FCC / CE version - maths time!

chriscuk · Nov 17, 2019

Okay so we've got the numbers from DJI and we know the specs for transmitters but they dont list antenna gain or sensitivity of the receiver so we'll have to assume they are using standard 1/4 wave antennas with 0 gain == 2.14dBi (this is very normal for wifi). Playing with the numbers I am struggling to make the FCC predicted range work.

I think we all know that the 2.4ghz wifi travels further in your home than 5.8ghz wifi and that is because "stuff" gets in the way of the signal. Air is "stuff" and the losses through air are greater at 5.8ghz than they are at 2.4ghz == same power you'll get more range with 2.4ghz.

I did wonder if DJI had higher gain antennas at 5.8ghz than at 2.4ghz but it'd mess up all the numbers for CE vs FCC too so it seems to make sense that they're all standard gain (no increase).

KNOWN VALUES
FCC
5.8ghz @ 30dBm transmitter (claimed 4000m)

CE
2.4ghz @ 19dBm (claimed 2000m)
5.8ghz @ 14dBm (claimed 500m)

Guesswork time
Using common values of 0 gain for the antenna, 14db link margin which is common and a Rx Sensitivity of 97dBm which DJI have quoted on a number of their products....
Rx Antenna Gain - 2.14dBi (Assumed)
TX Antenna Gain - 2.14dBi (Assumed)
Link Margin - 14db (Assumed)
RX Sensitivity - 97dBm (Assumed but common for other DJI products)

RF Calculators – ImmersionRC Limited

www.immersionrc.com

Maths time! - We get
FCC 5.8ghz - 3010m
CE 2.4ghz - 2050m
CE 5.8ghz - 480m

This seems to be fairly in line with what people are seeing(ish). I managed closer to 800m with CE @ 5.8ghz which would put the link margin down to 10db which is the lower limit that most systems might see as acceptable. Using this value you might see the FCC version hit out to almost 4700m! Using the same numbers though this means that the 2.4ghz CE version would reach 3250m which is far too far for a toy like this to go.

Either way the numbers quoted by DJI don't quite make sense. They seem okay for the CE version but seem over rated for the FCC version.

mavicn00b · Nov 17, 2019

chriscuk said:
Okay so we've got the numbers from DJI and we know the specs for transmitters but they dont list antenna gain or sensitivity of the receiver so we'll have to assume they are using standard 1/4 wave antennas with 0 gain == 2.14dBi (this is very normal for wifi). Playing with the numbers I am struggling to make the FCC predicted range work.

I think we all know that the 2.4ghz wifi travels further in your home than 5.8ghz wifi and that is because "stuff" gets in the way of the signal. Air is "stuff" and the losses through air are greater at 5.8ghz than they are at 2.4ghz == same power you'll get more range with 2.4ghz.

I did wonder if DJI had higher gain antennas at 5.8ghz than at 2.4ghz but it'd mess up all the numbers for CE vs FCC too so it seems to make sense that they're all standard gain (no increase).

KNOWN VALUES
FCC
5.8ghz @ 30dBm transmitter (claimed 4000m)

CE
2.4ghz @ 19dBm (claimed 2000m)
5.8ghz @ 14dBm (claimed 500m)

Guesswork time
Using common values of 0 gain for the antenna, 14db link margin which is common and a Rx Sensitivity of 97dBm which DJI have quoted on a number of their products....
Rx Antenna Gain - 2.14dBi (Assumed)
TX Antenna Gain - 2.14dBi (Assumed)
Link Margin - 14db (Assumed)
RX Sensitivity - 97dBm (Assumed but common for other DJI products)

RF Calculators – ImmersionRC Limited

www.immersionrc.com

Maths time! - We get
FCC 5.8ghz - 3010m
CE 2.4ghz - 2050m
CE 5.8ghz - 480m

This seems to be fairly in line with what people are seeing(ish). I managed closer to 800m with CE @ 5.8ghz which would put the link margin down to 10db which is the lower limit that most systems might see as acceptable. Using this value you might see the FCC version hit out to almost 4700m! Using the same numbers though this means that the 2.4ghz CE version would reach 3250m which is far too far for a toy like this to go.

Either way the numbers quoted by DJI don't quite make sense. They seem okay for the CE version but seem over rated for the FCC version.

Assuming it’s using the same system as the Air I find at range you need to be more careful with antenna orientation the further out you go. Personally I find 5.8ghz gives much greater range than 2.4 due to lower interference so while 2.4 may propogate further, there's much more interference from competitng sources in that spectrum.

sar104 · Nov 18, 2019

chriscuk said:
Okay so we've got the numbers from DJI and we know the specs for transmitters but they dont list antenna gain or sensitivity of the receiver so we'll have to assume they are using standard 1/4 wave antennas with 0 gain == 2.14dBi (this is very normal for wifi). Playing with the numbers I am struggling to make the FCC predicted range work.

I think we all know that the 2.4ghz wifi travels further in your home than 5.8ghz wifi and that is because "stuff" gets in the way of the signal. Air is "stuff" and the losses through air are greater at 5.8ghz than they are at 2.4ghz == same power you'll get more range with 2.4ghz.

I did wonder if DJI had higher gain antennas at 5.8ghz than at 2.4ghz but it'd mess up all the numbers for CE vs FCC too so it seems to make sense that they're all standard gain (no increase).

KNOWN VALUES
FCC
5.8ghz @ 30dBm transmitter (claimed 4000m)

CE
2.4ghz @ 19dBm (claimed 2000m)
5.8ghz @ 14dBm (claimed 500m)

Guesswork time
Using common values of 0 gain for the antenna, 14db link margin which is common and a Rx Sensitivity of 97dBm which DJI have quoted on a number of their products....
Rx Antenna Gain - 2.14dBi (Assumed)
TX Antenna Gain - 2.14dBi (Assumed)
Link Margin - 14db (Assumed)
RX Sensitivity - 97dBm (Assumed but common for other DJI products)

RF Calculators – ImmersionRC Limited

www.immersionrc.com

Maths time! - We get
FCC 5.8ghz - 3010m
CE 2.4ghz - 2050m
CE 5.8ghz - 480m

This seems to be fairly in line with what people are seeing(ish). I managed closer to 800m with CE @ 5.8ghz which would put the link margin down to 10db which is the lower limit that most systems might see as acceptable. Using this value you might see the FCC version hit out to almost 4700m! Using the same numbers though this means that the 2.4ghz CE version would reach 3250m which is far too far for a toy like this to go.

Either way the numbers quoted by DJI don't quite make sense. They seem okay for the CE version but seem over rated for the FCC version.

Looking at just the 5.8 GHz numbers one can compare the FCC and CE cases without any of those assumptions. 30 dBm compared to 14 dBm is an increase of 16 dBm, which is a factor of 2^(16/3) = 40 increase in power. Omnidirectional radiation would give a range that scales with the square root of the power, i.e. a factor of √40 = 6.3. That gives approximately the 3000 m value that you estimated. Any more directional radiation, including dipole radiation, will give a range that scales with >√P, and so 4000 m doesn't seem that unreasonable.

chriscuk · Nov 18, 2019

My point was to add any gain on the antenna side would increase the CE numbers also and considering they're actually pretty much spot on it seems hard to assume this.

Maybe they're showing conservative numbers for CE and then they're going for big numbers that sell for the FCC version and just using a really low link margin to calculate that value.

Just seems odd that that the CE version calculates quite closely yet the FCC does not

webvan · Nov 18, 2019

Good stuff !
I ran the calcs using the Spark numbers
2.4 GHz : 25 dBm (FCC) ; 18 dBm (CE) ; 18 dBm (SRRC)
5.8 GHz : 27 dBm (FCC); 14 dBm (CE) ; 27 dBm (SRRC)

and I get results that are consistent with what owners are getting (best case for 2.4 of course) so I suppose that means that the whole "enhanced WiFi" business of the Air and Mini comes down to...marketing ! Unless they mean better antennas on the Drone.