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made some RF discoveries today

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Hi people,

Being fairly new here, I thought it was time to contribute something helpful. I happen to have an RF spectrum analyzer that I use for my day job, so I thought I would put it to good use and answer some questions I had about the P4. I made a little video to provide some explaination of the RF transmission scheme on the P4, and some surprises I found. If you're confused when people start talking about channels and downlinks, hopefully this will help.


I'll try to answer any questions you have about this video in the thread below. And kudos if you find the 2 lies I told. oops. ;)
 
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I'll try to answer any questions you have about this video in the thread below. And kudos if you find the 2 lies I told. oops. ;)[/QUOTE said:
Hi James,

I think I understand your explanation and tests, so am I right in thinking that with the channel hack pushing the channel above 20, to say 25 would be beneficial for video tansmission.
 
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Hi James
Very enlightening post, many thanks for your analysis :)
PS where or how did you get the Channel hack, could you post a link..? and does the hack work with the Mavic

Many thanks
Waylander
 
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so am I right in thinking that with the channel hack pushing the channel above 20, to say 25 would be beneficial for video transmission.

Yes, that's the whole idea for the channel hack. It will lower your interference from wifi devices and your own rc. Also note, with the hack there are channels below 13 that achieve the same.
 
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PS where or how did you get the Channel hack, could you post a link..? and does the hack work with the Mavic

Here's a link to perform the hack:

Not sure if works on the Mavic, as it is a patch for the DJI GO App. I assume if you use DJI GO to fly, then it will work.
 
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Would have watched this but for some reason the video is unavailable to me in europe

huh. Sorry about that. I've limited the clip to only embed on phantomplots.com. Do you access the forum from a different url in Europe?
 
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Well, if i have this right then a summary would be that using DJI Go 4 app, channel 20 is the only one on which the video and control frequencies don't overlap. Channel 32 if by hack (or using Litchi?). The left xmitter antenna is used as a transceive antenna, but both antennas are used in a diversity receive configuration. Are both stock antennas then "dual band" and basically the same antenna electrically? As far as other interference I have learned to put my iPad Mini 4 into "airplane mode" as its wi-fi can give me an RF Interference flag. I'd be interested in seeing a simulated long range test with perhaps putting the left antenna into a dummy load and doing an "A/B" comparison between ch. 20 (or 32) and the others to see how the video quality holds up.
 
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Well, if i have this right then a summary would be that using DJI Go 4 app, channel 20 is the only one on which the video and control frequencies don't overlap. Channel 32 if by hack (or using Litchi?). The left xmitter antenna is used as a transceive antenna, but both antennas are used in a diversity receive configuration. Are both stock antennas then "dual band" and basically the same antenna electrically? As far as other interference I have learned to put my iPad Mini 4 into "airplane mode" as its wi-fi can give me an RF Interference flag. I'd be interested in seeing a simulated long range test with perhaps putting the left antenna into a dummy load and doing an "A/B" comparison between ch. 20 (or 32) and the others to see how the video quality holds up.

You've got the first part right. Using the stock app, Ch.20 is the only channel that doesn't overlap the rc control data. Using the hack opens Ch. 1-12, and 21-32, none of which overlap with the rc. I've heard that Litchi has eliminated the 32 ch. version of their app...

The term "Dual Band" (in our world) refers to antennas that operate on both 2.4GHz and 5.8Ghz. The Phantom just uses the 2.4Ghz band. The stock antennas are not dual band, as they don't need to be.

Your idea of a simulated long range test has some merit for sure. I may try that next. Also I want to hook up the sunhans amps to the analyzer and directly measure what they're doing. I've ordered some adapters to do that as the RP-SMA connectors don't connect to my analyzer.
 
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huh. Sorry about that. I've limited the clip to only embed on phantomplots.com. Do you access the forum from a different url in Europe?

Ah seems that the PhantomPilots app breaks that, will look at it in the morning from a web browser
 
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Hi people,

Being fairly new here, I thought it was time to contribute something helpful. I happen to have an RF spectrum analyzer that I use for my day job, so I thought I would put it to good use and answer some questions I had about the P4. I made a little video to provide some explaination of the RF transmission scheme on the P4, and some surprises I found. If you're confused when people start talking about channels and downlinks, hopefully this will help.


I'll try to answer any questions you have about this video in the thread below. And kudos if you find the 2 lies I told. oops. ;)
****************************
Capture.PNG
 
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Hi James - thanks for doing this & sharing.
Do you think the results are the same for the P3 Pro? My guess is yes it would. But not sure.

And - this is something I've always been fuzzy on - are you saying that the TRANSMIT freq. is pretty much constant in the middle of the 2.4~2.5 spectrum and the 32 channel hack only affect the Video downlink?
And does this affect the video data received AND written to the card - or only what we see on screen?
 
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You've got the first part right. Using the stock app, Ch.20 is the only channel that doesn't overlap the rc control data. Using the hack opens Ch. 1-12, and 21-32, none of which overlap with the rc. I've heard that Litchi has eliminated the 32 ch. version of their app...

The term "Dual Band" (in our world) refers to antennas that operate on both 2.4GHz and 5.8Ghz. The Phantom just uses the 2.4Ghz band. The stock antennas are not dual band, as they don't need to be.

Your idea of a simulated long range test has some merit for sure. I may try that next. Also I want to hook up the sunhans amps to the analyzer and directly measure what they're doing. I've ordered some adapters to do that as the RP-SMA connectors don't connect to my analyzer.

Got it. I was thinking that the P4 standard did use Dual Band (2.4 ghz control, and 5.8 ghz video), but thats the P3 Pro and P4P if it got it correctly. I was an Engineering Tech for 25 years or so and "Novice" Ham in 1971, we live in the same world. Except that as an old boss of mine (metallurgist) used to say, "I've probably forgotten most of what I learned in College, and the rest has been disproven...).
 
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View attachment 81623

What a great chart! Thanks for that! Is that something you'd be willing to share the original excel, as I would love to add some data to it.

I see you've marked ch.22 green, but I've found that is also part of the spectrum licensed by Sprint as BRS1. The entire band from 2495-2690 is licensed to various parties depending on location. The FCC calls these locations BTAs, which are basically like counties where you can operate. Currently, Sprint (who owns Clearwire) licenses about 80% of this spectrum, but the actual channels in use do vary based on the BTA.

I'd love to get some more info on use of the 2281-2401 spectrum. Yo've noted that a lot of it is for Sat/Space comm, but none of it is confirmed?
 
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Well if that chart is right he is wrong in what he is saying about channel 20??

Ch.20 is still a good option as your rc will not interfere with it. You will have some other wifi noise in that channel to deal with, as you can see in the video.
 

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