Anyone know what type of resistors were used in the DA-121/U? I almost presume 5% carbon comps but not sure if metal film would be okay. I presume so, especially at the frequencies where this is used, but would still like to know. I plan to make one and want to make it as close to original as possible.
Thanks, Barry - N4BUQ > Hi Jim, > > I feel that you went to a unnecessary elaborate demonstration to find the > (power > wise) attenuation of a DA-121/U. > I cannot understand why you had not replicated the resistor values used in the > DA-121/U because the discussion starting point was to evaluate the DA-121/U > output vs the output setting of the SG connected to it. > In your experiment, it is clear that two DA-121/U connected back to back will > provide a POWER attenuation being the double of what one unit will provide. > But the initial Larry Haney concern was about the VOLTAGE output of a DA-121/U > in the R-390A balanced input (125 ohms load). > The calculated attenuation IN VOLTAGE measured by Larry of the DA-121/U is > correct as being 0.56. > If all the resistances values are EXACT (68 and 100 ohms) the calculated > output > voltage across the 125 ohms receiver impedance is 0.5676 of the SG output > setting. > When Larry mentioned "5 dB" below, that is what his AC voltmeter tell him from > it's dB scale, (as being 20 log 0.56). > But what he used is a VOLTMETER, not a POWER meter: calling that -5 dBv will > be > correct, but it cannot be translated as a POWER ratio because the input and > output impedances are not the same. > The real POWER attenuation of the DA-121/U is (calculated) 8.896 dB, close > enough from the 9dB you expected as the result of your own experiment. > > When you mention an "experiment" about inserting a 50 ohms resistor in > parallel > with a 50 ohms coax, it will give the same result as loading the output of the > source SG with a 25 ohms value. > One of the 50 ohms resistors used will receive 3.52 dB less signal (10 log > 0.44444) than if it was left alone as the load of the SG. > Not even necessary to involve the 50 ohms coax SWR (or the Gamma, for that > matter) in the demonstration. > > About your statement: > " Transmission lines behave differently then DC circuits" > I have a funny conceptual experiment for it, that I used as the introduction > to > the course about transmission lines that I gave to my students at the time: > Let's figure that you have a PERFECT 50 ohms coaxial line (no power loss of > any > kind inside the line) for which the electrical signal velocity inside is 70% > of > the speed of light (Teflon dielectric) and that this "perfect" transmission > line is 210000 km long. > You take an ohmmeter (DC instrument) and connect it to one side of the line, > between the center conductor and the braid: > a) Does the meter reading will be 50 ohms ? > b) If yes, for how much time ? > c) If the other side (the end) of the line is shorted, what will be the > measurement ? > d) And what if the end is open ? > e) And what if the end is terminated in 50 ohms ? > > I will provide the answers later. > All the remaining of the course I gave was to explain these answers, and more. > > 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 > Jim Whartenby via R-390 > Envoyé : 22 octobre 2024 18:36 > À : Larry Haney <larry41...@gmail.com>; r-390@mailman.qth.net > Objet : Re: [R-390] Official specs > > Larry > I built a test fixture that is essentially two DA-121's connected back to > back. > Photos and drawing are enclosed. This does the conversion from 50 ohms to 125 > ohms and then back to 50 ohms. I used 1% resistors to make the attenuator > circuit with the values close to those found here: > https://k7mem.com/Res_Attenuator.html > > The closest I could come to the 64.18 ohms result from the attenuator > calculator > was 63.9 ohms. This is from the parallel combination of 3 each 237 ohm in > parallel with a 1k, in parallel with a 499 ohm resistor. Five resistors in > parallel, all 1% resistors. The result was 63.85 ohms, a 0.5% error. The sub > for the 96.83 ohm resistor is a 100 ohm 1% resistor (3% error) and the sub for > the R-390's 125 ohm impedance was a 121 ohm 1% resistor (3% error). This is > still much better then the 5% resistors used in the original DA-121. > > For a test oscillator I used a Helper SM-1000 signal generator and measured > the > insertion loss with a Stoddart NM-25T frequency selective voltmeter. The > insertion loss was measured at 10 MHz using two 4 foot BNC RG-58 coax cables > from Pomona Electric. 4 foot of coax from the SM-1000 to the test fixture and > another 4 feet from the test fixture to the NM-25T. > > The SG was set for a reading of 30 dB on the NM-25T signal strength meter when > measuring a BNC through connection and then measured 11 dB when the test > fixture was installed in place of the BNC through. The insertion loss for > the > test fixture is 19 dB. Dividing this by two since there are essentially two > DA-121s back to back gives an insertion loss of about 9.5 dB for a single > DA-121. This closely agrees with the attenuator calculator findings. > > So it seems that your DC circuit calculation do not agree with the RF > measurements. Transmission lines behave differently then DC circuits. You > calculate a 5 dB insertion loss, I measure a 9.5 dB insertion loss. > > Here is an experiment that you can try. Insert a 50 ohm resistor in parallel > with the 50 ohm coax. What do you think will happen? Perhaps nothing since > the coax is 50 ohms and the resistor is also 50 ohms? In reality, the coax > has > reactive elements, parallel capacitance and series inductance that make up the > coax impedance. Neither of which will dissipate the signal carried on the > coax. The only losses are from the resistance of the conductors that make up > the coax. Adding a parallel resistor will attenuate the signal to the > receiver > by 3 dB. > > If anyone on this list wants to make their own version of the DA-121, I can > supply the resistor values I used for a token $2 plus postage. Just DM me > with > your address and if you want one or two resistor sets. > > Regards, Jim > Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy > > On Friday, October 18, 2024 at 05:36:08 AM CDT, Larry Haney > <larry41...@gmail.com> wrote: > > Hi Jim, I just checked and I only have 1 da-121. As for insertion loss, my > coax is very short and the connections are very good so the loss there would > not be possible for me to measure. Now for the insertion loss due to > impedance > mismatch (due to resistance variations) would also not be possible for me to > measure, as I don't have the equipment required for that. But, because the 3 > resistors in the circuit are very close to the required values for a perfect > 50 > ohm match to the sig gen, I am sure that the insertion loss due to that very > slight impedance mismatch is extremely small. I have no way to measure that > loss as I don't have the 3 exact value resistors to compare it to. I could > calculate it, but I believe that would be a waste of time without being able > to > measure it. > After all the input you have given me and the research just done, I'm > satisfied > with my current measurements and calculations (IE: the output voltage of the > da-121 is 56% of the input voltage when the load is 125 ohms). > My biggest concern about making snr measurements is for those folks that don't > have a recently calibrated sig gen or calibrated rms AC voltmeter to verify > their readings with. > Regards, Larry > On Thu, Oct 17, 2024 at 1:55 PM Jim Whartenby <old_ra...@aol.com> wrote: > > LarryNo, just one SG and one 125 ohm load. You should be able to determine > the > total loss through two DA-121 attenuators connected back to back with an > o'scope and then divide the loss by two to solve for the insertion > loss.JimLogic: 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
