Hi If you take a look at the info that was posted several weeks back:
The input impedance goes into the “many hundreds” of ohms and well below 100 ohms. That’s after using the trim cap. We’re not talking about +/- 10% here. We’re talking about “near open circuit” down to “pretty good match for 50 ohms”. Bob > On Oct 27, 2024, at 12:59 PM, Jim Whartenby <old_ra...@aol.com> wrote: > > Bob > > I am sure that you are correct, the input impedance to the R-390 has a > tolerance so the balanced input varies around 125 ohms. There should be > enough of an adjustment in the "Antenna Trim" capacitor to get comfortably > close to 125 ohms or even close to 50 ohms, at any frequency. > > > But that was not the original issue. The test spec is not an assumption, it > is a mandate. It says to use the DA-121 in order to comply with the test > spec. So if you wanted to sell R-390s to the Signal Corps, you had to pass > their test spec as written and as witnessed by the government source > inspector. > > > This discussion started with a challenge to this test spec. The RF input to > the R-390, according to the spec, was higher then what the current owners > experience with their receivers. All I did was to point out was that the > DA-121 has an insertion loss which was not overtly accounted for in the spec. > Ever since then, the insertion loss calculation has taken over the > discussion. Most have stated that the DA-121 insertion loss is 5 dB which > appears to be true if the impedance change from 50 to 125 ohms is not taken > into account. > > > Look at the R-390 "Pearls of Wisdom": > https://www.r-390a.net/Pearls/sensitivity_alignment.pdf , check out pdf page > 684. > > "Because the DA-121 impedance adapter is a resistance 'L' pad and as such the > output voltage is a percentage of the input voltage (45% of input, 9.1 db > loss, or 4/9 of the input) for this adapter (each adapter is different). EG: > if the sig gen reads 1 uV out, the voltage seen at the rx will be 0.45 uV. Of > course, this is only true when the actual input impedance of the R-390A > balanced antenna connection is 125 ohms. Well, as you probably know, the > impedance varies from about 50 ohms to about 200 ohms, depending on the > received frequency. Although I have not calculated it, I believe the variance > is small enough to not make much difference. Regards, Larry" It should be > pointed out that the 45% mentioned above should actually be 55% or 5/9 of the > input voltage. Using the voltage divider rule, 125 ohms divided by (125 + > 100 ohms) = 0.556. So the 0.45 uV mentioned above should actually be 0.55 > uV. > > > To account for the DA-121 insertion loss, one can multiply the signal > generator attenuator dB reading by a little over 0.1 to get the actual power > applied to the R-390 input. If the DA-121 insertion loss was 10 dB then the > conversion factor would be exactly 0.1 but the DA-121 is a bit less. To > convert the SG voltage reading to what the R-390 "sees" the conversion factor > is 0.55 times the SG voltage. > > > As I have tried to point out, dB is always a power measurement. Power equals > the square of the voltage ratio IF and only IF the input and output > resistances are the same. When they are not the same then one has to use 10 > log ((Vout / Rout) / (Vin / Rin)) to find the insertion loss (or gain) of the > network. > > > I have read the above over perhaps a dozen times so hopefully any mistakes I > made today have been caught. > > Regards, Jim > > > Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy > > > On Sunday, October 27, 2024 at 09:09:46 AM CDT, Bob kb8tq <kb...@n1k.org> > wrote: > > > Hi > > We know that the R390() does not supply a 125 ohm load to the antenna > or the test setup. Based on doc’s shown earlier, it typically is way off from > 125 ohms. > > This is not at all uncommon in the world of receivers. > > Thus the *assumption* that the radio supplies a 125 ohm load is suspect. > > Welcome to why “1 uV” out of any signal generator probably is not what the > input to the radio actually has applied to it. > > Do people head off and work out what’s “really there”? You could work it out > various ways. That’s not how the spec on the radio is written. If the signal > generator says 1 uV that’s the correct number to use. > > How is this relevant? > > If I hook up a 50 ohm generator directly to the input of the R390(), it is > running > from a 50 ohm source. Based on the doc’s shown a wile back, the input to > the radio is *always* higher than 50 ohms (and often by quite a bit). Loading > will have a very different impact on that 50 ohm source than on a 125 ohm > source. > > If you *do* want to work this out in the “real case” ( = radio hooked up) > *and* you want to do it only based only on power : You have a whole lot of > work > to do. One (as yet unmentioned) part of that is the input to the radio has a > reactive > component. That messes a bit with power math. > > Bob > > > On Oct 27, 2024, at 9:40 AM, Larry Haney <larry41...@gmail.com > > <mailto:larry41...@gmail.com>> wrote: > > > > Jim, I agree with this posting of yours *except* for the 1st and last > > statements. > > > > 1. First you said: 'What has been overlooked is that there is an impedance > > transformation from 50 to 125 ohms.' We are all very aware of this fact. > > > > 2. Lastly you said: 'To convert the* SG voltage output* into the voltage > > actually seen by the R-390, multiply the SG reading by *0.1235* or divide > > the SG reading by 8.097, either way works.' That is *not right* at all. > > You just went through a nice step by step explanation about how to > > determine the power loss, then you use that power loss ratio (0.1235) to > > determine the voltage seen by the 390. *Wrong, wrong, wrong.* The last 3 > > steps in your procedure are: 1. dB = 10 Log ^ (.00247watts / 0.02 watts), > > 2. dB = 10 Log ^ 0.1235, 3. dB = -9.083. *No real disagreement there*. > > The input watts to the da-121 = 0.02 watts, the output watts from the > > da-121 = .00247 watts, that's a 12.35% loss of *power* in watts, not > > voltage. You *can not* use the 0.1235 *power loss* relationship to > > directly calculate the *voltage loss* relationship of the da-121 as you are > > doing in your last statement. > > > > One way to correctly calculate the voltage coming out of the da-121 (Vout), > > would be to use the formula: > > > > Vout = Sqr rt (Pout (watts) x impedance (ohms)) > > > > Where Pout is the power coming out of the da-121 (in this case, 0.00247 > > watts) and impedance is the da-121 load impedance provided by the 390, 125 > > ohms. > > > > Vout = Sqr rt (.00247 x 125) = 0.5556 Volts > > > > .00247 x 125 = 0.30875 > > Sqr rt 0.30875 = 0.5556 > > Vout = 0.5556 volts > > > > Vout is what's going into the 390 (in this scenario). > > > > Regards, Larry > > > > > > -------------------------------------------------------------------------------------------------------------------------------------------------- > > On Wed, Oct 23, 2024 at 9:35 AM Jim Whartenby <old_ra...@aol.com > > <mailto:old_ra...@aol.com>> wrote: > > > >> What has been overlooked is that there is an impedance transformation from > >> 50 to 125 ohms. Any calculation that ignores this transformation is in > >> error. The only solution that accounts for different impedances is by > >> looking at the respective powers at both input and output. > >> > >> > >> 1 volt into the DA-121 gives 0.556 volts out. Looking at the power-in > >> verses power-out using the respective impedances: > >> > >> > >> Power = voltage squared / resistance > >> > >> Pin = 1 volt ^2 / 50 ohms = 0.02 watts > >> > >> Pout = 0.556 volt ^2 / 125 ohms = .00247 watts > >> > >> dB = 10 Log ^ (Pout / Pin) > >> > >> dB = 10 Log ^ (.00247watts / 0.02 watts) > >> > >> dB = 10 Log ^ 0.1235 > >> > >> dB = -9.083 > >> > >> > >> To convert the SG voltage output into the voltage actually seen by the > >> R-390, multiply the SG reading by 0.1235 or divide the SG reading by 8.097, > >> either way works. > >> > >> > >> Regards, 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 <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
