External Antennas for Avionics

UhClem said:

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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After 4 1/2 decades in the aerospace/defense business, I fully appreciate the difficulties. Specialized antennas aren't "stock" and each one needs to be designed, fabricated and tested in the final configuration that it will be deployed in. Companies such as https://www.haigh-farr.com/index.shtml can certainly design, test and fabricate this type of antenna, but it's not cheap. The OP will be shocked when he sees the cost, but that's all part of the education process. I'm sure if he looked around MIT he could probably find a few EE majors that could help out since it's a lot more than just a rocketry problem, it's a team problem.

One more thing to consider is that generally the FCC Part 15 regulations call for a single-element antenna (whip, dipole, etc.) for transmitters, so any kind of patch array is out the window for the 900 mhz telemetry transmitter (assuming that it's an unlicensed Part 15 device, of course). His best bet is either to put the telemetry in a fiberglass nose cone (assuming that he's not going above mach 2 or so, and that any sensors can accommodate this), or to use fiberglass/kevlar for the AV bay (which is probably the easiest and most practical solution) and live with any signal degradation that may be caused by the other metallic stuff there.

bobkrech said:

I'm sure if he looked around MIT he could probably find a few EE majors that could help out since it's a lot more than just a rocketry problem, it's a team problem.

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Probably why the whole "team" concept went over my head in the first place. Why in the world would the op be here? Being an MIT team, one would think they would have access to all the tech resources and academic intellect they would ever need to knock this out no problem and bring their solutions to us rather than the other way around with queries to an amateur hobby rocketry internet forum.

CORZERO said:

Probably why the whole "team" concept went over my head in the first place. Why in the world would the op be here? Being an MIT team, one would think they would have access to all the tech resources and academic intellect they would ever need to knock this out no problem and bring their solutions to us rather than the other way around with queries to an amateur hobby rocketry internet forum.

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'Cause I think they like some input from here. Plenty of folks here not affiliated with an academic center have done some pretty interesting flights. That's a pretty good chunk of untapped experience. Kurt

NAR S&T performs our motor certifications at MIT and CMASS has 12 meetings a year in Cambridge. We also have the MIT rocket team at CMASS launches and unless I'm mistaken one of our L3's has worked with them in the past, but every year is different so we'll see if they contact us this year.....

Do we know yet what contest is this for?

bobkrech said:

I'm sure if he looked around MIT he could probably find a few EE majors that could help out since it's a lot more than just a rocketry problem, it's a team problem.

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Absolutely. The trick is finding them, and finding ones with free time. . . We have a few EECS majors who are indispensible, but antenna design and fabrication could be its entire own team.

cerving said:

One more thing to consider is that generally the FCC Part 15 regulations call for a single-element antenna (whip, dipole, etc.) for transmitters, so any kind of patch array is out the window for the 900 mhz telemetry transmitter (assuming that it's an unlicensed Part 15 device, of course).

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Very useful compliance info, thank you greatly.

bobkrech said:

NAR S&T performs our motor certifications at MIT and CMASS has 12 meetings a year in Cambridge. We also have the MIT rocket team at CMASS launches and unless I'm mistaken one of our L3's has worked with them in the past, but every year is different so we'll see if they contact us this year.....

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Every now and then we make it out to a CMASS launch. The low waiver and power lines at Amesbury usually make the MMMSC or URRF launches a better deal. We have three or four team members looking at getting L3's this year, so I'm sure we'll be in touch. We usually talk to Robert DeHate of AMW fame. He's been an invaluable resource, especially on our propulsion projects.

FredA said:

Do we know yet what contest is this for?

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IREC Advanced is the team goal this year (23,000ft with 10lb payload), but we're looking to the future hoping to go higher/faster/better/etc.

I have some interesting plans for the vehicle, but as ksaves2 noted, there is a lot of experience here I would be foolish to ignore. My plan right now is some combination of fin trace antennas for the 900 mHz system, and use antenna blisters for the GPS and 440 mHz systems. That saves us from running any new wires in the vehicle. The antenna aeroshells are not particularly aerodynamic, but I've seen Copenhagen Suborbitals use them successfully before, so I figure there's some benefit. I'm going to try and contact them regarding their system as well. We also had a team alum provide some interesting thoughts. It looks like avionics has its work cut out for itself.

23k with a 10# payload in no way requires CF....
Your team is making this way harder than it needs to be....which is not uncommon.

FredA said:

23k with a 10# payload in no way requires CF....
Your team is making this way harder than it needs to be....which is not uncommon.

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As was mentioned, not everything is a decision I can make, and not all the decisions we make are necessary for this year. If we try and build a CF rocket and fail this year, we already have all the supplies to make a fiberglass one in <2 weeks. If we get to January and avionics still can't make the antennas work, we'll descope to fiberglass. In that way we've learned a lot and haven't lost anything.

As was mentioned, not everything is a decision I can make, and not all the decisions we make are necessary for this year

Understand team dynamics....
I've seen FG goto 100k....CF is not needed, but realize it's "sexy."

Just realize that you'll want to paint over that "sexy" finish.
At ESRA this year, really nice CF rockets sat on the pad and cooked with internal temps exceeding 150 degrees which killed some of the onboard electronics.
Lesson learned the hard way - paint them white......

FredA said:

As was mentioned, not everything is a decision I can make, and not all the decisions we make are necessary for this year

Understand team dynamics....
I've seen FG goto 100k....CF is not needed, but realize it's "sexy."

Just realize that you'll want to paint over that "sexy" finish.
At ESRA this year, really nice CF rockets sat on the pad and cooked with internal temps exceeding 150 degrees which killed some of the onboard electronics.
Lesson learned the hard way - paint them white......

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I was going to point out to you Fred to blow off the paint as it'll get burned off anyways but you're right. If one can set it up fast and push the button, no problem. Who can do it that fast with a project like that? If it has to sit in the heat, the electronics/batteries will cook and fail like you point out.
Best to paint it white but still try to get it launched ASAP. Kurt

It's not about beauty, before or after....it's about survival.

ESRA (at least this year) was so poorly run that rockets sat out on the pad for HOURS in the ready state in the blistering Utah heat.
One team waited over 3 hours for the LCO to finally push the button.
The rocket was too hot to touch.
GoPro's and altimeters failed.

So - prepare for the worst - paint the rocket white - burnt paint is considered a "badge of honor."

Also, IREC/ESRA states that once you load your rocket you should have battery life for up to an hour because of their launch protocols and range setup.

Are you going to be winding the CF airframe or doing a cloth layup? The advanced category states that major systems components (which the airframe is one) shall be student constructed.

Edward

That has been our clubs experience that the launches are not run the best. They are great at a contest, but don't know how to run a range.

We have our local club that runs three three day launches a year. Our IREC team attends those and every year when they go to IREC they see opportunities on how IREC/ESRA could improve launch operations.

This year they had a hybrid that was fill at the pad, then needed to sit for a bit to raise internal tank pressure. It was very hard for them to convey the sequence to IREC to get them to launch.

Edward

FredA said:

At ESRA this year, really nice CF rockets sat on the pad and cooked with internal temps exceeding 150 degrees which killed some of the onboard electronics.
Lesson learned the hard way - paint them white......

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Not to mention the glass transition temperature of the epoxy holding that sexy CF together. Even if you don't exceed it on the ground it gives you a head start on aerodynamic heating.

AlphaHybrids said:

Also, IREC/ESRA states that once you load your rocket you should have battery life for up to an hour because of their launch protocols and range setup.

Are you going to be winding the CF airframe or doing a cloth layup? The advanced category states that major systems components (which the airframe is one) shall be student constructed.

Edward

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We do cloth layups now, but we're looking into a filament winder. Our lab isn't quite large enough so we are having to be creative. Our IREC rocket this year was all student constructed except the motor, which we've been working on now for over a year. We're ready to do advanced from a materials perspective. We've done both fiberglass and CF layups, they aren't that different.

FredA said:

As was mentioned, not everything is a decision I can make, and not all the decisions we make are necessary for this year

Understand team dynamics....
I've seen FG goto 100k....CF is not needed, but realize it's "sexy."

Just realize that you'll want to paint over that "sexy" finish.
At ESRA this year, really nice CF rockets sat on the pad and cooked with internal temps exceeding 150 degrees which killed some of the onboard electronics.
Lesson learned the hard way - paint them white......

Click to expand...

Our rockets are usually silver and red. Nice MIT colors with high visibility. We're also building some ground support equipment to provide conditioned air to the vehicle. We lost two cameras to the heat this year. Luckily two still worked and we got great footage. Our custom avionics works in 150 degree heat luckily. That wasn't a design feature, just a lucky accident. Our TeleMetrum and StratoLogger also worked fine. Our avionics bay was about 65C when we launched. We were even the first off the rail. . .

The "sexy" CF is nice, but our interest in the CF is for the stiffness and weight reductions.

FredA said:

It's not about beauty, before or after....it's about survival.

ESRA (at least this year) was so poorly run that rockets sat out on the pad for HOURS in the ready state in the blistering Utah heat.
One team waited over 3 hours for the LCO to finally push the button.
The rocket was too hot to touch.
GoPro's and altimeters failed.

So - prepare for the worst - paint the rocket white - burnt paint is considered a "badge of honor."

Click to expand...

ESRA runs a great contest, but their pad management could broke some improvement. We were ready to launch on Thursday, but the rails were backed up so we launched third salvo on Friday. That was after noon. We resorted to soaking rags and draping them over the rocket to keep it cool. We learned though. Our ground support team is building some desert proof handling equipment. The radio spectrum management was also sub-optimal. I was getting tons of people's 440 packets on my ground station.


Our original rocket used a linux OS on a beaglebone black. We moved all our code over to an ARM microcontroller, but kept the battery size the same. We had 4+ hours of endurance on all of the flight critical systems even in the heat, so we managed well.

I see it hard to justify the weight reduction from fiberglass to carbon fiber in this contest when your payload is required to be 10 pounds. That amount trumps a lot of airframe optimization.

Both teams I have helped have it hard to find a payload that weighs ten pounds- that is a lot of circuit boards, servos sensors and electronics. And still it did not come close to 10 pounds.

Edward

AlphaHybrids said:

I see it hard to justify the weight reduction from fiberglass to carbon fiber in this contest when your payload is required to be 10 pounds. That amount trumps a lot of airframe optimization.

Both teams I have helped have it hard to find a payload that weighs ten pounds- that is a lot of circuit boards, servos sensors and electronics. And still it did not come close to 10 pounds.

Edward

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We're hoping to fly to 100k ft next year. We just want the experience this year. We flew two payloads last year, a computer vision system and a solid state plasma generator which controlled vehicle roll by selectively stalling and unstalling canards. The plasma system is incredible. It used up about eight pounds after we completely overspec'd the batteries. There was a nice big heavy transformer to help with the weight.

Exactly - you had to *try* to meet the ten pounds by getting larger batteries and a transformer.

Edward