Fly away mystery NOT solved after all

[Capt Drone]

A month ago my Mavic was lost for 3 days after it was pretty much determined that the battery came loose in flight. Eventually with the help of everyone here, I found the Mavic in a tree, pretty much undamaged except for the camera.

Just received the Mavic back from Drone Nerds who replaced the camera and checked everything out. I put about 3 hours on it and it seems to work well. I decided to recreate my crash flight. Sorta wanted to complete the flight that never got completed I guess? So.... I take off from the same place and retraced the ill fated flight only just a little bit higher this time. 360 feet instead of 320 feet. It happened again at about the same exact spot!! Got a quick lost signal warning but got it back this time.

This time I noticed that I was making a turn a lot closer to a cell tower than I origionally thought. (probably 400 feet from it at about the same altitude) The Mavic was on a direct line between me and the tower when I lost signal. So the mystery continues??

Here is a link to my original post..
Mavic Fly Away this evening. Shes Gone!

 

[radio_flyer]

The lost signal was likely caused by the proximity of your Mavic to the cell tower. Cellular base stations transmit on frequencies close to what is used for the Mavic control link. The effective radiated power (ERP) of a cellular base station is typically around 500 Watts while the Mavic controller (US version) transmits only 0.4 Watts. You didn't say how far you were from the Mavic when you lost signal but if the Mavic was closer to the tower than it was to you then the ratio of interference to desired signal would be even higher. For instance if your Mavic was 400 ft from the tower and 1200 ft from you, the ratio would be more than 10,000 to 1. It's difficult for a relatively inexpensive receiver as is in the Mavic to detect the controller signal when interference 10,000 times stronger is on a nearby frequency.

Bottom line: don't fly close to cell phone towers.

 

[stratos]

Mate, there is no mystery here, you mentioned it yourself, CELULAR TOWER. If you see they have some mulit directional antennas as well as other to focus the beam between antennas, soyear pretty much if you fly on the beam you will get bombarded by signals.

 

[Robbyg]

While Radio Flyer is correct that cell phone towers can output as much as 500 Watts by law, the reality is not so simple.
Most towers Tx output is under 100W with somewhere around 10W being the norm. It all depends on how closely spaced the towers are and how many customers they need to serve. Each tower can only handle a limited amount of customers so the companies will put up multiple towers in close proximity, maybe 2-5 miles apart in city's and lower the output so the towers signals do not overlap by much.

Frequencies range from 800Mhz to around 1.9Ghz which is a huge jump away from the 2.4Ghz the Mavic operates on. None the less if you get too close to the tower the Harmonic frequencies can interfere with the Mavic.

I am only throwing this out there so people do not get overly spooked by cell phone towers. I typically fly about 800ft away from one and have never had an issue even when out at 5000ft plus mark from home point. I have also flown right over it on a return trip and did not notice any problems. YMMV but the further out in the sticks you go is the further you should fly from the tower because the output power is probably set to the higher end of the scale.

Rob

 

[boone]

While Radio Flyer is correct that cell phone towers can output as much as 500 Watts by law, the reality is not so simple.
Most towers Tx output is under 100W with somewhere around 10W being the norm. It all depends on how closely spaced the towers are and how many customers they need to serve. Each tower can only handle a limited amount of customers so the companies will put up multiple towers in close proximity, maybe 2-5 miles apart in city's and lower the output so the towers signals do not overlap by much.

Frequencies range from 800Mhz to around 1.9Ghz which is a huge jump away from the 2.4Ghz the Mavic operates on. None the less if you get too close to the tower the Harmonic frequencies can interfere with the Mavic.

I am only throwing this out there so people do not get overly spooked by cell phone towers. I typically fly about 800ft away from one and have never had an issue even when out at 5000ft plus mark from home point. I have also flown right over it on a return trip and did not notice any problems. YMMV but the further out in the sticks you go is the further you should fly from the tower because the output power is probably set to the higher end of the scale.

Rob

With you completely. I can't help but fly near towers sometimes, and I have yet to have a problem as well. But I mean...400 feet? That's pretty close, right? While I agree that it's impossible to unequivocally say the cause, we've got a decent idea going, right?

 

[Aerial Gopher]

Wireless Internet Providers co-locate on cell towers and provide service on none other than 2.4 Ghz Wifi Band.... Or even in the 5.8 Ghz band as well.
Not all cell towers are created equal.... they all can have more than one system on them. Probably had UHF and VHF frequency's for two way radio repeaters.

But yes, signal lost due to the higher RF power output from the "tower"

Its like when the mother in law is yelling in one ear, and you can't hear the football game.

 

[radio_flyer]

A number of cellular bands are uncomfortably close in frequency to the 2.4 GHz used by the DJI controller. LTE band 1 is widely used world wide and has a downlink frequency of 2.11 to 2.17 GHz. The band edge for bands 30 and 41 are 2.36 GHz and 2.496 GHz respectively. Both of these bands are in use in the US.

The main takeaway is to avoid close proximity to cell towers if possible. If you routinely fly in an area with cell towers nearby you might want to check what bands the carriers in your area are using and be extra cautious if any are above 2 GHz. And if you have to fly in the vicinity of a cell tower keep in mind that the base station antenna is directional and the field is concentrated in the horizontal direction. In other words, don't fly at the same altitude as the cellular antenna; if you are anywhere close to the tower, put as much elevation difference between your Mavic and the cellular antenna as possible.

"Its like when the mother in law is yelling in one ear, and you can't hear the football game." That's a great analogy. The solution is the same though - avoid the area the interfering signal is coming from.

 

[m0j0]

So let me understand this... you lost your Mavic on a particular flight path. Then you find it in a tree.... You then fly the same route.... again! Hmmmm... I think you should fly it again right where you lost it. That should crack the case.

 

[Robbyg]

A number of cellular bands are uncomfortably close in frequency to the 2.4 GHz used by the DJI controller. LTE band 1 is widely used world wide and has a downlink frequency of 2.11 to 2.17 GHz. The band edge for bands 30 and 41 are 2.36 GHz and 2.496 GHz respectively. Both of these bands are in use in the US.

The main takeaway is to avoid close proximity to cell towers if possible. If you routinely fly in an area with cell towers nearby you might want to check what bands the carriers in your area are using and be extra cautious if any are above 2 GHz. And if you have to fly in the vicinity of a cell tower keep in mind that the base station antenna is directional and the field is concentrated in the horizontal direction. In other words, don't fly at the same altitude as the cellular antenna; if you are anywhere close to the tower, put as much elevation difference between your Mavic and the cellular antenna as possible.

"Its like when the mother in law is yelling in one ear, and you can't hear the football game." That's a great analogy. The solution is the same though - avoid the area the interfering signal is coming from.

If I remember correctness the 2.11 to 2.17Ghz region is the Tx Frequencies of the phone in LTE 1 and the towers Tx are in the 1.9x range for Duplex use. You really do not need to worry about the 1 watt or less that a cell phone is putting out.

Also when we talk about these numbers it gives the impression that the frequencies are close! They are not close, they are miles away from each other in terms of broadcast spectrum. Even if we use 2.17Ghz that is over 230Mhz away from the WiFi Bands. To put that in perspective the huge analog TV towers of the 1990's were pumping out 100KW with only a 6Mhz spacing between channels.

If someone says there is a WiFi hot spot attached to the tower that is a concern but the actual cell phone signal is really not all that much of a concern.


Rob