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

[Jim Whartenby]

You will find answers in the "Final Engineering Report" starting on document page 54, "Antenna Input Circuit."  The receiver input impedance is a compromise and varies between 50 and 300 ohms instead of the published 125 ohms +/- 15%.  Same is true for the unbalanced input.

Terminating the coax in it's characteristic impedance tames the VSWR for all frequencies that are on the coax, not just the frequency of interest.  A VSWR of 10 implies that a 10 uV signal can appear to be as high as a 100 uV.  In other words, it's complicated and the report goes into the compromises that gave the best performance in real life conditions.  Sensitivity is only one of these considerations.

Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, September 22, 2024 at 09:30:46 AM CDT, 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
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