Hi James, thank you for sharing this information. I had bee thinking of doing the exact this using my Signal Hound that I use for my day job as well. So itappears the RC channels remain in the CH13- CH20 range but the Hack enables you to move the Video channel . Thanks for sharing this great information!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.
This is something I've wondered about for the P4P. So you're saying you confirmed that control signals are always using 2.4Ghz when flying auto 5.8, and the 5.8 is only for video downlink, correct?
Hi all,
So I took a first run with my Phantom 4 Pro and my RF Explorer.
Threw the results into a quick video:
~~~
Primary takeaways are:
1) P4P "Lightbridge 2.5" seems to match the same channel number <-> frequency layout as the P3P (and P4) "Lightbridge 2.0" systems.
2) The "control plane" aka, controller signal, seems to follow the video band on the P4P. Aka, if you manually select a channel in the 5.8 GHz band, the FHSS controller signal also seems to move entirely into that band. Conversely, if you select a 2.4 GHz channel, the controller signal moves back to the 2.4 GHz band.
3) Something interesting seems to be going on with the Lightbridge video signal. Check out the video at about 5:20 to see what I'm on about.
~~~
Hope this helps someone!
- Sam
So I took a first run with my Phantom 4 Pro and my RF Explorer.
Threw the results into a quick video:
~~~
Primary takeaways are:
1) P4P "Lightbridge 2.5" seems to match the same channel number <-> frequency layout as the P3P (and P4) "Lightbridge 2.0" systems.
2) The "control plane" aka, controller signal, seems to follow the video band on the P4P. Aka, if you manually select a channel in the 5.8 GHz band, the FHSS controller signal also seems to move entirely into that band. Conversely, if you select a 2.4 GHz channel, the controller signal moves back to the 2.4 GHz band.
3) Something interesting seems to be going on with the Lightbridge video signal. Check out the video at about 5:20 to see what I'm on about.
~~~
Hope this helps someone!
- Sam
So looks like the controller actually does follow the same band as the video signal.
Obviously, @james_hunt didn't have a P4P to test with to confirm this... but I do. ;-)
Nice analysis. This is very interesting to see and confirm what the radio is doing. Thanks for sharing this.
Glad to help! Hope you get your hands on one soon as well, as I think your signal analyzer might be much less noisy and allow for some closer analysis.
Also, any chance you have thoughts on that wonkiness with the lightbridge video signal stuff?
GadgetGuy
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When the RC left most LED turns red, you no longer have RC control of the aircraft, and RTH will initiate after 3 seconds, if control is not restored before 3 seconds elapses. As long as the LED stays green, no matter the video feed status, you still have full control of the aircraft. I am often prompted with the P4P that "Video transmission has been lost---Do you want to RTH?" which I reject, until the control signal is truly lost, while attempting to restore video by elevating and flying on.
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Thanks for that analysis @Shammyh! Very interesting results with the lightbridge rolling like that. Now I have to find a P4P to see what the 5.8 spectrum looks like. Do you know if it's using 20 or 40 MHz channels? I imagine there's no hack for the 5.8 band due to the requirement of DFS. Are you using any amps or just stock for that test? It looks like DJI is really pushing the envelope here for the video downlink. I've never seen anything like that rolling pattern before. My initial thought is that it is a way to enable higher transmit power. The FCC limits the power output based on the spectral efficiency, so if they use less of the channel, they can put more power into the parts they do use.
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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.
great video
Glad to help!
- 10 MHz channel width for the LB video feed, though it only seems to be using maybe 8 MHz or so at a time.
- Just stock for the tests, though I aligned the antenna polarity throughout the tests, so I expected pretty high levels.
Interesting thoughts about the "rolling" aspect of the LB video signal, wouldn't surprise me at all that DJI is doing whatever they can to boost the legal signal levels.
Hope you get your hands on a P4P so we can see what's going on in the 5.8 GHz space!
So here's that new video I promised about measuring the various antennas. The VSWR is a complicated measurement for sure, but a few basics are:
1) Most manufacturers pass or fail their products compared to a baseline VSWR measurement of >2. Meaning if it is higher than 2 across the frequencies it is intended to work in, it will fail and be rejected.
2) A VSWR of 1 is perfect and never achieved in the real world.
3) VSWR is logarithmic. A VSWR of 2 represents a loss of 10% of the transmit power. A VSWR of 3 represents a loss of 25% of the transmit power. Example: If you have a 100mW transmitter (like the P3A/P4), using an antenna with a VSWR of 3 for the channel you're operating on, can only output 75mW.
1) Most manufacturers pass or fail their products compared to a baseline VSWR measurement of >2. Meaning if it is higher than 2 across the frequencies it is intended to work in, it will fail and be rejected.
2) A VSWR of 1 is perfect and never achieved in the real world.
3) VSWR is logarithmic. A VSWR of 2 represents a loss of 10% of the transmit power. A VSWR of 3 represents a loss of 25% of the transmit power. Example: If you have a 100mW transmitter (like the P3A/P4), using an antenna with a VSWR of 3 for the channel you're operating on, can only output 75mW.
GadgetGuy
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Excellent Information!
Selecting the right manual channel makes all the difference!
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Excellent Information!
Selecting the right manual channel makes all the difference!
thanks man! brilliant...
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Excellent Information!
Selecting the right manual channel makes all the difference!
So, we have the 5.8 option on the P4& P4P . What advantage if any is there over 2.8? ( I realize this is beyond the scope of your research). Thanks
So what are the best channels to use with the a stock P4 controller, or doesn't it matter since the stock omni's are good all over?
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Hi, not trying to hijack your quest. but it is believed to be channel 20. I use directional slip on's for right now as I have trees to push thru and aiming the antenna directly straight at the drone. 360 directional is fine, but my range is up on channel 20 and the cheap slip which block the 360 and push forward the signal.
Nice and thanks for the info. I have some parabolic antennas too, but have yet to try them as I can get very decent range most of the time unless obstacles are in the way.
Only the P4P and Inspire 2 adds the 5.8 option. It has the advantage of moving your control and video downlink outside of the heavily used 2.4G wifi bands. But if you're using the channel hack, you're already able to get outside of the wifi bands. So that's less of an issue.
Another advantage is the channel width in the 5.8G band is twice as wide (20Mhz), meaning your higher bit rates (10Mbps) will work better.
A further advantage of 5.8 is reduced loss due to moisture (breaks thru foliage better, and a lower fresnel zone and better scattering (gets around objects better). One drawback is that 5.8 has more loss thru the air, so it doesn't carry as far, but if more gain is added this is easily overcome.
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While I haven't been able to test the vswr of the stock antennas (they get destroyed during the mod), but I know they are actually not a simple omni. They are a 3.5dBi directed omni, so they add gain, which brings the 16dBm output of the RC up to 19.5dBm in the US.
I would imagine they are a bit wider of a match over the 2.4 range than the 6dBi I have. So I would think the antennas themselves probably work fine over the whole 32 ch range. That being said, I think my first video makes it clear that ch 20-22 are the preferable options for stock.
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I think you and I talked about tis already didn't we GadgetGuy??Excellent Information!
Selecting the right manual channel makes all the difference!
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