[R-390] Official specs

Mon Oct 14 18:27:53 EDT 2024

Larry

You are really splitting hairs here!  The T-Attenuator IS the solution used for the DA-121.  One can assume that 0.22 ohms is a short circuit.  Given this, the solution found using 9dB attenuation is good enough.  If you revisit the link, the next box down gives resistor values using 5% and 1% resistors.  The 5% resistors are 68 ohms shunt and 100 ohms series.  Which are exactly the values which the DA-121 uses.

Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Monday, October 14, 2024 at 01:03:11 PM CDT, Larry Haney <larry41gm2 at gmail.com> wrote:   

 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_radio at 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 <larry41gm2 at 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_radio at 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 <jacques.f at videotron.ca> wrote:   

 The chart TM 856-91 attached.

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 Jacques Fortin
Envoyé : 13 octobre 2024 16:13
À : 'Larry Haney' <larry41gm2 at gmail.com>; 'Jim Whartenby' <old_radio at aol.com>
Cc : r-390 at 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 at 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
> 

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