[Milsurplus] RBA RBB RBC receiver multicoupler

Mon Dec 17 02:05:46 EST 2018

It seems that the very best one can do at HF is about -136 dBm or 0.0354 uV for a 75 ohm receiver input impedance.  This is the noise floor which will drown out any signal below this level.  No known HF receiver can do any better then this, it is the physical limit as I understand it.  To copy a signal with a 10 dB margin, you will need an input signal level of 0.1121 uV or -126 dBm in the best of all possible worlds.  
If the Extron amp does indeed have a 30 dB noise figure, then the input signal will have to be at -96 dBM or 3.55 uV to be copied with that same 10 dB margin.  This does not take into account the ability of the Extron to actually be able to amplify a signal this low since the specs say that the minimum video signal is 0.3 volts and nothing is said in the spec about microvolt signal levels.  Then there is the problem of cross modulation and the ability of the Extron to actually handle large signals without wiping itself out or the desired received signal.
Bob's suggestion of a four way power splitter with a 6 dB loss seems to be the better solution.  An input signal to the splitter will need to be at a minimum of -120 dBm or 0.2236 uV.  I would think that the average tube based HF receiver is not anywhere near this sensitivity and free of noise so it will need an input signal of somewhere near -107 dBm or about 1 uV.  Then band conditions and local noise will have to be considered which will require even higher signal levels to overcome the total noise at the antenna.  
Robert's comments about what the Navy did in WW2 as stated in Navships 91469 is perhaps the best solution for hooking up a bunch of the RB(X) receivers to the same antenna.
I don't have any experience on how low in frequency the 75 ohm cable TV splitters can go so that could be another issue to worry about.  Perhaps Al Klase can comment on this?  He has done a lot of work on MF and HF antenna systems using home made splitters.  See: http://www.skywaves.ar88.net/ANTENNA/antsys.htm
The links below are what I used to get a rough idea of what receiver performance can be expected.

http://www.jamminpower.com/main/noise.html

https://vk6ysf.com/uV-dbm_conversion.htm

If I am way off base on the above, please let me know,Jim I wonder why people argue over the 10% of their differences and ignore the 90% they agree on?

      From: Bob kb8tq <kb8tq at n1k.org>
 To: JEFF CICCONE <kg2bz at comcast.net> 
Cc: Military Surplus Mail List <milsurplus at mailman.qth.net>
 Sent: Sunday, December 16, 2018 3:25 PM
 Subject: Re: [Milsurplus] RBA RBB RBC receiver multicoupler

Hi
A 30 db noise figure is considered “pretty quiet” in terms of a video distribution amplifier. That’s not to say that they all have that sort of noise, but that the oneyou get could easily be that noisy. In terms of getting to the sort of 6 db or so NF you probably would want in an antenna distribution setup …. look elsewhere. 
A 4 way power splitter should be pretty easy to find with 6db loss per port. Thatwould likely do better for a coupler than one of the video gizmos and would beat least as cheap.
Bob

On Dec 16, 2018, at 3:51 PM, JEFF CICCONE <kg2bz at comcast.net> wrote:

FolksI have been fortunate to have this nice receiver lineup however I am using an 3 position antenna switch to make the connection from my rf bnc antenna patch panel
I saw an article written by a british ham about using an Extron DA2 RGBHV wideband video distribution amp in lieu of rf multicouplers for my 3 navy receivers
http://gm1sxx.blogspot.com/2018/01/the-poor-mans-receiver-multicoupler.html
 Has anyone tried this ?   Those units are always on ebay and are inexpensive. I wonder how well they handle weak signals, nearby transmitters, IMD etc? I oredered one and will let everyone know how it performs
What about using Extron Ultra-Wideband Matrix Switchers for RGB and Stereo Audio as a crossbar switch to route and select multiple receivers to multiple antennas?  I realize they cannot be used for transmitting. 
By the way I echo the comments about the magnificent beast RBC.  Its smooth tuning , its squelch/silencer and noise limiter circuits really work. The audio filter in the sharp mode is great for crowded CW signals. 
Jeff KG2BZ ______________________________________________________________
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