This balanced to unbalanced adapter is also shown in TB SIG 319 aka Electronic
Equipment Maintenance Kit MK-299/URM.
As for the Navy's reasoning, I am not sure that this actually holds water, pun
intended. <grin>
A balanced line will better reject common mode noise then an unbalanced line.
This is the main reason why Twinax was used for high speed data communications
back in the day. Cat 5 balanced cable has less loss then either coaxial type
and is much cheaper so it became the preferred transmission line for high speed
signals.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Sunday, October 13, 2024 at 10:04:08 PM CDT, Jacques Fortin
<jacque...@videotron.ca> wrote:
Gianni,
There is a modification instruction from the US Navy that connects the C
connector directly to one end of the balanced antenna 125 ohms input.
The other end of the 125 ohms input is shorted to GND (see Navships_p41
attached) Purpose of the mod is to increase the S/N ratio.
See also P. 59 in the attached eimb-1-13 document.
I applied this mod. in my R-390A the following way:
Connect P207 to J105
Connect P206 to a shorted socket, or short the end of cable anyway you can.
Connect P205 to J107
Use J103 (the C connector) for input.
For your question below, you are correct.
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 Ing. Giovanni Becattini via R-390 Envoyé : 13 octobre 2024 17:47 À : Bob
Camp <kb...@n1k.org> Cc : Jacques Fortin <jacque...@videotron.ca>;
r-390@mailman.qth.net; Larry Haney <larry41...@gmail.com> Objet : Re: [R-390]
Official specs
Is it possible that without the front tuned circuits there is more noise and
thus the S/N ratio is worse?
From my side, the unbalanced antenna (with the DA-124) gave worse results. In
this case, however, I did not applied any correction factor (like for the
DA-121) because I thought the DA-124 on a high impedance had little loss. Is it
correct?
> Il giorno 13 ott 2024, alle ore 23:21, Bob Camp <kb...@n1k.org> ha scritto:
>
> Hi
>
> When they are measuring “AM Sensitivity” are they running 30% modulation or
> 100%?
>
> If by some chance, they are running 30% and you are running 100% …. the
> numbers will off a bit. Will it be 3:1? Probably not. Still it’s going to
> impact things quite a bit.
>
> I’d doubt they switched techniques between the two plots. Switching them
> between a 1953 document and one from 1961 …. yup, could indeed happen.
>
> Bob
>
>> On Oct 13, 2024, at 5:12 PM, 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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