Antenna Placement in EBay for GPS

I am finishing up a minimum diameter project and starting to layout the electronics bay and I wasn't sure if the placement of the tracker in the bay with the threaded rod and aluminum bits would mess with the tracker. I can't put it into the nose very easily as that's where the some of the chute will be. The entire rocket is fiberglass including the bay. I know that the fiberglass is not an issue.

I have not permanently placed anything so it is still a blank canvas.
The 2 pictures are of the bay, one of each side. This is what I am thinking so far.

Looking forward to your opinions on this.
I know I will loose some but hopefully not too much.
Also any issue using this antenna verses a tracker with the wire?
The tracker is the one that Derek designed here on this forum.
I will have everything snugged down for flight.

I wouldn't worry about the metal in the av-bay as much as I'd worry about the cross interference between the SL100 and the transmitter. I have placed the BRB tracker in very close proximity to the Raven in MD builds but the behavior of the two is well-known. They don't radiate RF and interfere with each other. How much do you know about the transmitter you are using? Does the manufacturer provide any RF characterization?

Your antenna is parallel to your altimeter's power supply, as well as to your match leads. This may or may not lead to problems.

Based on my experience with testing some configurations (I've never done it with an SL100), I would suggest you hook up "charges" (I use christmas tree bulbs), with leads run where they normally would be. Power things up and let it sit for at least an hour, and see if you encounter any problems.

Using a specific altimeter/tracker combination, I've had it get spoofed and fire matches after less than 5 minutes, and I've had it take over 30. Thus the recommendation to let it sit and "cook" for at least an hour.

Afterwards, I'd suggest downloading the data from your altimeter, even if the matches don't fire, and see if you see irregularities in the data.

-Kevin

Just wondering, what are the issues with trackers next to altimeters?

blackbrandt said:

Just wondering, what are the issues with trackers next to altimeters?

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You need to get farther along in Physics then you'll know: https://en.m.wikipedia.org/wiki/Electromagnetic_interference

I wasn't even thinking of those issues.
Good point.
I was concerned about the threaded rod and the aluminum end caps and that blocking / decreasing the transmission range of the GPS, since the antenna and threaded rods are parallel. The antenna has a joint at the mounting and I may be able to give it a slight slant.

dixontj93060 said:

They don't radiate RF and interfere with each other.

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They both "radiate rf" - especially the brb otherwise it would be a pretty worthless tracker. The issue is whether or not the rf is harmful to one another. The test troj suggests will hopefully indicate if this is the case.

Derek said:

They both "radiate rf" - especially the brb otherwise it would be a pretty worthless tracker. The issue is whether or not the rf is harmful to one another. The test troj suggests will hopefully indicate if this is the case.

Click to expand...

Of course! Would have thought that would be blatantly obvious. But thanks for clarifying for those that no absolutely nothing about electronics.

blackbrandt said:

Just wondering, what are the issues with trackers next to altimeters?

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In a nutshell, confusing the altimeter.

We have a guy in our club who had several flight failures we couldn't figure out. Then I started testing altimeters and trackers in close proximity to one another, after which I asked about his configurations. Voila! We think we know the source, now....and he has changed his approach, based on that information.

-Kevin

I flew my 3" Hawk Mountain Talon "Back in Black" many times with no tracker. One weekend I installed a BRB GPS into the AVbay on the opposite side of the sled from the Raven altimeter. It flew once in this configuration to 9300 ft on a LOKI L-1300 (video here). The next day it flew again in the same configuration - except this time, despite the raven indicating 4V and 4 good charge continuities on the pad, absolutely no deployments occurred. It came screaming back down at us and buried itself into the ground. All the electronics (a Raven, a BRB GPS, a BRB beacon and a Contour camera) were utterly destroyed - so I have no way to verify what happened. But given that I am a veteran of flying the Raven many, many times I am fairly certain there was nothing amiss with it, the power perch or the battery. The altimeter, perch and battery were secure with no chance of disconnection during flight. Given the battery voltage and continuity beeps on the pad, I am left to assume that the transmitter in the avbay somehow interfered with the raven. The BRB GPS transmits every 5 seconds or so for a brief burst. I think I got lucky the first flight, but on the second the transmitter burst came at just the right time to freak out or confuse the raven.

So, even if you let the combination sit and cook - and you don't see any unintentional deployments - remember that it is also possible for the transmitter to interfere with the altimeter such that nothing deploys when it is supposed to. Believe me, this is terrifying - especially if you've never experienced such a failure before. I was sure that someone was going to be hurt by my rocket. That would have destroyed me.

Could you mount that tracker such that the antenna is outside the end plate of the avbay? The end plate may be too thick to allow the rp-sma connector to protrude far enough to install the antenna. If it is possible, so long as there is sufficient airframe attached to that end of the avbay after deployment, the antenna should not be beaten up by the recovery hardware. The ejection charges will foul the antenna, but it can be replaced if this ever becomes too big a problem. The aluminum end plate would block the vast majority of the radiation from the antenna being injected into the altimeter. There will still be some radiation in the bay from the tracker.

A better solution would be to put the tracker in the nosecone. Then it's far away from the altimeter. If you use an aluminum bulkplate or aluminum foil on the inner surface of the nosecone bulkplate, the chance of interfering with the altimeter should be all but eliminated.

I will never fly a tracker in close proximity (on the reverse side of the sled) to my altimeter again.

Why not install a shield around the Altimeter ? On a train layout with computer control , the signal pass true the rails and when a rail pass over another one we have interference, we just install a grounded mesh under the track on the top of the other one and it fix the problem

Area66 said:

Why not install a shield around the Altimeter ?

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The ejection charge leads can act as antennae, directing the radiation from within the bay into the altimeter. One could perhaps employ ferrite beads on the wires to choke the RF, but this would require some design expertise that is outside my comfort zone.

sti_ffy said:

The ejection charge leads can act as antennae, directing the radiation from within the bay into the altimeter. One could perhaps employ ferrite beads on the wires to choke the RF, but this would require some design expertise that is outside my comfort zone.

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you just have to twist pair the ignitors leads.

I would recommend getting a short RP-SMA cable and getting the BRB antenna outside the AV bay. You can also put your altimeter in a small aluminum box if you're worried about RF affecting it. All the metal bits inside an AV bay can reflect RF in unpredictable ways, so even twisting the igniter leads together might not totally prevent RF from being conducted back into the altimeter. You can always try using shielded twisted pair cable, and properly grounding the shield.

troj said:

Your antenna is parallel to your altimeter's power supply, as well as to your match leads. This may or may not lead to problems.

Based on my experience with testing some configurations (I've never done it with an SL100), I would suggest you hook up "charges" (I use christmas tree bulbs), with leads run where they normally would be. Power things up and let it sit for at least an hour, and see if you encounter any problems.

Using a specific altimeter/tracker combination, I've had it get spoofed and fire matches after less than 5 minutes, and I've had it take over 30. Thus the recommendation to let it sit and "cook" for at least an hour.

Afterwards, I'd suggest downloading the data from your altimeter, even if the matches don't fire, and see if you see irregularities in the data.

-Kevin

Click to expand...

Hmmmmmmm, I thought altimeters don't start recording to memory unless the electronics detect some sort of starting event like g's or a rapid ascent? The mere fact that the unit started recording in a ground test tells me something like Rf triggered the unit to store data. That in and of itself should warn someone
something is not right. Kurt

sti_ffy said:

I flew my 3" Hawk Mountain Talon "Back in Black" many times with no tracker. One weekend I installed a BRB GPS into the AVbay.....

Click to expand...

Snip for brevity's sake.

Which BRBGPS did you use? 900Mhz? 70cm? If 70cm (400Mhz) was it the low powered unit, 12mW or the high powered one, 100mW?
Higher powered trackers can lead to more problems. 2 watt dog trackers can be the worst with certain deployment electronics.

I have a Wildman Jr. that works fine with the low powered BRBGPS 70cm 12mW riding on top of the Raven II. The antenna wire projects through a hole the bulkhead into the main chute bay. The hole is close tolerence and I seal it with a bit of clay. I then found a use for the tubes that Aerotech igniters come in.
I wrap the outside with tape to shield against the ejection charge flash and put it over the antenna so is doesn't get squished by the chute. Hold it on with
a bit of clay.

Also, do not, do not, do not......... paint a rocket with a metallic paint if one is going to seal up a tracker inside. At least for 70cm trackers, the paint will soak up Rf and you won't get a signal at altitude. I found that out twice in one day with the 12mW Beelines. In fact, I stripped the metallic paint off of the WMjr upper bay for that reason.

My 3" metallic painted Wildman, I did a flight with one packet received at altitude. I made a longer ebay and the tracker rode with the MAWD. Fortunately it came back within eyeshot. The rocket is too big for me to strip and repaint easily. I was lucky my 10" long ebay is totally symmetrical so I put the tracker in the bottom and used a bulkhead connector like this: https://www.ebay.com/itm/RF-pigtail...ltDomain_0&hash=item58939d5dce#ht_3841wt_1105
to have the antenna project out the aft bulkhead. This isn't the "exact item" but is a means to connect an antenna at the bulkhead. At least Rf can get out after apogee. Yeah it could break on landing but I can live with that. Kurt

cerving said:

I would recommend getting a short RP-SMA cable and getting the BRB antenna outside the AV bay. You can also put your altimeter in a small aluminum box if you're worried about RF affecting it. All the metal bits inside an AV bay can reflect RF in unpredictable ways, so even twisting the igniter leads together might not totally prevent RF from being conducted back into the altimeter. You can always try using shielded twisted pair cable, and properly grounding the shield.

Click to expand...

you resume what I said, Shield around the altimeter ( aluminum box ) and twist pair every cable. In make case I will make it more easy as I plan to install all my tracker in the nose not in the altimeter bay, so the altimeter bay can be all shielded.

Best practice is simply to not place RF transmitting devices in the altimeter bay. If you must, test assuming the altimeter has live signal output or recording so you can see what happens. Test with the RF device twice as close to the altimeter as the maximum distance the RF device can be mounted from your altimeter. If it passes move the RF device to it longer distance location.

If it fails, do not attempt shielding, twisting wires, aluminum foil, tin hats or other incarnations, your system is not robust and cannot be trusted with RF power your device is transmitting nearby. Simply move the RF transmitters to a nosecone compartment or other place well away from your altimeter.

jderimig said:

Best practice is simply to not place RF transmitting devices in the altimeter bay. If you must, test assuming the altimeter has live signal output or recording so you can see what happens. Test with the RF device twice as close to the altimeter as the maximum distance the RF device can be mounted from your altimeter. If it passes move the RF device to it longer distance location.

If it fails, do not attempt shielding, twisting wires, aluminum foil, tin hats or other incarnations, your system is not robust and cannot be trusted with RF power your device is transmitting nearby. Simply move the RF transmitters to a nosecone compartment or other place well away from your altimeter.

Click to expand...

Ahhhhh, That's the best advice above. If one has to live with the tracker in the ebay, consider the lower powered tracker. It doesn't take much
to track at altitude. Kurt

Area66 said:

you just have to twist pair the ignitors leads.

Click to expand...

While the firing FET is not being gated, one lead of the e-match is connected to the battery supply and the other (the FET lead) is open. So you are still injecting any radiation received by the twisted pair directly into the power supply of the altimeter - unless you are using a separate firing supply. I don't think this would be totally effective.


edit - I was concerned that I was being too critical of the wire-twisting RFI rejection concept. So I performed a somewhat-less-than-scientific test. I have a Futaba FP-R1380P receiver and T7CAP transmitter that aren't otherwise occupied. So I doubled the receiver's wire antenna back upon itself so that it was half as long as usual, then twisted it tightly together with itself. In theory, all the electromagnetic energy should cancel - and nothing will enter the receiver. Surprise - it still works, and at surprisingly large distance, even with the transmitter antenna completely collapsed. This is what I expected, and why I think that a twisted wire with one end unterminated may not be effective at reducing RF induced interference.

ksaves2 said:

Snip for brevity's sake.

Which BRBGPS did you use? 900Mhz? 70cm? If 70cm (400Mhz) was it the low powered unit, 12mW or the high powered one, 100mW?
Higher powered trackers can lead to more problems. 2 watt dog trackers can be the worst with certain deployment electronics.

Click to expand...

It was the 70cm (430MHz) 100mW.

I was stupid and assumed that since the tracker didn't seem to affect the altimeter on the ground, that it wouldn't affect it during flight. It even survived one flight without issue. But then everything went horribly wrong. There's no more sick feeling than watching your rocket threaten everyone's safety.

Fly safe, avoid putting transmitters in the avbay if you can. (You can!).