[R-390] Doubts about R-390A sensitivity

[Ing. Giovanni Becattini]

Glad to read you..
Thanks, very complete answer!

> Il giorno 22 set 2024, alle ore 18:39, Larry Haney <larry41gm2 at gmail.com> ha scritto:
> 
> Hi Gianni,  Nice to hear from you.
> 
> You are right in your suspicion.  The Navy documents assume that the Navy mod 5 is on the 390A that changes the balanced input so that it is connected to the unbalanced C connector on the back of the 390A.  But still, the da-121 should be used to match the impedance because it should be connecting to the balanced antenna input circuit (whether the Navy mod is on or not).  And, since you are using the da-121, it has a rather large loss that needs to be taken into consideration when calculating the s/n.  For more information, see my document in our tutorials section: R-390A Measuring Signal to Noise Ratio Report <https://www.r-390a.net/R390A%20Measuring%20Sig%20to%20Noise%20Ratio.pdf>, Why the Navy did Field Change 5.  A well tuned 390A should get < 1 uv s/n (after accounting for the da-121 loss).
> 
> Regards, Larry
> 
> On Sun, Sep 22, 2024 at 7:30 AM Ing. Giovanni Becattini via R-390 <r-390 at mailman.qth.net <mailto:r-390 at mailman.qth.net>> wrote:
>> Hi Friends,
>> 
>> I believe that this is a theme already afforded, but I could not find a precise answer, so I ask kindly if someone would help to cure my ignorance.
>> 
>> The manual NAVSHIPS 0967-063-2010 (4. 3.1 Procedure Q1- Measure Receiver Sensitivity) states: “Connect the RF OUTPUT jack of Signal Generator AN/URM-25 to receiver ANTENNA UNBALANCED jack J103”.
>> 
>> This doesn't seem correct to me, as I believe that in this way the signal generator and receiver’s impedances don't match. To confirm this, the TM 11-856A manual (94. Over-all Receiver Gain Test) states that the DA-121/U dummy load must be used for the connection from the signal generator to the R-390A antenna when you check the overall gain.
>> 
>> According to my measurements, the difference between using it and not using it is not small: more than 10 dBm, as I could expect. 
>> 
>> Using the DA-121/U at 7,500 kHz, I measured for my R-390A a sensitivity of -100 dBm (2.1 µV) for 10 dB S/S+N ratio. 
>> 
>> It is compliant with the official specification, but I read that it should be much better.
>> 
>> Thanks
>> 
>> Gianni
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