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Noise Floor Questions, etc.

rmrrgs
 

Hi,

This is just one of those "trying to learn" questions:

Using a RSPdx.

a. At around 10 MHz, am, my noise floor is typically
    about -90 dbm

    How "typical" and "good" is this ?
    (have a simple 40 foot long wire outside, goes to a Balun right there, and then a "long" run of coax to the RSPdx input)

b.  Thought I'd experiment some, so grounded the input of the RSPdx (at the radio).
     Surprised to see that the Spectrum doesn't show a straight line.

    The "floor" is now around -50 dbm or so, but lots of broad peaks, noise etc. show.

    Why ?

c.  Any exp with a floating input is probably meaningless, but If I leave the radio input floating,
    noise floor is also around -90 dbm.  Surprising; at least to me.

Regards,
Bob

Larry Dodd
 

Hi Bob,
You didn’t explain what your ground consists of but it sounds like it’s acting more like an antenna picking up additional noise. A poor ground is worse than no ground at all as your experiencing. 
Larry

On Jan 13, 2020, at 9:01 AM, rmrrgs <rgsrose@...> wrote:

Hi,

This is just one of those "trying to learn" questions:

Using a RSPdx.

a. At around 10 MHz, am, my noise floor is typically
    about -90 dbm

    How "typical" and "good" is this ?
    (have a simple 40 foot long wire outside, goes to a Balun right there, and then a "long" run of coax to the RSPdx input)

b.  Thought I'd experiment some, so grounded the input of the RSPdx (at the radio).
     Surprised to see that the Spectrum doesn't show a straight line.

    The "floor" is now around -50 dbm or so, but lots of broad peaks, noise etc. show.

    Why ?

c.  Any exp with a floating input is probably meaningless, but If I leave the radio input floating,
    noise floor is also around -90 dbm.  Surprising; at least to me.

Regards,
Bob

Kriss Kliegle KA1GJU
 

A) Using ‘new’math, a 40’ wire will be a 1/2 wave on 11.14 MHz with the 95% velocity factor in coated wire. One should see a rise in noise floor near the resonant freq, and succeeding multiples thereof. I’m assuming you are usin an UNUN, not a BALUN.

B) The ground wire noise you see is the other noisy devices such as SPS’s, wall worts, computers, etc attached to it. Better to short the SMA connector at the RSP, and sweep the noise floor.

C) Leave it floating, and enjoy the hobby!


73 de Kriss KP2/KA1GJU/MM
Large Lampshire Bay, St John, USVI

David Jackson
 

On a long wire, you're better of using a counterpoise vs. a ground.

And if you've tied into your mains ground (i.e. that convenient ground rod outside your house that has your electric meter, cable box, satellite, internet etc all connected to) you're going to pick up a tremendous noise of RF noise.