Electronics > RF, Microwave, Ham Radio

Bandwidth VS Power VS Distance

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Foxxz:
My brain is having a hard time wrapping itself around a concept here and people have told me I'm wrong and I can't find equations that say otherwise so maybe you can help me.

You have a 100W transmitter. You transmit a 100Hz wide signal. I feel like the amplitude is 1W/Hz and your signal strength is strong and should go further than say...
You have a 100W transmitter. You transmit a 1,000,000Hz wide signal. I feel like the amplitude is 0.0001W/Hz and your signal strength is weaker overall for the same distance.

Calculating RF path loss doesn't seem to take into account signal bandwidth. It says for a given distance/frequency/power both of those signals will attenuate the same (or thereabouts given the low/high frequency edges).

So is the only difference then on the receiver side where the Nyquist noise floor comes into play based on receiver sensitivity VS bandwidth? To get better receiver sensitivity do I need to narrow my receiver bandwidth? IE - Am I going to better hear that 100Hz signal better if I'm not pulling in 1,000,000Hz? I'm guessing yes since I'm having to deal with the additive or average noise floor of the entire 1,000,000Hz VS just 100Hz of noise floor?

Even given receiver/bandwidth sensitivity I just can't see how the signal attenuates the same no matter if the power is spread over 100Hz or 1,000,000Hz. Help me out here.

Foxxz:
Is this what I'm looking for? It seems close but im not quite sure

https://www.itu.int/en/ITU-R/space/WRS16space/WRS-16%20avg%20BW.pdf

Kim Christensen:
Think of a 25watt CW signal... That's 25 watts power at "one" frequency.
Then think of an 25 watt FM modulated signal where the baseband signal is a 0.1Hz sinewave that shifts the carrier slowly between 100Mhz to 100.1Mhz  (100Khz bandwidth)
Basically you have a CW signal that drifts back and forth between 100Mhz and 100.1Mhz every 10 seconds. Now do it faster.

tggzzz:

--- Quote from: Kim Christensen on April 13, 2024, 05:31:18 am ---Think of a 25watt CW signal... That's 25 watts power at "one" frequency.
Then think of an 25 watt FM modulated signal where the baseband signal is a 0.1Hz sinewave that shifts the carrier slowly between 100Mhz to 100.1Mhz  (100Khz bandwidth)
Basically you have a CW signal that drifts back and forth between 100Mhz and 100.1Mhz every 10 seconds. Now do it faster.

--- End quote ---

Now do the same over a much wider bandwidth, so that the CW power at any wavelength is below the receiver noise level.

Provided you can determine what the signal ought to be at any time (i.e. lock onto the signal), you can recover the signal from below the receiver's noise level. Yup, that's called spread spectrum :)

The OP may care to understand the definition of "signal" in the Shannon and Nyquist equations. It is not related in the slightest to the carrier frequency :)

gf:

--- Quote from: Foxxz on April 13, 2024, 05:09:44 am ---To get better receiver sensitivity do I need to narrow my receiver bandwidth?

--- End quote ---

The problem is: If you make the receiver bandwidth narrower than the occupied bandwidth of the useful signal, then you will also lose parts of the useful signal (which you probably don't want). So the only thing that really helps is a narrower bandwidth of the signal (then the receiver bandwidth can be narrower, too), but this means that it can carry less information per unit of time.