RC 2.4 & 5.8 and acceptable interference via Spectrum Analyzer

[Dougcjohn]

The radios and associated transmission for sUAV, at what surrounding dBm does it become a concern for possible interference for RC flights?

Using a Spectrum Analyzer 6G Combo Plus by RF Explorer (50-6100MHz full spectrum), a quality handheld $400 meter. As an example, focused on 2.4 GHz range and using DJI’s specs for 2.4GHz range of 2.400-2.483 (Same question applies for 5.8 freq).

Scanning a downtown construction site, it’s a pretty full RF spectrum, but in the lower -80 to -40 dBm for my work area. Relatively low power concern or threat even at the -40dBm. That RC freq spectrum does fall into the 2.4GHz Network WiFi Channel 13 bands of 1 to 4, primarily 2 & 3. The DJI radio’s EIRP is spec’ed at 2.4GHz: < 26dBm FCC (USA Example) & 8km for transmit power.

Maybe I’m overlooking the obvious, but at what measured dBm of surrounding frequency power does it become a concern and need to insure a selected freq or no-fly due to strong interference? Not a concern in countryside, but often pondered if playing with possible interference problems in downtown areas or dense home neighborhoods heavily transmitting Wireless WiFi, or various hardware in buildings, garages, hospitals, etc.

On rare occasion, I’ve noticed a few blips during a flight that appears drone received an alternate signal when flying a downtown construction survey off to one end closer to 2 Hospitals at higher ground, still multiple blocks away and no Hell’s in visual. Once drone appeared it totally changed, I felt I lost hold for a few seconds, had a short interval of no-response to flying irratic. I questioned a jam for an instant... little butt pucker effect, flipped it to abort flight, once over to atti mode and regained. Brought it back to LZ, freq scanned area and noticed a few high dBm peeks near selected freq that quickly faded and washed away. Relaunched without issue to complete mapping. That persuaded a newer spectrum analyzer... but still questioned what pre-launch levels were a concern. The thought of losing control and a fly away is a dreaded concern when downtown... especially when the Construction Rep told me they don’t fly their own sUAV anymore, their Insurance no longer covers them due to 3 previous issues on other sites, but he felt it may have been them not operating correctly too... was a Phantom 3 I believe he indicated 2 years ago when this project broke ground. Had 2 plus years of no failures at this site, although I agree, that’s not an indicator.

Reading RF probably isn’t a common thing for the majority, although with the increasing RF saturation... others might find it useful. I started out more of curiosity than concern... or to gain some practical knowledge or benchmarks. Those that are more into the RF spectrum, any input or knowledge would be appreciated. My military days of late 1970’s early 80’s offered a foundation, but I’m not knowledgeable... more hobbyist.

I’d welcome a bit more understanding if others have experimented or have a much better knowledge base to interpret the spectrum analyzer... or if I need to tune the meter with different dBm limits.

https://www.amazon.com/Explorer-Digital-Handheld-Spectrum-Analyzer/dp/B089QBXX6C/ref=pd_sbs_1?pd_rd_w=MaTWs&pf_rd_p=3ec6a47e-bf65-49f8-80f7-0d7c7c7ce2ca&pf_rd_r=ZNMEG80RVQTC31R1TFJZ&pd_rd_r=74f49e69-657b-4d52-bede-fdcf2a821a9f&pd_rd_wg=AJ2y5&pd_rd_i=B089QBXX6C&psc=1

 

[Repaid1]

The radios and associated transmission for sUAV, at what surrounding dBm does it become a concern for possible interference for RC flights? <SNIP>Maybe I’m overlooking the obvious,

I’d welcome a bit more understanding if others have experimented or have a much better knowledge base to interpret the spectrum analyzer... or if I need to tune the meter with different dBm limits.

I think your getting way to much into it. DJI's App has a built in analyzer, you can see what freq/channel it is on and anything on any available channels. You simply go manual, watch the channels and traffic/interference and optimally select a channel that has zero to little on either side of that channel.

 

[Dougcjohn]

I think your getting way to much into it. DJI's App has a built in analyzer, you can see what freq/channel it is on and anything on any available channels. You simply go manual, watch the channels and traffic/interference and optimally select a channel that has zero to little on either side of that channel.

True, it does a little frequency scale, not a spectrum analyzer to any degree. A spectrum analyzer sensitivity picks up much more. Comparing DJI’s Freq screen is more like a simple phone wifi analyzer app... low accuracy and limited.

But, that has value and useful... in many situations; but it won’t show instruments coming on & off and doesn’t show dBm magnitudes.

Several are beginning to use a spectral analyzer in congested areas and to find freq least concerning after letting it collect a period prior to flight. Some areas don’t have a non-used freq. Others have “fly aways” without an explanation... many can be avoided with a pre-flight check.

 

[Camino Ken]

What you’re trying to diagnose will be a bit tricky but I think you are on the right track. Occusync utilizes OFDM (orthogonal frequency division multiplexing) for video and FHSS (frequency hopping spread spectrum for data. Both of these very robust transmission architectures that are inherently relatively immune to interference problems. In addition, they are packet based systems. If a data packet get corrupted it just gets re-transmitted.

I would also suggest that you select the option to automatically switch between 2.4 and 5.8. It seems to do a pretty good job of finding the least congested band.

What you may have seen is some packet corruption to cause the temporary loss of control.

Since it is also a SDR (software defined radio), it can potentially have fairly good out of band rejection filters. If I was looking for interference (noise) I would measure the average in-band noise levels using the built in analyzer and start to be concerned when it reaches the 1km threshold line. If you want to be more precise you could compare readings between your spectrum analyzer instrument with the one built in the app to determine the level of noise for concern.

By the way, as a retired electronics engineer my knowledge is a bit dated as well but I recently looked into this issue and refreshed my understanding.

Additionally, interference cannot cause “fly aways”. Worst case you will lose the connection with the drone and it will either RTH or hover depending on your settings.

 

[RadioFlyerMan]

What you’re trying to diagnose will be a bit tricky but I think you are on the right track. Occusync utilizes OFDM (orthogonal frequency division multiplexing) for video and FHSS (frequency hopping spread spectrum for data. Both of these very robust transmission architectures that are inherently relatively immune to interference problems. In addition, they are packet based systems. If a data packet get corrupted it just gets re-transmitted.

I would also suggest that you select the option to automatically switch between 2.4 and 5.8. It seems to do a pretty good job of finding the least congested band.

What you may have seen is some packet corruption to cause the temporary loss of control.

Since it is also a SDR (software defined radio), it can potentially have fairly good out of band rejection filters. If I was looking for interference (noise) I would measure the average in-band noise levels using the built in analyzer and start to be concerned when it reaches the 1km threshold line. If you want to be more precise you could compare readings between your spectrum analyzer instrument with the one built in the app to determine the level of noise for concern.

By the way, as a retired electronics engineer my knowledge is a bit dated as well but I recently looked into this issue and refreshed my understanding.

Additionally, interference cannot cause “fly aways”. Worst case you will lose the connection with the drone and it will either RTH or hover depending on your settings.

I need education... and it sounds like you're just the knowledgable person to help. I've always wondered how this whole frequency thing worked.

Whenever I fly my MPP or my M2P, the limiting factor at distance has always been the transmission of the image, not the control of the aircraft. The strength indicator for the RC control signal as I interpret it, is always very strong, maybe 4 to 5 bars as compared to the HD strength of 2 to 3 bars at distance. From that experience, I've always thought that the control signal was transmitted differently, on a separate frequency of 2.4, and that of the telemetry and image data was flip flopping between 2.4 and 5.8. When you say "spectrum for data", does that also include "control commands"? In other words, when the frequency band of 5.8 is selected, does that mean for the transmission and reception of every thing between the RC and the AC?

 

[Dave Maine]

The notion of finding a frequency that will stay interference free during a 30 minute flight is tricky for 2 reasons. The first - as the drone flies it moves, and the interference strengths may change with that movement.

2nd - new interference sources can easily come on line during the flight, for example someone in a nearby building reboots his router.

DJI’s auto feature updates in real time, picking the best frequency at that moment at that particular place. What’s not to like?

 

[Dougcjohn]

What you’re trying to diagnose will be a bit tricky but I think you are on the right track. Occusync utilizes OFDM (orthogonal frequency division multiplexing) for video and FHSS (frequency hopping spread spectrum for data. Both of these very robust transmission architectures that are inherently relatively immune to interference problems. In addition, they are packet based systems. If a data packet get corrupted it just gets re-transmitted.

I would also suggest that you select the option to automatically switch between 2.4 and 5.8. It seems to do a pretty good job of finding the least congested band.

What you may have seen is some packet corruption to cause the temporary loss of control.

Since it is also a SDR (software defined radio), it can potentially have fairly good out of band rejection filters. If I was looking for interference (noise) I would measure the average in-band noise levels using the built in analyzer and start to be concerned when it reaches the 1km threshold line. If you want to be more precise you could compare readings between your spectrum analyzer instrument with the one built in the app to determine the level of noise for concern.

By the way, as a retired electronics engineer my knowledge is a bit dated as well but I recently looked into this issue and refreshed my understanding.

Additionally, interference cannot cause “fly aways”. Worst case you will lose the connection with the drone and it will either RTH or hover depending on your settings.

Thanks, very helpful. I learned a few things with Ocusync. Most sites I'm using Inspire2, M210 or M600Pro as primary due to payloads, M2P as secondary..

Tapping your knowledge base, what can you share on Lightbridge 2 and it's operation?

 

[Dougcjohn]

The notion of finding a frequency that will stay interference free during a 30 minute flight is tricky for 2 reasons. The first - as the drone flies it moves, and the interference strengths may change with that movement.

2nd - new interference sources can easily come on line during the flight, for example someone in a nearby building reboots his router.

DJI’s auto feature updates in real time, picking the best frequency at that moment at that particular place. What’s not to like?

Not attempting to find a free freq... highly agreed, not going to happen. Which is why I was pondering dBm "thresholds of concern". Turning on a spectral analyzer arriving on site collects the environment to help identify possible frequencies of concern... likely able to catch oddities that may cycle on / off... the GoApp Freq Screen not able to duplicate. An additional tool to help identify and be aware of environment concerns. A Router powering up with a -70dBm isn't a concern... a device appearing every few minutes at high dBm covering a broad spectrum is a concern... a jammer creates a flat line at high dBm across the usable frequency... others move through freq segments. Not to debate "jammer", failing hardware, or various devices can produce similar conditions.

It's a tool for questionable areas, in addition to RC features... sits with a Scanner in kit.

 

[Camino Ken]

I need education... and it sounds like you're just the knowledgable person to help. I've always wondered how this whole frequency thing worked.

Whenever I fly my MPP or my M2P, the limiting factor at distance has always been the transmission of the image, not the control of the aircraft. The strength indicator for the RC control signal as I interpret it, is always very strong, maybe 4 to 5 bars as compared to the HD strength of 2 to 3 bars at distance. From that experience, I've always thought that the control signal was transmitted differently, on a separate frequency of 2.4, and that of the telemetry and image data was flip flopping between 2.4 and 5.8. When you say "spectrum for data", does that also include "control commands"? In other words, when the frequency band of 5.8 is selected, does that mean for the transmission and reception of every thing between the RC and the AC?

Yes, the control signals (the data) are transmitted differently than the video. That is the FHSS part. It is my understanding that they coexist in the band and can both switch betweven the two primary frequencies although I can’t confirm this. It is important to note that they are both digital information sent in a packet based system but the video requires multiple orders of magnitude more packets per second compared to the data and is typically the first to suffer from interference. As to the signal strength differences I don’t know the details of how they are derived but could venture a guess that it is relative to the down converted baseband signal size.

 

[Camino Ken]

Thanks, very helpful. I learned a few things with Ocusync. Most sites I'm using Inspire2, M210 or M600Pro as primary due to payloads, M2P as secondary..

Tapping your knowledge base, what can you share on Lightbridge 2 and it's operation?

I’m sorry but I have not been able to find any technical details about Lightbridge. I think that it is similar in the basic approach but to what extent I do not know. From what I can find I can infer that Occusync is an improvement in terms of resistance to interference and capable of higher data bandwidth. For example, Occusync can transmit up to 1080p while Lightbridge is limited to 720p. The maximum stated range is more as well.

 

[RadioFlyerMan]

Yes, the control signals (the data) are transmitted differently than the video. That is the FHSS part. It is my understanding that they coexist in the band and can both switch betweven the two primary frequencies although I can’t confirm this. It is important to note that they are both digital information sent in a packet based system but the video requires multiple orders of magnitude more packets per second compared to the data and is typically the first to suffer from interference. As to the signal strength differences I don’t know the details of how they are derived but could venture a guess that it is relative to the down converted baseband signal size.

Thank you! Your explanation helps resolve a question regarding the application of RF amplifiers on a dual band system. I have amps on my MPP which is single frequency. But the M2P is of course dual, and since they coexist, amplification cannot be applied to just the “video” signal.
Then a whole other question crops up regarding the diversity antenna system. I choose to amp both antennas since I really don’t know how diversity is applied at the RC.

 

[Dougcjohn]

Thank you! Your explanation helps resolve a question regarding the application of RF amplifiers on a dual band system. I have amps on my MPP which is single frequency. But the M2P is of course dual, and since they coexist, amplification cannot be applied to just the “video” signal.
Then a whole other question crops up regarding the diversity antenna system. I choose to amp both antennas since I really don’t know how diversity is applied at the RC.

Separate subject, but what Amp did you decide on?
Does it conceal inside RC or mount externally?

The NLD has introduced a very small sized 2.5W Amp for Both Ends... on-board Aircraft and inside RC... which is much better than Amp on RC only. It’s also switchable, able to turn off when needed.

I was considering for both M2P and the M600pro LB2 that provides dual video channels for 2 payloads... but a bit weak & spotty on video at times.

NLD 2.5W Bi-Di Signal Booster Kit Released

nolimitdronez.com

 

[Camino Ken]

Thank you! Your explanation helps resolve a question regarding the application of RF amplifiers on a dual band system. I have amps on my MPP which is single frequency. But the M2P is of course dual, and since they coexist, amplification cannot be applied to just the “video” signal.
Then a whole other question crops up regarding the diversity antenna system. I choose to amp both antennas since I really don’t know how diversity is applied at the RC.

There is some debate and disagreement by folks on this forum as to whether the two antenna on the standard remote are using array or diversity configuration. Used as an array they would increase the transmitted power in one direction (front and back) but reduce the power to the sides. Used as in a diversity configuration both antenna receive the similar signal but then cancel out slight differences due to mult-path and reflections. I have not been able to find conclusive proof of what is being used. Regardless of the configuration if you amplify both equally it seems like it should not make a difference. I believe amplifiers exist that can work at both the specified bands. You are correct that you cannot amplify just the video portion without access to the guts of the radio in the controller, if at all.