RF Blocked by Carbon Fiber Fins?

[lkal32]

I am planning a build where a BRB 70cm GPS will be mounted with the antenna pointing down along the motor mount and between fins. If the fins are made of carbon fiber, I would assume they would tend to block the signal from radiating past them, so in the case of the design with four fins, limiting the signal to about 90 degrees.



What are your thoughts? Would they block the signal significantly?

 

[CarVac]

More than the fins, the motor case will likely eat the signal.

 

[rocketkid88]

Having the BRB that close to the motor case and the CF fins is definitely going to limit and perhaps completely destroy your signal... if that seems like the best place for your build, then I'd be interested in seeing your results of ground testing!

 

[CCotner]

More than the fins, the motor case will likely eat the signal.

^ what he said. I was considering doing this, in a 2.5" rocket with 54mm MMT, and I talked to a HAM expert at my college. We talked about the relative positioning, and he said that maybe at best an eighth or so of the power output from the transmitter would actually radiate away from the rocket, and not into the motor case and be lost in heating effects. I wish I had better news, because that's how I wanted to mount my antenna as well, but I don't think it's gonna work.

 

[lkal32]

but I don't think it's gonna work.

Bah humbug!

Thanks for all the help guys. I will have to hide the antenna in a more crafty fashion... At least the good news is I can use carbon fins though!

 

[JLRockets]

The other issue here is heat. Commercial motor cases can get up to 200 degrees F on the outside, and still meet certification restrictions. Electronics don't like this level of heat. Adrian from Raven told us to keep it below 150 degrees F. I believe that Greg from BRB recommended to stay below 130 degrees F, but I can't find the exact number.

 

[CarVac]

I suspect that the RF oscillators are more sensitive to temperature, hence the lower temperature limit.

 

[lkal32]

Well, with the motor in a motor mount tube, and then the electronics mounted above it, I cant see how the electronics themselves would see a noticeable rise in temperature.

 

[bobkrech]

The other issue here is heat. Commercial motor cases can get up to 200 degrees F on the outside, and still meet certification restrictions.....

Actually it's 200 C, or 392 F.

Bob

 

[lkal32]

Do you guys think the signal would be ok if the antenna was mounted into a fin (not carbon fiber of course) or is the proximity of the GPS to the motor still an issue (assume proper insulation was used and no temperature increase was seen by the unit)?

 

[CCotner]

The big long conductor parallel to the big long antenna will likely be a problem. If you run the wire at an angle to the motor case, it'll be much better but probably still be a problem. I honestly don't know.

 

[CarVac]

Do you guys think the signal would be ok if the antenna was mounted into a fin (not carbon fiber of course) or is the proximity of the GPS to the motor still an issue (assume proper insulation was used and no temperature increase was seen by the unit)?

You could use coaxial cable to put the transmission and gps antennas wherever you want.

 

[bobkrech]

The GPS antenna needs to have a clear "view" of the sky to function accurately. Putting it beside a motor casing will block the GPS signals from more than 180 degrees. Similarly the transmitter antenna will be effected by the metal motor casing.

What is driving your unconventional design? The usual location for this device is in a payload compartment or in a nose cone?

Bob

 

[lkal32]

The GPS antenna needs to have a clear "view" of the sky to function accurately. Putting it beside a motor casing will block the GPS signals from more than 180 degrees. Similarly the transmitter antenna will be effected by the metal motor casing.

What is driving your unconventional design? The usual location for this device is in a payload compartment or in a nose cone?

Bob

Well the rest of the rocket is only the nosecone. That was pretty much reserved for long motors and parachute volume. Hence the reason for having the GPS where it is. I think putting the antenna in one of the molded fins would work, but I think the GPS may still have a problem.

 

[bobkrech]

Well as the old saying goes, something has to give. You can't put 10 pounds of stuff in a 1 pound can.

Bob

 

[stealth6]

This might be a minor thread derail, but only slightly....

For an upcoming build of mine - a 38mm minimum diameter - I've been planning on using a Rocketry Warehouse fg nose cone that has a metal tip. I was also going to put my BRB-900 in that cone (like I do with most of my rockets). I've got the nosecone, and the metal tip on that size is MUCH bigger than I expected, relative to the whole cone. The tracker antenna won't be inside the metal tip, but it will be immediately below it, and very close. Other than that of course it will be completely surrounded by fibreglass.

I don't think this will be a problem, but I'm not 100% sure. Anyone who knows better than I do wanna chime in with assurance and/or dire warnings?

thanks, s6

 

[bdale]

It won't help you directly, but I thought I'd relate a few details of the fun I had making a UHF downlink antenna for a fin-can ebay that's part of my YikStik3 project. I call it a "fintenna".

On the predecessor, 2YikStik, I had a TeleMetrum in an ebay between two of the fins of a fiberglass skinned 3-fin rocket with fins skinned in CF. The wire whip antenna just ran up inside the airframe, parallel to and very close to the motor casing (75mm phenolic motor mount inside 98mm airframe). As expected, the downlink signal from this board was lousy. I could hear it well enough with a gain antenna nearby to confirm the board was ready to fly, but we were really counting on the TeleMetrum mounted in the nose cone for recovery.

After that flight suffered a structural failure elsewhere in the airframe, I had a brainstorm for YikStik3. Instead of fighting with the fins and motor casing, I decided to use them!

This time the motor mount was 75mm phenolic, the airframe was 98mm convolute glass, and the fins were birch ply substrates with skins of multiple CF layers and a glass sanding veil. The "front" fin (opposite the rail buttons) was loaded up with thermistors attached to a TeleScience board. So I loaded up the other two fins, that straddle the launch rail, as the elements of something akin to a 70cm dipole! This involved copper foil tape under the CF on each fin and across the motor mount, slit in the center with a piston trimmer cap for tuning and a piece of small diameter coax with an SMB connector to attach to the slightly modified TeleMetrum board. The fins alone tuned up reasonably well, but with the motor case in place, I was able to achieve nearly a perfect match, and WOW, the radiation efficiency was AMAZING. Super strong signals, I had solid reception all through the flight from both ends of the airframe through Mach 2.15 up to 21660 feet AGL, a personal best!

This only worked because the airframe and motor mount were non-conductive (not CF!), but perhaps my experience doing this will inspire someone else to do something similar. I should probably write this up in more detail for one or more of the magazines... if I can just find the time! In the meantime, the build photos linked from the YikStik3 page have plenty of photos of my "fintenna" for others to try duplicating it if they want!

Enjoy!

 

[bdale]

The GPS antenna needs to have a clear "view" of the sky to function accurately. Putting it beside a motor casing will block the GPS signals from more than 180 degrees.

That's not strictly true. Mounting a TeleMetrum against a motor mount, for example, will have the top of the GPS antenna pointing outward through the side of the airframe. While this is not an optimal geometry, it's still typical to lock 6-8 satellites on the rail in this orientation, which is more than sufficient to track an airframe through flight and recovery.

Similarly the transmitter antenna will be effected by the metal motor casing.

Absolutely! You're spot-on, here. I think the only real choices are to figure how to take advantage of the motor casing and/or airframe as radiating elements, or just assume you won't get much signal.

 

[CarVac]

I always wondered if there is a motor case that is the right length to be the radiator for a legal frequency... sand through the anodizing on the outside and solder a wire to it.

 

[bdale]

I always wondered if there is a motor case that is the right length to be the radiator for a legal frequency... sand through the anodizing on the outside and solder a wire to it.

I'd just inductively couple to it... that way you're not modifying the case (enough heat to solder or weld a wire to aluminum will affect the material properties .. don't think that's a good plan without thinking through all the consequences!). And frankly, any big piece of metal can be loaded up and tuned even if it's not a perfect radiant length and provide a lot better radiation efficiency than a 1/4 wave wire very close to a large hunk of metal, I suspect....

 

[lkal32]

I appreciate the insight, Bdale, however, most of that is over my head. Sounds like a good plan though, especially for min diameter rockets.

I did some testing where I put the GPS next to a K motor casing and into a fiberglass tube and threw it outside, so that the GPS was about 1/16" away from the casing and facing the ground with the casing on top. It got a lock and transmitted good coordinates. I have no idea how valid the testing is to a flight profile but it seemed to work better than I thought it may. I understand transmission may not be optimal, but I dont see this rocket flying higher than about 10K feet. With the antenna at a ~35-40 deg angle away from the casing, do you guys think I can still get ok reception on the ground? I dont really know how I can test the unit in realistic conditions without actually flying it, and if the signal lets me down, well then the chance of losing the entire system isnt worth it.