Radio Questions for Telemetry Transmission

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jetsman97

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Hello!
I am in my university's Rocket Club and am trying to design our telemetry transmission system for our Avionics Team. We are building our own hybrid-engine rocket, and our first launch is expected to reach an apogee of approximately 500 meters. We want to send data from sensors (accelerometer, gyroscope, altimeter, magnetometer, GPS) in real time to us on the ground.

We have been looking into using the XBee 3 RP-SMA Module which transmits at 2.4 Ghz with a maximum transmission power output of 6.3 [mW] (+8dBm). I thought to use the RP-SMA module because I understand the UFL connection is less sturdy/reliable, especially if we plan to put unplug/replug the connection often (this is likely due to the many tests we plan on doing). My question is which antenna should I use, and how can I connect it to the Xbee? The maximum transmission power for the country we are located at 2.4 {GHz] is 125 [mW]. I understood that having an antenna which is either half or a fourth of the wavelength is recommended, meaning either 3 or 6 centimeters. Any information on the antenna to use on the rocket/the connection from the module to the antenna would be greatly appreciated.

Additionally, we need a ground antenna for our receiver on the ground, are there any recommendations for the size/specs of the ground antenna?

Here are some links:
XBee 3 SparkFun
XBee 3 DataSheet - page 29 has relevant information
 
Here are links to some antennas you may want to consider. Once on the Digi-Key site you can download spec sheets.

Base Unit/Station Antenna:
https://www.digikey.com/en/products...x8fLZYQwX4WRjZeNYlwmQgfZnBgzGNhjDt5Ht4qvPg5i3
Rocket Unit Antenna - Modest Length & Good Performance:
https://www.digikey.com/en/products...MHceoChetXL5LGkTpY0id9OqMjE1oDkgfwyNRSrGOJ05H

Rocket Unit Antenna - Stubby Length & Average to Low Performance:
https://www.digikey.com/en/products...oxtpJptIhrz9j9rxAWZnADoRzgS9KO1X_Z3t1_cqPCUrt
 
Work on calculating the "Link Budget". Lots of info on web about this.
Then if gain is required add this to the ground station receiver with a directional antenna.
On the RX end is the easiest place to get link improvements..
 
depending on range requirements you'd be better off with a higher power 900 MHz Xbee like the 900HP (or 868 MHz if you are in a country that doesn't allow 900). But definitely learn how to do a link budget.
 
Here are links to some antennas you may want to consider. Once on the Digi-Key site you can download spec sheets.

Base Unit/Station Antenna:
https://www.digikey.com/en/products...x8fLZYQwX4WRjZeNYlwmQgfZnBgzGNhjDt5Ht4qvPg5i3
Rocket Unit Antenna - Modest Length & Good Performance:
https://www.digikey.com/en/products...MHceoChetXL5LGkTpY0id9OqMjE1oDkgfwyNRSrGOJ05H

Rocket Unit Antenna - Stubby Length & Average to Low Performance:
https://www.digikey.com/en/products...oxtpJptIhrz9j9rxAWZnADoRzgS9KO1X_Z3t1_cqPCUrt

I see these aren't the exact antennas that are mentioned in the data sheet. Do you think there'd be a problem not using them? I prefer to use the ones you sent me because I can find where to order them from, unlike the ones mentioned in the data sheet.

Thanks for your help!
 
Work on calculating the "Link Budget". Lots of info on web about this.
Then if gain is required add this to the ground station receiver with a directional antenna.
On the RX end is the easiest place to get link improvements..

I read a bit about this.
However, after looking through the data sheet of both the XBee and the linked antennas, I wasn't able to find any figures describing the tranmission, connector, nor antenna loss. Is this "loss" subtracted from the gain and thus we are shown a "net gain" number? How is it possible no figures describing gain are included in any of the parts? Additionally, if only a max gain is given, how can I know the actual (or approximate) gain of the antenna?
 
connector loss may be in the Antenna's Gain spec.
All three antennas listed above have a spec but only the 'base station' antenna has radiation patterns show in the data sheet.

On first cut of link budget calculations assume no loss in antenna connector (antennas directly connected to TX and RX) and see what the RX strength and SNR at the distance expected.
If antenna has a length of coax then you need to estimate or measure the loss and include this in the link budget.
 
I see these aren't the exact antennas that are mentioned in the data sheet. Do you think there'd be a problem not using them? I prefer to use the ones you sent me because I can find where to order them from, unlike the ones mentioned in the data sheet.

Thanks for your help!

Sent you a PM.
 
@jetsman97
Just a little bit about gain :
Gain in an antenna simply means that RF is enhanced in some directions and decreased in others. For a rocket that’s a speck in the sky having a high gain can mean that in some attitudes the rocket’s signal might not be detectable.
 
@jetsman97
Just a little bit about gain :
Gain in an antenna simply means that RF is enhanced in some directions and decreased in others. For a rocket that’s a speck in the sky having a high gain can mean that in some attitudes the rocket’s signal might not be detectable.
That is the biggest reason to keep the TX gain lower and omnidirectional then add gain at the receiver if needed.
 
I see. How should I orient my antennas to maximize the signal between them. I would think to have the ground antenna pointing straight up toward the rocket, and have the rocket antenna protruding from the side, so that it is oriented perpendicular to the rocket. Is this correct? If not, what data can help me determine how to orient the antennas on the ground/rocket? @waltr @Steve Shannon
 
I see. How should I orient my antennas to maximize the signal between them. I would think to have the ground antenna pointing straight up toward the rocket, and have the rocket antenna protruding from the side, so that it is oriented perpendicular to the rocket. Is this correct? If not, what data can help me determine how to orient the antennas on the ground/rocket? @waltr @Steve Shannon
An antenna that protrudes is easily damaged. I would suggest a simple omnidirectional antenna (even a wire of the correct length should be fine) inside the rocket, aligned with the length of the rocket. If you look at commercial rocket telemetry systems that’s usually what they use. I would also use a Yagi for the ground station. Do some experiments to find out whether you need to rotate the Yagi to get a better signal.

Are you required to come up with your own telemetry system? If I were you I would look into adapting an existing telemetry system. There are some good ones on the market.
 
An antenna that protrudes is easily damaged. I would suggest a simple omnidirectional antenna (even a wire of the correct length should be fine) inside the rocket, aligned with the length of the rocket. If you look at commercial rocket telemetry systems that’s usually what they use. I would also use a Yagi for the ground station. Do some experiments to find out whether you need to rotate the Yagi to get a better signal.

Are you required to come up with your own telemetry system? If I were you I would look into adapting an existing telemetry system. There are some good ones on the market.

What sort of wire? What is considered the correct length? (1/2 or 1/4 of the wavelength)? I was under the impression that the antenna needs to be located outside of the rocket, so that "line-of-sight" is maintained. The only issue with adapting an existing telemetry system is that my University is located in Israel, so I can only use the frequency of 2.4 [GHz] without dealing with the complex process of getting very special permissions. A lot of the resources I find use frequencies in the 433 or 900 [MHz] range.
 
First, you really need to study radio antennas in more detail. There is much info easily available if you look for Ham Radio stuff.

As to frequency, do consider getting a Ham Radio License from your country. This opens up a huge spectrum of choices including 70cm (440MHz) and many more.
 
First, you really need to study radio antennas in more detail. There is much info easily available if you look for Ham Radio stuff.

As to frequency, do consider getting a Ham Radio License from your country. This opens up a huge spectrum of choices including 70cm (440MHz) and many more.
This!!! ^^^
 
What sort of wire? What is considered the correct length? (1/2 or 1/4 of the wavelength)? I was under the impression that the antenna needs to be located outside of the rocket, so that "line-of-sight" is maintained. The only issue with adapting an existing telemetry system is that my University is located in Israel, so I can only use the frequency of 2.4 [GHz] without dealing with the complex process of getting very special permissions. A lot of the resources I find use frequencies in the 433 or 900 [MHz] range.
Line of sight isn’t the same as visible. An antenna can be placed within or behind something that is transparent to RF.
The length of the wire antenna will depend entirely on the type of antenna you use, the impedance required by the transceiver module, and the velocity factor of the wire. The data sheet for the transceiver module should provide you with at least some of the necessary information.
More importantly, it’s simply not possible to teach you all you need to know here. @waltr ’s idea about becoming a ham is really a great idea.
 
If you want to use the XBee 3 PRO, your max range will be about 3.5 km with omni-to-omni antennas, assuming 1.9 dB antennas and a fade margin of 15 dB. You'll need to use a directional antenna on the ground to do any better range-wise.

If you use RP-SMA, then you can buy antennas off the shelf. https://www.sparkfun.com/products/145 would work.

It's entirely possible to get these to work without knowing much of anything about antenna design or even radio, though it's more fun to understand what you're doing. :)
 
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