insertion loss of > either of the two dummy antennas mentioned in Mil-R-13947B. A search for > the word "record" finds that the SG output is what is entered on the test > data sheet, not the actual signal level applied to either the balanced or > unbalanced RF input. Paragraph 4.11 is an example of this with the SG > output being recorded on the test data sheet and not the actual or > calculated signal voltage for either the balanced or unbalanced receiver RF > inputs. > > > So the balanced input is always some 2.82 microvolts higher then the > unbalanced input because only the SG levels applied to the dummy antennas > are recorded for either input. If you want to get a feel for the > sensitivity of the balanced input with the insertion loss of the dummy > antenna accounted for, just multiply the SG output by 0.36 or divide by > 2.82. Either method will remove the calculated dummy antenna insertion > loss. > > > As said previously, the spec is correct but I am at a loss to explain why > there is an error in paragraph 3.13.5 on document page 6 were the "Balanced > input" heading should read "Unbalanced input". I looked for but did not > find a copy of Mil-R-13947B with a date later then 26 October 1960 to see > if this error was ever addressed. > > > Jim > > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > > On Sunday, October 13, 2024 at 04:13:03 PM CDT, Jacques Fortin < > jacque...@videotron.ca> wrote: > > > The chart TM 856-91 attached. > > 73, Jacques, VE2JFE in Montreal > > -----Message d'origine----- > De : r-390-boun...@mailman.qth.net <r-390-boun...@mailman.qth.net> De la > part de Jacques Fortin > Envoyé : 13 octobre 2024 16:13 > À : 'Larry Haney' <larry41...@gmail.com>; 'Jim Whartenby' < > old_ra...@aol.com> > Cc : r-390@mailman.qth.net > Objet : Re: [R-390] Official specs > Importance : Haute > > Well... > I found two diverging statements.... > In the first R-390/URR manual (Collins Radio, 23 October 1953) the Figure > 95 on page 172 (Also IDed TM 856-91) shows that the sensitivity is higher > for the Unbalanced input, compared to the Balanced one. > This is the only manual I know that gives a "sensitivity" plot for the > R-390/URR. > On the R-390A/URR side, the TM 11-856A, on pages 173 and 174, seems to > tell us the reverse from the charts M and J (More sensitivity on the > balanced antenna input than on the unbalanced one) I do not exclude the > possibility that one of those "sources" could tell the truth in reverse.... > > Bottom line: we have to test ! (once again). > > 73, Jacques, VE2JFE in Montreal > > Hi Jim, My thinking in why the second 'balanced' entry is actually the > Unbalanced info is that the Unbalanced antenna input is the high impedance > input and therefore would have the higher microvolt readings. And in the > numerous other places in the spec where both are listed, the 'balanced' one > is first. > > Regards, Larry > > On Sun, Oct 13, 2024 at 8:27 AM Jim Whartenby via R-390 < > r-390@mailman.qth.net> wrote: > > > I agree that one list is mislabeled but I would think that the first > > "Balanced" list is the one that is actually the Unbalanced spec. > > > > My reasoning is that the CW input voltage spec is lower in the first > > list and higher in the second for all frequencies. We may disagree on > > the actual insertion loss of the DA-121 but it would still be higher > > for all frequencies then the DA-124 which uses a series capacitor to > > mimic a short antenna instead of a voltage divider to transform > impedance. > > > > Jim > > > > Logic: Method used to arrive at the wrong conclusion, with confidence. > > Murphy > > > > > ______________________________________________________________ > R-390 mailing list > Home: http://mailman.qth.net/mailman/listinfo/r-390 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:R-390@mailman.qth.net > > 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:R-390@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
