[R-390] Official specs

Mon Oct 14 14:02:57 EDT 2024

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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