[R-390] R390A noise floor-whats sufficient?

[B Riches]

 Hi Perry,
Thanks for the memories - was stationed at 2140 comm sq Athens 1962-1964 radio maint - still working on R-390a!!
73,
Bill, WA2DVUCape May, NJ
    On Thursday, April 25, 2019, 7:34:29 PM EDT, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:  
 
 Yo Bubba Dudes!,
Wrote: Suffice it to say, the R390a's noise floor - especially if you optimizethe tube locations - is more than sufficient for our modern day, noise filledenvironment.  Disconnect your antenna,put the receiver in MGC mode, then short the antenna balanced inputstogether.  Compare that to what you hearto what you hear with the antenna connected on a dead frequency.  If it's dramatically different - which I'msure it will be - you needn't worry about the R390a's noise floor.  -143 dB is crazy low (even compared to a thunderclap) and will almost certainly never beachievable in real life except in a screened lab.
As my good friend Bob KB8TQ would say: It's complicated.
Now as a generalization that's a fairly accurate assessment.  The problem becomes with all the common exceptions that occur.
Those exceptions are common enough that Ham transceivers routinely offer receivers specs far better than what *surveys* deem necessary for HF receiver needs.
Now we won't call the general ham radio operator *cheap* but extremely thrifty fits well.  Hams aren't going to pay for such stuff (say $9K) unless it provides tangible benefits.
Now let me take you back to yesteryear of mid 1965 to Nov. 1999 at Karamursel  Air Station TUSLOG DET 28 Turkey, located on the south shore of the sea of Marmora.  It was one of a number of USAF Security Service radio intercept base along that south coast.
Cutting to the chase.  I spent two-thirds of my time there in the base receiver site.  We had a 3 bay Hughs synthesized receiver for most of our radio traffic which was IDSB. One side was 16 channel encrypted RTTY and the other a voice reserved channel that went from the eastern Turkey border to ENT AFB in Colorado Springs, CO.
We also had banks of A's from some other sites as well as a few for our shop RTTY and a couple of spares.
Now we were in a radio quiet area and had directional rhombic antenna's but we routinely picked up AM pirate radio rock and roll stations off the English coast to feed to the ditty-boppers in the intercept radio building.
These stations were about 1,575 miles away on a ground wave.  They were not 50 KW clear channel behemoth's with umpteen hundred feet tall verticals as are common in the USA but more like above average peanut whistle stations found on the higher end of the USA AM broadcast band.
We also routinely kept out HP 103B(?) shop master oscillator calibrated by using the one second ticks on WWV 10MHz during daytime hours with a Oscope triggered by the 103.  The ground distance is around 6,175 miles so we had to have at least one, or maybe two skips to get to Turkey.
We had no newspapers (Stars and Stripes) Playboy magazine (the base chaplains) and just a small AM station AFRTS with all the Viet Nam and US news that we should need to hear,
So on 2nd (mostly) and 3rd (a little) shifts we ran wire service RTTY.  Ruters, AP, UPI whatever.  We went through rolls of yellow print paper as if it was TP in a diarrhea ward.We were on standby maintenance except for changing the bay receiver frequencies very infrequently.
So watching the RTTY feeds was one way to pass the time.  There would be long periods of time where the RTTY was printing plain text (using the line output) but when you'd turn up the audio output all one would hear is mud.  For varying periods of time the RTTY would be quite and then many times the output would just be some minor update, so we tended to surf the different services for more/new info.
So there were times when we could hear a flea fart in Figi or have a hard time receiving AFRTS in Greece just 600 hundred or so miles away with clear reception.

The other issue to consider with minimum noise floor figure is that the receiver ages and the original noise floor decreases.

So whatever noise floor one strives for goes back to  It depends (on what is important to you as no one size fits all)
Regards,
Perrier









 


  





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