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Gary Evans

Cause of horizontal banding

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Any ideas on what is causing the intermittent horizontal banding in this video clip. Motor run has no affect.

Equipment includes –

900MHz 500mW on board video Tx with a 191 camera operating on channel #1 (910MHZ).

Ground station has two patch antennas and a diversity receiver.

433MHz RC.

http://vimeo.com/13469726

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Most likely external interference, some foreign digital equipment working on the same frequency band. Where are you located?

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yes if your in the UK it will be from mobile phone masts.

Terry

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Good advice, has the look and feel of external interference.

I can see that the noise was present when your model was on the ground (visible at end of video). This is helpful since you don't have to fly to check it out. So head back to the site and recreate the noise bands. Then turn off the 433MHz R/C Tx; Does that help? If not, then check out ALL the other things that you brought with you. If still no solution, then there is something at the field that is getting you (digital data, cellular, evil RF).

BTW, Gary is in the USA; operating close to a USA cellular tower will indeed cause video problems. My gut feeling is this one isn't cell phone tower related, but that is something that should be investigated.

Edited by Mr.RC-Cam

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Glad to hear that. Overall, one of the best interference troubleshooting tricks in the tool box is to move to a different field. :)

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You are right of course. We have been using this field for a couple of years with no problem so it didn't immediately jump out as the first suspect.

Would this type of interfence be coming in through the ground station antenna or the aircraft electronics? In other words any chance of filtering?

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Good flying locations can turn on you without notice. However, there is always a chance that the RF source will be absent on your next visit that field. So, try again later on, perhaps at a different time of day. If still no luck, then it's time to call it quits there (if you don't want to switch to another video system RF band).

I'd say the problem is on the receive end. Filtering the Rx's RF signal may adequately attenuate the noise (if it is sufficiently out of band and the filter is narrow enough). Finding the magic band-pass filter may be a challenge.

But still one more thought. The noise seems to have a rep rate that is similar to a R/C frame period. Are you absolutely sure that all R/C links were off (perhaps a buddy still had his on)? Or maybe something else you (or your pals) brought along to the field? For sure, it is important to kick all the rocks over when you are looking for grubs.

Edited by Mr.RC-Cam

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If you can, try to check another channel... Here at my place CH1 on 2.4 has become unuseable all around the town (even quite far away enough so that I suspect some unlawful industrial equipment...), but CH3 works perfectly.

Edited by Kilrah

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Good flying locations can turn on you without notice. However, there is always a chance that the RF source will be absent on your next visit that field. So, try again later on, perhaps at a different time of day. If still no luck, then it's time to call it quits there.

I'd say the problem is on the receive end. Filtering the Rx's RF signal may adequately attenuate the noise (if it is sufficiently out of band and the filter is narrow enough). Finding the magic filter may be a challenge.

But still one more thought. The noise seems to have a rep rate that is similar to a R/C frame period. Are you absolutely sure that all R/C links were off when you did your tests? Perhaps a buddy still had his on? For sure, it is important to kick all the rocks over when you are looking for grubs.

There was no one else at the field except me that day and the next closest flying spot that I'm aware of is about 5 miles away. This is another clip where I fly from the problem field to the second one 2 miles away. I've reviewed the flight and it has intermittent banding up to about 1 mile. At 2 miles the banding is gone although the video signal has degraded.

Also interesting that the banding even near the first field isn't constant but turns on and off.

http://vimeo.com/12892074

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At 2 miles the banding is gone although the video signal has degraded.

If the noise bands go away as the model's distance increases, and it is not a fluke observation (and nothing else has changed), then I'm not sure what it could be. So hopefully someone has tackled this and can offer some help. For sure, try moving to another frequency per Kilrah's suggestion.

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I know this will sound a bit wild but when I was using 2.4Ghz I often found wifi lines on the picture that were not detectable when I was not flying. Before the flight I would sweep the area with my patch looking for any interference so I could avoid the area which as expected would be out towards open fields. BUT when I flew there up poped the lines on the screen and yes they would often fade as I flew further away. Now for the wild bit, do you think the video gear in the plane could have been picking up the interference as it flew through a strong RF beam?

Terry

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I know this will sound a bit wild but when I was using 2.4Ghz I often found wifi lines on the picture that were not detectable when I was not flying.

With the video TX on or off?

If the TX is on, and it's close to the RX (I'd say <10m for 10mW, scale appropriately for your TX power), the signal is strong enough to cover the wifi signal, AGC on the RX is low, attenuating the interference, so S/N ratio is high enough for the interference not to be visible. Once you take off, go further and your TX's signal gets weaker, S/N drops and the interference becomes noticeable.

With the TX off you should see the interference over the "snow" if your monitor doesn't bluescreen or otherwise squelch the noise. But it can be misleading too, as with no signal from your TX the RX's AGC will try to amplify the input as much as it can, and in the process reveal interference that's so weak it might actually not be visible in flight.

So basically there's no miracle solution to check if a channel is useable before takeoff, unless you walk some couple hundred meters away from the RX with the TX on.

and yes they would often fade as I flew further away

That's a little more strange though.

Edited by Kilrah

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I have noticed that there is no visable banding when the plane is sitting close to the ground station. I had previously tried the other channels and noticed no differance but will try it again.

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I have noticed that there is no visable banding when the plane is sitting close to the ground station.

That is the normal behavior we expect from the usual external interference source. This is nicely explained in Kilrah's information. But your observation that the interference goes away as you fly farther away has thrown a monkey wrench into the mix.

If the flight direction had substantially changed then that would offer an explanation to this. But as I understand it, the direction is the same but the distance was increased. I'm definitely interested in what is causing that; Suspect everything, even the things it cannot be.

Perhaps someone will have an ahh-ha moment if you fully describe the FPV installation. Things like how power is provided to the Tx and camera, types of other onboard electronics, and so on, all can matter. Along with a concise description, photos of the aircraft's installation and the ground station (during actual use) may be helpful.

Now for the wild bit, do you think the video gear in the plane could have been picking up the interference as it flew through a strong RF beam?

Some digital RF systems are frequency agile and will actively skip around competing RF signals to avoid RF band collisions. Perhaps this trickery is creating the oddball symptom. For sure, proximity of the flying model could matter.

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If the flight direction had substantially changed then that would offer an explanation to this. But as I understand it, the direction is the same but the distance was increased. I'm definitely interested in what is causing that; Suspect everything, even the things it cannot be.

I'm not following your thought process on this statement. I would have thought this would be an expected reaction. As the plane flys further away from the interference source would not the problem is reduced?

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As the plane flys further away from the interference source would not the problem is reduced?

If an external interference source is strong enough to impact reception when your model is flying close to you then the problem will normally get worse as the model travels further away. Kilrah's explanation in post 13 describes the reason for it.

Now if your ground station was moving away from the interference source then that would indeed help reduce interference from an external RF offender. That is why moving to another field is handy solution. :)

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If it is interference affecting just the ground station that would make sense but it seems the planes location changes it or maybe the interference intensity is just changing and it seems that way.

One other possible factor that I forgot to mention is that this year for the first time the park service has started watering the basins early in the morning during the peak temperatures in summer. They turn the sprinkler off at 8:00am and I'm usually out by 9:00 so the grass is dry but the ground is still soaked.

Here is a clip from March using the same video equipment. The only time the video gets bad is when the plane is off to the side of the patch antennas pattern and low.

http://vimeo.com/13650811

Edited by Gary Evans

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If it is interference affecting just the ground station that would make sense but it seems the planes location changes it or maybe the interference intensity is just changing and it seems that way.

One other possible factor that I forgot to mention is that this year for the first time the park service has started watering the basins early in the morning during the peak temperatures in summer. They turn the sprinkler off at 8:00am and I'm usually out by 9:00 so the grass is dry but the ground is still soaked.

Here is a clip from March using the same video equipment. The only time the video gets bad is when the plane is way off to the side of the patch antennas pattern and low.

http://vimeo.com/13650811

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If it is interference affecting just the ground station that would make sense but it seems the planes location changes it or maybe the interference intensity is just changing and it seems that way.

That is the reason for the head scratching. If it is a real issue then it does not follow the normal observations of external interference. So if you are confident that this was not just a coincidence then it will be interesting to hear how you solved it.

Keep in mind that the plane's attitude and travel direction will have an influence in the symptoms. For example, with close flying there tends to be more attitude changes due to the turns. And there are more turns to make. So antenna fades and cross polarization issues are more likely, which can help the external interference creep into the RF link (because it becomes more dominant when the video link signal is attenuated). But the further you fly, the more likely the model will be flown level and in a constant direction. This can accentuate or mask external interference. All the little details matter.

Here is a clip from March using the same video equipment. The only time the video gets bad is when the plane is off to the side of the patch antennas pattern and low.

Perfectly good fields can turn on you in an instant. That is to say, newly installed or existing data/voice/telemetry RF systems can pounce at any time. For example, some large installation sprinkler systems are managed by RF signals. However, public parks don't have the budget for such things, so I can't imagine it is the bug-a-boo in this case. But, there are 999 other RF gadgets that can do the same thing. :)

Something just does not seem right. So, you might think about tracing over your troubleshooting steps using a different 900Mhz Tx/Rx in a different plane (or have a buddy fly his 900Mhz setup there). This will confirm it is indeed external interference and not some unexpected problem with the equipment that is trying to mess with your head. Suspect everything.

One other possible factor that I forgot to mention is that this year for the first time the park service has started watering the basins early in the morning during the peak temperatures in summer.

Subtle changes in the earth can affect the RF propagation. Since anything is possible, maybe you should fly later in day at the site when the ground is dry and see what happens. If the results are different then it could be due to the wet ground or a RF source that goes into hibernation at certain times.

Edited by Mr.RC-Cam

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