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BlazingAngel665

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A new school year is coming up and that means a new project. This rocket is anticipated to be all carbon fiber, which is of course, RF opaque. It will also be going nearly five miles into the sky. This means that the avionics has to step up its game. I'm looking into externally mounted antennas. There are a lot of systems that will need external antennas. We have 2 GPS receivers (telemetrum and a custom computer) a 900 mHz telemetry transceiver and a telemetrum. We discussed putting things in the nosecone and making that from fiberglass, but then the wiring gets complicated. I'm wondering if anyone has any ingenious systems or tips on ways to get telemetry off of this vehicle. Any favorite antennas or mounting methods? Anybody found a way that just doesn't work?
Thanks.
 

mpitfield

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A new school year is coming up and that means a new project. This rocket is anticipated to be all carbon fiber, which is of course, RF opaque. It will also be going nearly five miles into the sky. This means that the avionics has to step up its game. I'm looking into externally mounted antennas. There are a lot of systems that will need external antennas. We have 2 GPS receivers (telemetrum and a custom computer) a 900 mHz telemetry transceiver and a telemetrum. We discussed putting things in the nosecone and making that from fiberglass, but then the wiring gets complicated. I'm wondering if anyone has any ingenious systems or tips on ways to get telemetry off of this vehicle. Any favorite antennas or mounting methods? Anybody found a way that just doesn't work?
Thanks.
I recommend that you approach Bedale Garbee from Altus Metrum. He had a project where he embedded antennas in his fins http://gag.com/rockets/airframes/YikStik3/ I exchanged a few emails with Bedale on the topic and at the end of day Bedale recommended a simple wire 1/4-wave whip. Having said that Bedale included someone else in the email who was also very knowledgeable on the topic and taught antenna classes. Below is a cut and paste of that part of the email

"For what it's worth, one of the antenna design projects I offered in my antenna class last year was a "telemetry antenna for high power rockets". One team took up the challenge, and produced a quadrifilar helix that could be mounted on the skin of a 4" fiberglass airframe; the idea being that circular pol was a good idea, and that the interior of a high power rocket was precious/dangerous.

I think I might try antennas embedded in fins as a student project for next year."

Way over my knowledge base.
 

CORZERO

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http://www.digikey.com/product-detail/en/molex-llc/1052620001/WM9669-ND/3918562

Below link is just an example, but all popular extensions in different lengths and conversion types (e.g. uFL to RP-SMA) are available inexpensively from popular sources should you choose to mount all antennas in the nose cone and electronics in a separate AV bay.

http://www.getfpv.com/20cm-sma-male...7DFBCBE2736F&gclid=CKfei-rx580CFckehgodQIcIgA

Active GPS antennas are also available through this vendor if needed:

https://www.adafruit.com/product/2461
 

bobkrech

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A new school year is coming up and that means a new project. This rocket is anticipated to be all carbon fiber, which is of course, RF opaque. It will also be going nearly five miles into the sky. This means that the avionics has to step up its game. I'm looking into externally mounted antennas. There are a lot of systems that will need external antennas. We have 2 GPS receivers (telemetrum and a custom computer) a 900 mHz telemetry transceiver and a telemetrum. We discussed putting things in the nosecone and making that from fiberglass, but then the wiring gets complicated. I'm wondering if anyone has any ingenious systems or tips on ways to get telemetry off of this vehicle. Any favorite antennas or mounting methods? Anybody found a way that just doesn't work?
Thanks.
Justify your design. Can you make a rational case why you rocket must be 100% CF?

If not, there's no reason for a rocket that will be going to only 5 miles to be 100% CF.
 

markkoelsch

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Bob brings up a very good point. Carbon fiber has a lot of strong points, but also a few drawbacks. Radio opacity and cost are a few negatives.

What makes carbon fiber a mandatory choice over a glass airframe, which is less expensive and does not suffer the Rf issues.
 

AlphaHybrids

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My guess is this is an ESRA/IREC team that is shooting for the 25,000' goal. Fiberglass will definitely work for this type of rocket. From being involved in past competitions, I would say the down portion has needed more work than the up portion.

Edward
 

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"Justify" his design? What is this, the Supreme Court of Rocket Materials Usage? "Rational" is subjective. Perhaps the OP sleeps on a mattress stuffed with hundred dollar bills and thinks CF is pretty. Who cares? OP has no obligation to answer to anyone but the Feds when deciding what materials he uses. His choice in materials is his concern and totally irrelevant to anyone else other than to serve as a basis for his dilemma when asking for guidance with his project. The "why" is not important, so long as the OP does not specify such concern.

It is rather frustrating to see responses like this, which is quite common in this forum. A hobbyist states his intentions, poses a question regarding a specific issue he is attempting to resolve, then gets shot down by opinions denouncing the hobbyists original method by recommending (or making no recommendation at all) solutions unrelated to the root of the stated issue entirely.
 

cerving

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I recall seeing somebody that took some stick-on copper foil, trimmed it to the tuned length of the frequency (for 900 MHz about 80mm) and stuck it on the outside of the rocket, essentially making a dipole antenna out of the outside of the AV bay. You'd need two 80mm-long pieces, of course... one for the antenna and one for the ground plane. I haven't personally tried it, but it looks intriguing. This will work fine for the telemetry, but GPS reception is a different story... you're going to need an external patch antenna. Look them up on Mouser or DigiKey, there's quite a few of them around with u.fl cables attached. Good luck with your build, it sounds like fun!
 
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FredA

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PSAS (Portland Aerospace) made a really nice conformal antenna that was on the outside of their EBAY for a project a few years back.
This was for their ground-link.
I don't think they had one for GPS.
But you could start looking there.
 

vance2loud

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http://store.rfdesign.com.au/rfdflex1-900mhz-flexible-pcb-antenna-no-cable/
something like this could be mounted to the outside and sealed in place.
Will carbon tube create a ground plane and/or interfere with any antenna mounted on it? testing might be required, if so please post results.
You said nosecone mounting would be difficult, can you explain your project a little more? It might help for giving you worthwhile advice.
 

cerving

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http://store.rfdesign.com.au/rfdflex1-900mhz-flexible-pcb-antenna-no-cable/
something like this could be mounted to the outside and sealed in place.
Will carbon tube create a ground plane and/or interfere with any antenna mounted on it? testing might be required, if so please post results.
You said nosecone mounting would be difficult, can you explain your project a little more? It might help for giving you worthwhile advice.
That should work, it's a dipole so you don't need a ground plane. The CF tubing isn't really very conductive, so it would make a poor ground plane anyway.
 

timbucktoo

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That should work, it's a dipole so you don't need a ground plane. The CF tubing isn't really very conductive, so it would make a poor ground plane anyway.
Cris - do you know if these are available in the USA?
 

ksaves2

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I recall seeing somebody that took some stick-on copper foil, trimmed it to the tuned length of the frequency (for 900 MHz about 80mm) and stuck it on the outside of the rocket, essentially making a dipole antenna out of the outside of the AV bay. You'd need two 80mm-long pieces, of course... one for the antenna and one for the ground plane. I haven't personally tried it, but it looks intriguing. This will work fine for the telemetry, but GPS reception is a different story... you're going to need an external patch antenna. Look them up on Mouser or DigiKey, there's quite a few of them around with u.fl cables attached. Good luck with your build, it sounds like fun!
The other alternative is a colored "glass" nosecone ("any thing other than black") and stuff the tracker in there. I agree with Mr. Krech. Why make this more complex than it needs to be? How does the wiring become complicated with a NC tracker? Turn the tracker on, button up the nosecone and go fly.

Can try some unconventional stuff like like using an FG ebay coupler tube and have the GPS tracker/antenna ride in the aft portion. Would have the tracker get a lock onsite, set it on the CF rocket quickly and launch hoping the tracker would get a lock when ejected at apogee for the ride down. Would also need an aft bulkhead antenna so it would be exposed at apogee so the Rf can get out "there". Might be prudent to slap an RDF tracker to boot on the harness for backup sake. One really shouldn't expect very many telemetered position packets on the "up" side anyways as the high "G's" and doppler effects at the GPS receiver affect the decoding along with the imposed
speed limitations. It's near apogee and the ride down one wants to see the positions streaming in.

Also if insisting on totally CF, would be easy enough to embed some copper tape on the exposed surface of the tube to make an external dipole like others have mentioned. If going only 5 miles up, me thinks surface heating of the airframe would not be enough to worry about delamination of the copper foil. I'd use a simple clear laminating epoxy without any additives though. The only other hangup is "getting" the the GPS satellite signals to the receiver and
that would entail an external "radome", patch GPS antenna or hole in the wall of the carrier with some sort of window. If you ask me, gets a bit complicated.

I just don't know why this fascination of real time data acquisition. The only thing that's needed is to have adequate position information to recover the rocket. Once found, the onboard electronics can be accessed and the data off loaded.
Real time is only necessary if one doesn't expect to see the project again, but that of course might take more work and cost to achieve.

It only takes one or two GPS position packets to find a rocket although it's cool to be able to see a graphical presentation of a flight in real time on a map. For now, that's a PITA to do and the only thing out there that's easy enough and doesn't cost an arm and a leg is the Altus Metrum software/hardware. There are a couple of ham radio apps that can be hacked for NMEA trackers but it's a lot of work to achieve. GPS Rocket Locator for NMEA trackers is getting close to off line map caching.

Yeah, yeah. I know about "Kate" but she's a high dollar flight escort. Those that have "used" her some are plenty happy about the affair. If flying a very high cost project, the cost of Kate (Multitronix http://www.multitronix.com/) is probably not much compared to the value of the other flight components. For those with a more limited budget there are some pretty good choices out there.

I expect the flying community is getting close to off line real time tracking that's economical to do soon in the future. Kurt
 

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If you google "conformal GPS antenna " you will find numerous milspec suppliers ( Antcom, Haigh-Farr, Ball, etc.) In the end you would need to weigh the cost of a system that likely far exceeds the cost of the rest of the project. I looked at some of these for a project a couple of years ago and opted instead to use a fiberglass nose cone.
 

bobkrech

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"Justify" his design? What is this, the Supreme Court of Rocket Materials Usage? "Rational" is subjective. Perhaps the OP sleeps on a mattress stuffed with hundred dollar bills and thinks CF is pretty. Who cares? OP has no obligation to answer to anyone but the Feds when deciding what materials he uses. His choice in materials is his concern and totally irrelevant to anyone else other than to serve as a basis for his dilemma when asking for guidance with his project. The "why" is not important, so long as the OP does not specify such concern.

It is rather frustrating to see responses like this, which is quite common in this forum. A hobbyist states his intentions, poses a question regarding a specific issue he is attempting to resolve, then gets shot down by opinions denouncing the hobbyists original method by recommending (or making no recommendation at all) solutions unrelated to the root of the stated issue entirely.
You totally misunderstood my comment.

In my career I been involved in hundreds of design reviews, some of which required spending a week in another country, and this had happened more than once. When you are spending someone else's money to build a system, the folks paying for it want to know if the proposed system will meet their requirements, how it will work, how it will be made, how long it will take and how much will it cost. In order to do this, each design team member has to justify their designs,

This is a university rocketry team consisting of a number of members with multiple backgrounds and experiences. They don't have to justify their design to me, but rather the various members of the team have to justify their design to the other members of the team who might have conflicting requirements, and to the team manager who is the final arbitrator.

In any commercial/manufacturing setting, a preliminary and critical review of a team design is, or should always be, conducted to make sure that the proposed sub system designs and concepts meet and are compatible with the overall project goals, can be manufactured within the budget, and on a schedule. Since in any design there are multiple ways to make things work, there are always trade-offs on which approach to take and what materials need to be used so the device meets all performance specifications. That's what engineering is all about, and students especially have to get use to it because that's what they are going to be doing in their professional lives.

In this case, the airframe designers might have a desire to make the airframe from carbon fiber for whatever reason. The avionics/comm electronics team need to receive and transmit RF signals to and from the rocket. As CF is radio-opaque, this will require the electronics team to use external antennas. If the payload compartment uses fiberglass composite, the electronics team can use internal antennas. Both teams have conflicting requirements so each team has to analyze their designs, and conduct the performance/cost tradeoffs to justify their requirements to the other teams and the project manager. If CF strength is required for the mission, then the electronics team would be required to use external antennas instead of external antennas. If CF strength is not require and FG is good enough, then the electronics team would not need to use external antennas. The aero team also needs an input as external antenna could effect the aerodynamics of the rocket, and the finance team also need an input because external antennas cost more as does CF tubing compared to FG. It should be apparent from this simple presentation of the problem that the airframe team needs to justify the need for a CF payload compartment versus a FG payload compartment to the other team and the project manager because the CF solution adds to the cost and complexity of the project.
 

BlazingAngel665

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My guess is this is an ESRA/IREC team that is shooting for the 25,000' goal. Fiberglass will definitely work for this type of rocket. From being involved in past competitions, I would say the down portion has needed more work than the up portion.

Edward
Good guess ;) The down portion is always the trick. As they say at URRF "Well you've got the up part down." We have good success with dual sep dual deploy, we're working on getting single sep dual deploy working, but we've had a time trying to figure out tender descenders (pretty obvious mistakes in hindsight though). We also have some experimental parafoil recovery systems built, they haven't flown though. We're also working on CO2 recovery systems. We'll chose a robust system for the IREC vehicle to make sure we have a good chance of getting it back. Probably a single sep dual deploy CO2 based system.

Justify your design. Can you make a rational case why you rocket must be 100% CF?

If not, there's no reason for a rocket that will be going to only 5 miles to be 100% CF.
No, I couldn't. There really isn't a good reason for it to be CF. Fiberglass would work fine. We did fiberglass last year, but the structures team is building a CF rocket and avionics needs to work around that. We already needed to consider new antennas to get better range than wire whips, so we figured we'll just learn to handle them on the outside of the vehicle. There is a vague roadmap for future years to fly vehicles that would require the mass fraction and structure that CF can afford us. If we don't learn how to deal with it now we'll learn later. Might as well start now.

PSAS (Portland Aerospace) made a really nice conformal antenna that was on the outside of their EBAY for a project a few years back.
This was for their ground-link.
Great tip, thanks.

Also if insisting on totally CF, would be easy enough to embed some copper tape on the exposed surface of the tube to make an external dipole like others have mentioned.

I just don't know why this fascination of real time data acquisition. The only thing that's needed is to have adequate position information to recover the rocket. Once found, the onboard electronics can be accessed and the data off loaded.
Real time is only necessary if one doesn't expect to see the project again, but that of course might take more work and cost to achieve.

It only takes one or two GPS position packets to find a rocket although it's cool to be able to see a graphical presentation of a flight in real time on a map. For now, that's a PITA to do and the only thing out there that's easy enough and doesn't cost an arm and a leg is the Altus Metrum software/hardware. There are a couple of ham radio apps that can be hacked for NMEA trackers but it's a lot of work to achieve. GPS Rocket Locator for NMEA trackers is getting close to offline map caching.

Yeah, yeah. I know about "Kate" but she's a high dollar flight escort.

I expect the flying community is getting close to off line real time tracking that's economical to do soon in the future. Kurt
Avionics in the nose cone was evaluated in a trade study. It makes the wiring a huge pain. We'll have payload and sensors in the fin can. We're potentially using telemetry for more than just location, so we'd prefer not to jettison the nosecone with the avionics to get the parachute out. We've thought about using bluetooth to talk between boards wirelessly, but that's not a good option. Our fascination in real time telemetry is pretty much academic. We have plenty of bandwidth in our link, might as well put something interesting in it.
We have several TeleMetrums for live graphical presentation, but our custom computer can also do .klm files as well as 9 DoF, barometric data, and dozens of pyro channels and digital I/O. We've looked at Multitronix, it's a cool looking system but our custom one is cheaper and will be just as good once we get the radio link improved. We're hoping to make our "Pyxida" more available in the future. I've flown it on a couple of L1/L2 flights. It's pretty nice already, and having 12 pyro channels is good for doing whatever crazy project you want.


Thanks for all the tips guys. I'm aware we are pretty well over complicating this. The basic plan is that we can always "descope" to a simpler set up if we hit a block. Bob Krech is of course correct about design trades. There are many groups that handle different parts of the rocket. This particular problem is an interesting one, even if unnecessary. Keep the suggestions coming!
 
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UhClem

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The PSAS antenna is a poor example. Most people build corporate arrays where there are a number of small square patches that get fed by a splitter network. PSAS feeds a dingle very wide antenna with a splitter. I have seen no one else do it that way.

Mounting a dipole on the airframe works very poorly. It changes the center frequency giving low VSWR unless you can retune it. It also changes the pattern and radiation efficiency. The Molex part recommends mounting at least 5mm from the board/gound plane. Just think what that would do to your aerodynamics.
 

CORZERO

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You totally misunderstood my comment.

In the context of a team approach -which apparently went way over my head when reading the original post-, your rebuttal makes perfect sense. Mia culpa. I retract and yield to the experts.

In light of the recent posts, however, it does seem that the team is still interested in an external antenna solution. In regard to UhClems response, mounting dipoles on radio-opaque airframes is a bad idea (presuming he meant radio-opaque only). Perhaps embedding dipoles in the fins could be a solution, granted enough surface area was available and CF or metallic materials weren't used in the fin construction?

What about dipoles embedded in fin leading/trailing edges? What are the fin dimensions?
 

FredA

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The PSAS antenna may not have been the best, but it worked.

"Better is the enemy of good enough...."
 

John Beans

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I'd be tempted to go "fiberglass payload" rather than a constructed, dedicated conformal antenna. The wiring, application, and maintenance of that would be fussy (hard to tell if it's cracked), and a payload can be re-used more easily for a subsequent project.

I think it's always good if stuff can be easily re-used and shared among rockets, rather than dedicating it to one airframe. In fact, if there was one thing I'd like to see changed about rocketry, it's how disposable everything is. Things, including fins, would be better if they were more easily replaced.
 

BlazingAngel665

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I'd be tempted to go "fiberglass payload" rather than a constructed, dedicated conformal antenna. The wiring, application, and maintenance of that would be fussy (hard to tell if it's cracked), and a payload can be re-used more easily for a subsequent project.

I think it's always good if stuff can be easily re-used and shared among rockets, rather than dedicating it to one airframe. In fact, if there was one thing I'd like to see changed about rocketry, it's how disposable everything is. Things, including fins, would be better if they were more easily replaced.
Our team is working on standardizing things across airframes. Two years ago we built a rocket out of a shopping cart on apogee (COTS everything), and it performed very well. Last year nothing was purchased. Fiberglass mats, chips, solder, wires, ripstop nylon, sheet aluminum, and 3D printer filament came in the door and a rocket walked out. We did buy a motor though. We're doing custom prop this year. We've standardized our booster design and are working to build an iterated version of last years. You'd like our fincan, any fin is hot swappable with four screws, but it'll break mach no problem. We crashed our first test flight and re-flew several undamaged components on the competition flight.

Believe me, I'd love a fiberglass rocket. I'm linking this thread to our structures lead, I'll see if I can convince her to see things avionics' way. In the future we may actually need the CF though, so this discussion is still worthwhile.
 

ksaves2

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I'd be tempted to go "fiberglass payload" rather than a constructed, dedicated conformal antenna. The wiring, application, and maintenance of that would be fussy (hard to tell if it's cracked), and a payload can be re-used more easily for a subsequent project.

I think it's always good if stuff can be easily re-used and shared among rockets, rather than dedicating it to one airframe. In fact, if there was one thing I'd like to see changed about rocketry, it's how disposable everything is. Things, including fins, would be better if they were more easily replaced.
Hmmmm, That didn't occur to me. Get some FG tubing the same external diameter as the CF and put couplers on either end so there's an ebay made out of FG tubing. They can choose the necessary length and the rest of the rocket could be CF.
I understand about the wiring issue. Need tracker data to be integrated to the onboard data acquisition. It's a college exercise. If just for being able to find the rocket, the tracker in the the nosecone is all that's needed. Unless you could engineer say a
bluetooth/wireless device to send the data from the NC tracker to another device wirelessly inside the rocket.
I don 't know how that would go on the interior of a CF rocket. Might be doable. Kurt
 
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RocketHunter

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Our team is working on standardizing things across airframes. Two years ago we built a rocket out of a shopping cart on apogee (COTS everything), and it performed very well. Last year nothing was purchased. Fiberglass mats, chips, solder, wires, ripstop nylon, sheet aluminum, and 3D printer filament came in the door and a rocket walked out. We did buy a motor though. We're doing custom prop this year. We've standardized our booster design and are working to build an iterated version of last years. You'd like our fincan, any fin is hot swappable with four screws, but it'll break mach no problem. We crashed our first test flight and re-flew several undamaged components on the competition flight.

Believe me, I'd love a fiberglass rocket. I'm linking this thread to our structures lead, I'll see if I can convince her to see things avionics' way. In the future we may actually need the CF though, so this discussion is still worthwhile.
One thing I'm surprised no one has mentioned is to look into making a kevlar/aramid airframe - very similar strength to weight ratio as carbon fiber, stiffer than fiberglass, radio transparent, and still has the 'coolness' factor that CF has. It's a bit more difficult to work with post-layup, but nothing too bad - just make sure to do a veil layer of fiberglass that you can sand into partially for finishing.

This one went well over 3 miles and mach 2.3, for long enough to melt away the clear coat - all on only 1,500 n-s. Top airframe houses recovery system and electronics for real-time telemetry. 1.5" OD, wall thickness of ~0.04". Fins are ~0.06" quasi-isotropic unidirectional plate with carbon-nanotube reinforced epoxy - nice thing about tiny rockets is even the most exotic materials don't cost a whole lot :smile:
FullSizeRender 10.jpgFullSizeRender 11.jpg

Which competition is the rocket for?
 

FredA

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Believe me, I'd love a fiberglass rocket. I'm linking this thread to our structures lead, I'll see if I can convince her to see things avionics' way. In the future we may actually need the CF though, so this discussion is still worthwhile.

What competition is this for?
None of the ones I'm familiar with (USLI and ESRA) do....you could achieve those flight profiles with cardboard and most certainly FG...
I think your structural lead has gone haywire....
 
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cerving

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I'd be tempted to go "fiberglass payload" rather than a constructed, dedicated conformal antenna. The wiring, application, and maintenance of that would be fussy (hard to tell if it's cracked), and a payload can be re-used more easily for a subsequent project.

I think it's always good if stuff can be easily re-used and shared among rockets, rather than dedicating it to one airframe. In fact, if there was one thing I'd like to see changed about rocketry, it's how disposable everything is. Things, including fins, would be better if they were more easily replaced.
You should be able to share a given size nose cone with a tracker in it amongst rockets, too. Put a 29mm tube in it and build a sled that can slide into any of the various sized NC's. Problem is, different NC's have different sized shoulders, and tubes from various manufacturers aren't the same inside diameter.
 

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Just a couple of comments/questions.

You are planning to use a CF airframe and a plastic or FG NC? That really doesn't make a lot of sense as the NC can have the highest thermal and strength requirements on the rocket..... That's where a high temperature resin CF really beats everything else if you're going fast........ I know you're not going very fast so it is probably not an issue however beyond some Mach number, plastics melt, and FG chars and then becomes RF absorbing.

Band antennas and/or patch antennas can work, but a question is how close can they be to a conductive surface. Again as was mentioned earlier, a FG/coupler with a narrow 1" to 2" airframe diameter FG band will make the antenna problem go away, especially when using band/patch antennas.

Missiles with radar seekers use ceramic nose cones. That however is a very expensive solution for hobby rockets.
 

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You should be able to share a given size nose cone with a tracker in it amongst rockets, too. Put a 29mm tube in it and build a sled that can slide into any of the various sized NC's. Problem is, different NC's have different sized shoulders, and tubes from various manufacturers aren't the same inside diameter.
and thus , I learned how to spin bulkheads on my drill press ...which I use all the time .

Mostly for utilizing plastic nose cones for tracker bays...Coker style .

Kenny
 

UhClem

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Band antennas and/or patch antennas can work, but a question is how close can they be to a conductive surface. .
The ground plane under the visible part of the patch is an integral part of the antenna and should extend for some distance beyond the visible patch. This can be either more of that double sided PCB or structure. (Changing the extent of that ground plane alters the center frequency. Fortunately, after a certain point, adding more doesn't change it much.) Lots of patch antennas are mounted to conductive structures. One example that springs to mind is on the nose of flight test versions of the ATACMS Block II. (Visible in pictures as a dark band on the light nose.) That had not one but three patch antenna systems to cover C, S,and L bands. (5GHz, 2.3GHz, and 400MHz)

Adding a patch antenna to a payload section is not simple. You need multiple patches and the simplest way is to fab them as a single wrap around PCB. But for aerodynamics the top of the antenna radome should be flush with the surface of the airframe. Which means you have to cut a band out of that CF airframe to create a recessed area and have structure for the antenna to mount to. Repeat for any additional antennas.

Designing and building these antennas is both a science and an art. The science will get you close but because of various details that are difficult to model (the radome for example) tuning the antenna is an art. Requiring test equipment and a sharp knife.
 

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