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DaveThomasPilot

Narrow Beamwidth, 2.4-2.525 Ghz Receive Antenna

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I used to actively monitor this forum. I learned a LOT from Mr. RCCAM!

I'm now working on a project that has wireless "triggers" on a 1 Mhz channel between 2.4 and 2.525 Ghz, using Nordic NRF24L01 chips.

Everthing works well, except occaisonal interference from Wifi (I think).

I'm doing lots of stuff to minimize the interference, like scanning the channels for RSSI, sending a bunch of packets on each channel to find the ones with minimal packet loss, etc.

Another option I've thought about (and remembered this Forum) was use of a "high gain" antenna. In this context, I don't need higher receive signal strength, but rather just a narrow beam width.

What do you think? If I use something with a narrow beam width and aim the receive antenna at the omni transmitter, am I likely to reduce problems from interference?

The application is indoors (lots of multi-path) with potentially hundreds of Wi-Fi users.

I was thinking something like a pringle can might be work. The receiver is always "aimed" at the transmitter anyway, since it's a camera that is taking a picture of the area around the trigger.

(using auto-ack, retransmit, etc doesn't help, since the trigger needs to sub 1 msec latency--resending the trigger later doesn't help).

Thanks,

Dave Thomas

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I think the 2.4Ghz band is just too full and you will always get interference there. The use of narrow aerials may help to some extent but also may just cause you more problems.If even the tiny delay of resending the signal is too long then I think your using the wrong frequency.

Terry

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Changing to a different frequency band really isn't an option at this point. And the interference is not THAT much of an issue. Missing an occassional trigger isn't a killer (say, one out of a thousand). I'm just trying to make it as robust as practical given my constraints.

I've added a second "diversity" receiver so the trigger can be sent (nearly) simultaneously and receiverd on two separate 2.4X frequencies. So this should decrease the missed triggers dramatically (haven't tested this yet).

Also, eerything is now "frequency agile" -- the frequencies (2.4x Ghz) used for triggers can be changed on the fly based on congestion metrics or packet loss statistics. Haven't tested this in the "real world, demanding environment yet either". Won't happen until a big event late January.

And, my application is not nearly so demanding as wireless video from a flying model. In my case, the transmitter and receiver are in fixed positions, and only 20 or 30 feet apart. The signal from my transmitter can always be much stronger than any noise source.

I might be able to improve the S/N by simply using a "bad" antenna. Bascially, anttenuate both signal+nosie, then crank up the transmit power to achieve higher S/N. Without some attenuation at the receiver, I worry that excessive transmit power might saturate its front-end.

So, that brought me to a consider a directional antenna. I really don't care how much "gain" it has--lossy may even be good. I just need something highly directional.

Thanks for the reply!

Dave Thomas

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Using a directional antenna on the receiver will mask some of the interference sources, so it is worth a try. If both ends have directional antennas then that should help a bit more. Be careful with using excessive Tx power because I've found that doing it can make things worse in environments that are prone to multipathing. You only need to go 20-30 feet, so a couple milliwatts should be more than enough for a LOS application. :)

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OK we are getting a fuller picture now. Try a couple of circularly polarised aerials such as the helical, may well give you what you need.

Terry

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I can't really do much at the transmit end. That's pretty space constrained. The transmitter along with my custom pcb has to fit in a 2" pvc cap. This sits on top a 3' piece of PVC which is full of stuff I can't mess with.

My pcb senses a led turning from red (light curtanin interrupted) to green and transmits a trigger for a "photo finish" sort of thing (I'd have to talk about flyball here to better explain).

I've used both an antenna on the pcb layout as well as a "rubber ducky" antenna that extrudes from the top of the pvc cap. I haven't noticed a difference in how they work with interference, but I'm only now adding firmware that will count missing packets.

The receiver end is not so space constrained. That is typically mounted on the ceiling and cosmetics don't really matter like they do on the transmit end.

So, I was thinking a Yagi at the receive end my help. Don't know much about circular polarization pros/cons, but I'm guessing it wouldn't make sense to use a CP receive antenna if the transmit isn't only CP?

I also have a LVDS transmiter and receiver on the custom transmit/receive pcbs to use when hard wiring is an option or the interference problem really can't be solved. But it's not very practical to hard wire from the transmitter to the receiver in many venues.

Thanks!

Dave Thomas

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Mr RCCAM, I haven't really tried reducing transmit power. I recall the same thing when I was trying to eliminate video drop-outs due to multi-path. I guess I thought since my baseband was digital instead of analog, things might be different.

But, I can dynamically change the transmit power and see if that helps. Thing is, this type of characterization is only valid at the real venue. Maybe I should add a command that counts missing packets on a given RF channel at various power levels. Shouldn't be that hard to do.

Thanks,

Dave Thomas

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It shows the passes.

An important part of flyball is to time release of your dog so that he doesn't cross the start line before the previous dog has crossed it on his way back.

The release is 40 or 50' away from the start line so your dog gets to full speed before he gets to the start line. You waste precious time if you release 100 msec too early, and get a big penalty (you have to rerun your dog) if you pass too soon.

A fast dog will take less than 4 seconds to go from the start line 55' over four jumps, grab his ball, and return those 55' in less than 4 seconds. Frequently, a team wins or looses based on how well the handler's pass.

Typically a team member will be dedicated to "calling passes". He watches for when the returning dog breaks the start line plane and notes how many feet away from the start line the outgoing dog is at that instant of time. A good pass is 1 or 2 feet. That corresponds to about 50 msec per foot.

I have lots of video showing humans can't really do this. And most people don't believe how bad they are, like calling the pass 1 or 2 feet when it's really 6 or 7 feet. But, without good feedback from the pass caller, the team can't be truly competitve.

So I got a system working that takes a snap shot and a few additional frames for slow motion replay starting at the instant the returning dog breaks the start line plane. This is displayed on monitors competitors and spectators can view. Everything is saved on a pc so people can later proudly point to their great pass.

Anyway, this has been up and working for a little less than a year. But, I've had some intermittent missed triggers at some events that might be due to WiFi interference. Hence, the diversity receiver, hard wire option, and firmware to identify good/bad channels and missed packets. And the question whether a directional antenna is likely to help too.

It's all just a hobby for me--I'm paid nothing. But it's a really fun project and it is really appreciated by the competitors.

Thanks,

Dave Thomas

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Yep sounds good Dave. I would have thought a few ms to resend a trigger would be better than a complete miss but I guess you know better. Good luck and be sure to let us know how you get on.

Terry

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If I get a packet gets through, everything for the next 3.5 seconds is ignored. So, rather than bother with the extra bits required for the packets that support auto-ack, I just resend the packet several times, until it's too late to matter anyway.

The receiver throws away all packets it receives for 3.5 seconds after it receives the first one.

So, having the receiver send an ack packet and the transmitter conditionally resending based on whether the receiver got the packet just adds more latency.

Didn't want to cloud the issue with this detail, but "now you know the rest of the story".

Thanks,

Dave

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So, rather than bother with the extra bits required for the packets that support auto-ack, I just resend the packet several times, until it's too late to matter anyway.

How long is it "until it's too late to matter"?

This sounds like a good scheme, only interference that is there for a "really long" time would completely block the trigger signal. It would be interesting to compare your useful time frame to the hopping rate of common systems.

Now, if you could change frequency for each of your packets, it should be quite robust... do it like now i.e. send a stream of trigger packets, BUT changing frequency every packet (or every few packets, depending on how long your RF transceivers need to lock on a new freq). Then, having interference on all those channels during the whole trigger "event" should be a very rare occurrence. Others hop, you hop, so you should have less overlap.

Edited by Kilrah

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