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Author Topic: Understanding noise floor?  (Read 344 times)

KT0DD

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Understanding noise floor?
« on: July 09, 2021, 04:51:26 AM »

I've been looking at Rob Sherwoods table and the first column listed is noise floor. Sorry if I sound stupid, but which is better, a lower number or a higher number? Is -133 vs. -123 better, or is -123 vs. -133 better? Constructive comments are always appreciated. Thank you.

Todd - KT0DD
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W1VT

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Re: Understanding noise floor?
« Reply #1 on: July 09, 2021, 05:57:00 AM »

For most hams on 160 through 6, it doesn't matter.  The local noise totally dominates the system sensitivity.

Zak W1VT
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W8FN

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Re: Understanding noise floor?
« Reply #2 on: July 09, 2021, 06:25:54 AM »

The noise floor is specified in dBm (dB relative to 1 mW), measured in a 500 Hz bandwidth. –133 dBm is therefore 1/10 as much (10 dB less) power as –123 dBm. That being said, W1VT is correct that for most folks in urban and suburban locations the noise from various electronic and power-line related sources will usually be overwhelmingly higher.

73...
Randy, W8FN
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K1KP

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Re: Understanding noise floor?
« Reply #3 on: July 09, 2021, 06:27:24 AM »

Noise floor is usually expressed as dBm, which is the noise power relative to 1 milliwatt into 50 ohms. Because the usual numbers are much lower than 1 milliwatt, the numbers are negative. A greater negative number is indicative of a more sensitive receiver.

Short Answer: More negative numbers are better.

Now for the caveat: Sensitivity of most modern receivers is so good that the noise floor of the receiver is much lower than the typical noise level coming from the antenna. This noise from the antenna can be 'real' noise, i.e. thermal or galactic noise; but much more likely in today's environments it is man-made noise from all the electronic crap nearby.

This means that having a receiver sensitivity of -133dBm doesn't really matter if the noise floor in your location is -103dBm, which is a typical number for 20 meters in a residential environment.

This also means that if you do not add attenuation on the low bands, you are wasting dynamic range of the receiver. One should almost always use 6-10db attenuation on low bands (80m or lower) unless you are using some sort of very low gain antenna such as a beverage or flag.

-Tony, K1KP
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W1VT

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Re: Understanding noise floor?
« Reply #4 on: July 09, 2021, 06:38:22 AM »

I saw lots of noise on the short Beverage that I used to work Japan on 80 CW from Connecticut.  Beverages are great beginner antennas because they pick up a lot of signal.  It is best to decouple common mode noise when installing Beverages.

I took advantage of the noise with my 8 element Flag array--I downsized the flags in my noisy directions as there was no benefit to picking up a lot of noise.  This made it easier to juggle the locations to dodge noise sources in preferred directions. This allowed me to work Japan on 160!

The Waller Flag, two closely spaced flags with a phasing line for better pattern, often needs a feedpoint mounted preamp for optimum performance.  The preamp in the rig is best turned off if you have one at the feedpoint.

Zak W1VT
« Last Edit: July 09, 2021, 06:43:20 AM by W1VT »
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KT0DD

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Re: Understanding noise floor?
« Reply #5 on: July 09, 2021, 03:15:19 PM »

Noise floor is usually expressed as dBm, which is the noise power relative to 1 milliwatt into 50 ohms. Because the usual numbers are much lower than 1 milliwatt, the numbers are negative. A greater negative number is indicative of a more sensitive receiver.

Short Answer: More negative numbers are better.

Now for the caveat: Sensitivity of most modern receivers is so good that the noise floor of the receiver is much lower than the typical noise level coming from the antenna. This noise from the antenna can be 'real' noise, i.e. thermal or galactic noise; but much more likely in today's environments it is man-made noise from all the electronic crap nearby.

This means that having a receiver sensitivity of -133dBm doesn't really matter if the noise floor in your location is -103dBm, which is a typical number for 20 meters in a residential environment.

This also means that if you do not add attenuation on the low bands, you are wasting dynamic range of the receiver. One should almost always use 6-10db attenuation on low bands (80m or lower) unless you are using some sort of very low gain antenna such as a beverage or flag.

-Tony, K1KP

Thank you. Got what I was looking for. The more negative number, the more sensitive a receiver. I know it's probably irrelevant in real world conditions due to the normal background noise in urban and suburban areas. However, for me making a choice, I tend to look at it as a "quality of design / quality of engineering" factor, even if I will never realize it. It seems like most radios in my mid price range are quite close, so I may be splitting hairs, but when shelling out $1500-$3000 on a new radio, I look for the best figures as one of my parameters. I also look at layout / arrangement of controls, styling, E-Ham reviews etc. I don't just go by Rob Sherwood numbers alone.

Thanks everyone for your replies.73.

Todd - KT0DD
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N6YFM

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Re: Understanding noise floor?
« Reply #6 on: July 10, 2021, 08:04:17 AM »

You guys are making this WAY too complicated.

Understanding noise floor?
Go to the beach, lay down on the sand, and hold a shell to your ear.
We call that the noise floor.

[Neal ducks under the work bench to avoid hams throwing transformers...]
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