You are here

Free Space Path Loss

5 posts / 0 new
Last post
Free Space Path Loss

I watched the mesh meeting video from yesterday and have a question regarding Andre's Free Space Path Loss table. It appears to be off, but perhaps it is accounting for something that I'm not. For example, he has a path loss of 114 dB for a 1.5 mile link at 2 GHz. I calculate 107.7 dB for 2.397 GHz (Ch -2). All the numbers seem to be off by similar amounts. The equation I'm using is 20*log(4*pi*(distance in m)/(wavelength in m)). Am I missing something?

BTW, enjoyed the meeting content. Thanks for providing!

Dave K3GX

K6AH's picture
More than free-space loss

Thanks Dave.  Yeah, Radio Mobile adds 6.29 dB of "Statistical" loss.  So my figures add that to the Free-Space loss derived from your equation.


Thanks Andre, that explains

Thanks Andre, that explains it... and it's probably more realistic.


More on Link Calculations


Doing some more link calculations here and thinking about the 6.29 dB of statistical loss. Is the statistical loss purely a function of atmospherics between two nodes, or could there be more to it? For example, could it also account for where the receiving station is between the TX antenna's 3 dB points that define its beam width?

As for output power and sensitivity, the Ubiquiti datasheets are not totally clear with respect to how their units would perform on an AREDN non-shared channel. For example, the NSM2 lists an output power of 28 dBm, and I know 28 dBm can be selected in the AREDN software. However, it also lists output powers for Data Rate/MCS that varies from 22 dBm (MCS7) to 28 dBm (MCS0) and from 24 dBm (54 Mbps) to 28 dBm (1-24 Mbps). Is there a certain modulation format we should assume when we look at this table, or is our output always 28 dBm, regardless of bandwidth chosen (hence, data rate) or channel chosen? Intuitively, I would think the bandwidth chosen or modulation format would not affect the total output power, as it would just be the same power spread out more or less through the spectrum. However, I would think that the output power would be affected by where the channel is located in the band, but this isn't shown in the table.

The same was noted for sensitivity. For the NSM2, AREDN shows a sensitivity of -96 dBm. However, it varies between -74 and -96 dBm in their datasheet depending on the data rate and MCS.

I used the NSM2 above as an example since I own an NSM2 and know what AREDN reports. However, I don't know what to assume for a PBE-M5-300. The datasheet table header says the output power is 26 dBm, yet the average TX power ranges from 23-26 dBm, depending on data rate / MCS. The sensitivity varies between -74 and -96 dBm. What should I assume for output power and sensitivity for the PBE-M5-300?

Dave K3GX

KE2N's picture
path loss

The statistical part of this results from the fact that path loss is not just a single number but rather a range of numbers, each with a probability - known as a statistical distribution in math language - usually called a "bell curve" for the kind of distribution we are talking about here (see figure).

If you are happy with 50/50 reliability, you pick the central value.  Most designs expect to deliver something better than 50/50, so they pick a value out on the tail of the curve to get 75% or 98% or whatever.  That is where the 6.29 dB comes from. 

In terms modulation scheme, the usual approach is to see what path loss can be tolerated for each pair of units and then see what MCS the unit can be expected to operate at.  If it is not fast enough for you, then you need better/higher antennas, etc.  This is the approach that  Ubiquiti uses in their  (optimistic) online tool.

For example with the NS M2 to pick a few numbers

MCS0     TX   23 dBm  RX   -96 dBm  ->   path -119 dB
MCS5     TX   20 dBm  RX   -83 dBm  ->   path -103 dB​
MCS14   TX   18 dBm  RX   -78 dBm  ->   path -96 dB​

So, if your path loss is 110 dB, you can expect it to run at MCS0.  If you can reduce your path loss to 103 dB or less, then it might run at MCS5 and so forth.

If the two units are different models, you grab the transmit number from one unit and the receive number from the other unit, for a given MCS.

Although you can set the power level higher than the TX numbers shown in the specs, you should use the spec number in your planning.

AREDN's recommendation (I think I read this) is that, when running at higher than spec power,  you should back down the power until the data rate just starts to drop, then leave it at that setting. 

Lastly, I would note that Radio Mobile does not have a good model for microwave attenuation due to trees.  And the tree height data shown in the terrain model is very rough and may not be accurate for your particular location.  There are ways around these deficiencies, but it is hardly worth the trouble since, once you path gets into trees the signal disappears quickly.


Image Attachments: 

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer