[R-390] Official specs

Wed Oct 23 11:35:24 EDT 2024

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-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de Jim Whartenby via R-390
Envoyé : 22 octobre 2024 18:36
À : Larry Haney <larry41gm2 at gmail.com>; r-390 at 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 <larry41gm2 at 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_radio at 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