Anyone know about multiple antennas and same frequency issues?

[slodave]

My little UAV project is a nightmare. After many years of trying to get both telemetry (GPS, altitude...) and live video to work simultaneously on my R/C glider, I finally have current and working equipment but....

If you really want the back story, I'll post tomorrow...

The short of it:

I briefly had both video and limited telemetry working a couple of months ago, GPS failed. The setup didn't have any real interference issues. Two 900 MHz transmitter antennas within 6" inches of each other.

Last night I replaced the Tx and a recorder unit. The recorder has some static instruments on it, but does not transmit a radio signal, it's the hub for all the sensors/inputs and can record the data. The Tx plugs into this device.

I ruled out the GPS as causing interference.

Somehow, I think the recorder is the culprit - receiving interference from the video Tx. I switched out the new telemetry Tx for the old one and the same problem. The telemetry receiver loses the signal. Unplug the video power and the telemetry comes right back. I have tried separating the Tx's as far from each other as I can, with the same result.

Any ideas, thoughts? I was planning on being at the field at around 9:30 in the AM...

Thanks!

[svandamme]

They all use the same battery pack?

[Jim Richards]

Dave, please provide a bit more information. First, what transmitters do you have (just the two or more on the ground), and what receivers? Where are each located? Are the two transmitters with antennas 6" apart operating on exactly the same frequency, or are they separated by some amount?

A simple test might be to separate the two transmitter frequencies by some amount. Video is a wider bandwidth application than telemetry, so you're going to have to account for that by using enough frequency separation. As far as GPS failing, perhaps you're overloading the GPS receiver with the two 900 MHz transmitter signals. The GPS receiver might not have enough out-of-band rejection to reject signals outside it's normal operating range of ~1227 to 1575 MHz. Increase the distance separation between the transmitters and the GPS receiver, or you're going to have to reduce transmitter power, or add some additional filtering between the GPS antenna and the GPS receiver.

Good luck with your project!

[fingpilot]

A bit of aluminum foil separating the bits might help. Could be RF 'leaking' inside the boxes. The GPS is looking for a signal at microvolt strength, as I remember from my UAV days. Our 2KW Microwave live directional transmitter datalink was swamping the GPS receiver.

Once the ROE were locked down, our little project starting knocking Mercedes and Toyota trucks off of dirt roads, and providing live TV to the funny shaped building in Virginia.

[slodave]

Stijn: Two batteries, a Li-Po for the video equipment (GPS, recorder, Tx) and a Ni-Mh pack for the receiver and servos.

Jim: On the glider, there is the receiver for the control Tx, it's in the 72 MHz band and does not seem to be causing or receiving interference. There is the Tx for the telemetry which runs on the 900Mhz, but I'm not sure exactly what Fq. There is also a 5Hz GPS module, it has a very short cable, but I took this out of the loop last night and the same issues occurred. The Tx for video has 2 dip switches to toggle between four 900 frequencies, but the manufacturer sets the channel and then seems to glue the dip switches. I think I ruined one by trying to force it to another channel. I don't have the exact frequencies in front of me. It'll take a little time to find.

I do know, that as soon as I turn off the video equipment on the glider, the telemetry receiver picks up the signal again. I am assuming that it is the video Tx that is causing the issue, but I will have to double check. The Tx and video camera have identical power cables and I'm not sure which is which. I will label and check later today. I will also hook up the old telemetry equipment again and see what happens - although I have used both without issue.

Here are a couple of pix from December. Old telemetry that was working with the video.


The old GPS receiver is the red square on top of the wing




I'll dig up the frequencies later. Time for lunch and a bit of work.

Thanks!

[svandamme]

any chance there could be some interference via the wiring?
since the tx, recorder and gps are all on the same pack?

you said it cuts out the moment you hook up the recorder, maybe it's sending some bad noise over the dc wiring?

just guessing here, i know absolutely nothing about radio transmissions/antennas

[slodave]

The GPS was removed from the loop and the issues still were there. It's the moment were I disconnect power for the video Tx and camera, that the telemetry picks up the signal again and it becomes solid. Plug the video equip in and the telemetry signal cuts in and out. I have thought about wire interference, but at the moment the wire layout is exactly the same as before. It's a pain to remove all the wires, but I have to anyway for another issue.

The GPS has no effect.

[slodave]

OK, I just plugged in the old telemetry system, minus the GPS as well and all is working.

The telemetry gets displayed on a "Real-time Data Dashboard", it gives me a number of two line text screens with the info I choose. I am wondering if this issue with the new system has to do with the firmware updates. The new system is a slightly newer revision.

I think I solved it. I can use the old Dashboard with the new Tx and recorder and new GPS, but there seems to be a small incompatibility with the new firmware update in the recorder and the old in the Dashboard - I may try this tomorrow anyway. Things may clear up a little at the field. As soon as I turn on the new Dashboard, the signal keeps dropping.

Looks like I get to have another talk with Don (support) at the company that is making the telemetry equipment.

[slodave]

More on the issue. I pulled all the cables tonight, rerouted them, twisted them, braided them... After getting everything back in, I played with the camera and it's Tx. It turns out that when both are powered, the Dashboard drops and reconnects its signal. I pulled the power from the camera, but left its Tx on and the Dashboard is stable. Plug the camera back in and signal goes. Next, I left the camera powered, but unplugged the camera. Once again, the Dashboard becomes stable.

I don't get it.

I did talk with the telemetry company and they will probably end up sending me the 2.4 Ghz Tx and Rx (Dashboard). This should solve the problem.