Electronics > RF, Microwave, Ham Radio

Bandwidth VS Power VS Distance

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If you're coming to FDIM I've organized a SIG meeting on CCW/QRSS/DSSS.

I'll start the meeting with a 10 minute review of Fourier theory in graphical form to make certain everyone is using the same terminology.  No equations more complex than vector addition and multiplication.  The messy stuff will simply be sketches of the time and frequency domain results.

There are 3 relevant parameters, signal BW, data rate and ERP.  I usually state this as time, BW and ERP.  To avoid having 2 BWs, data and signal, I substitute the inverse of the data rate in the form of symbol duration time.

I'm working on algorithms to implement antipodal transfer of a 2 kB message transfer at << 1 W ERP overnight.  However, primary SIG focus will be CCW/QRSS.

I'm presenting pieces of this in the qex@groups.io list.  Please join if you're interested.

Have Fun!


--- Quote from: Foxxz on April 13, 2024, 05:09:44 am ---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.
--- End quote ---

yes, the signal power is spread across signal bandwidth.
If you want to keep the same power for all frequencies within more wide bandwidth, you're needs to increase power.

--- Quote from: Foxxz on April 13, 2024, 05:09:44 am ---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).

--- End quote ---

The RF path calculation just estimates how power from isotropic radiator is reduced with distance. It is reduced because radiated power is spread across entire sphere surface all around isotropic radiator. For real antenna with some radiation pattern it just add some gain in the direction of the main lobe. Longer distance means more large sphere and as result the power received with fixed area receiver antenna will be less.

If you build receiver antenna as a sphere all around transmitter, the power loss on the receiver will be zero, because it will receive all radiated power independently on the distance between transmitter in the center and receiver sphere around it.  :)

As you can see, the signal bandwidth is not needed for RF path loss calculation.

--- 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 ---

Yes, the receiver sensitivity is limited with environment noise floor and noise figure of receiver RF frontend, but you can improve sensitivity by reducing bandwidth. This is possible, because noise power is spread across entire spectrum. So, if you reduce bandwidth, you're cutting part of noise power outside bandwidth of your interest. Since noise power is reduced, you're get better sensitivity. More narrow bandwidth means that you remove more noise power and get more high sensitivity. This is how processing gain works.

But there is a con of more narrow bandwidth - it means that you're needs more time to transmit the same amount of information, so it leads to smaller transmission speed.

Also more narrow bandwidth become more sensitive to precise tuning on exact frequency and requires more longer time to detect signal.

--- Quote from: Foxxz on April 13, 2024, 05:09:44 am ---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.

--- End quote ---

The signal attenuated because less power arrived on the same size antenna placed at longer distance. This is happens because radiated power is flying as a sphere from transmitter, so if you want to not loss signal power on longer distance, you're needs to use more large antenna to collect the same signal power. If you use antenna with the same effective surface, you will get smaller part of power.

See picture:

As you can see, the same power from transmitter is distributed across more large surface area at longer distance.

Signal bandwidth and carrier frequency doesn't matter here. When you calculate RF path loss, you're just calculate relation between two surface areas - for a sphere with radius equals to the specific distance and for receiver antenna.

When using antenna with high directivity, you can use the same calculations for entire sphere as used for isotropic radiator just apply antenna gain correction for transmitter power at specific direction.


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