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
<larry41...@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_ra...@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
<jacque...@videotron.ca> wrote:
The chart TM 856-91 attached.
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 Jacques Fortin
Envoyé : 13 octobre 2024 16:13
À : 'Larry Haney' <larry41...@gmail.com>; 'Jim Whartenby' <old_ra...@aol.com>
Cc : r-390@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@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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