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

[Larry Haney]

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> 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
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html