900Mhz GPS transmitter and Receiver

Greetings.
So this has been done on here before. and there is a GREAT thread on the forum already of this being done. That said my ultimate goal is to test off the shelf components first and design a compact surface mount version as I test out the various parts.

How much of this gets done, and how fast will depend on funds and other rocket projects of course.
As of today, I tested it at 3k feet successfully. I'm sure it would have gone much farther but I didn't feel like walking all day. Specs say it should go 30k feet with high gain antenna. I hung the nose cone on a fence post and started walking away from it.

Breadboard mounted as a functional proof of concept:


Transmitter (mounted in nose cone):
900MHz, 250mW xBee Module
900Mhz whip antenna
adafruit GPS module
3.7v LiPo battery



Receiver (mounted in project box):
900MHz, 250mW xBee Module
3D clover antenna
sparkfun serial Bluetooth module
3.7v LiPo battery
Project Box



So Today was our clubs first launch of the season. It was scrubbed due to crappy weather. April is the next launch date so I will have more info on range then.

Cool! I'm working on my own project as well, same xbees, 3dB dipole antennas, ublox max7 GPSs, some other telemetry bits, and Teensy 3.1 microprocs. I haven't tested range yet, but hoping to get at least a few miles in the air without going to a yagi.

A question - why are you using a circularly polarized antenna on your receiver, and a linear dipole on the transmitter? I had wondered about the circular antennas (HobbyKing) - but one web page I found said you loose some range (used on both trans & rec), but gain because there is no required antenna orientation - good for things like planes, copters, etc that dont go as far. I didn't try it because the circular pol antenna was not going to fit in my rocket. Do you know how the combination you are using effects signal strength, and how much immunity does it really give you from polarization if the transmitter is still linear? I'm asking because I know little about antennas and honestly have no idea.

1) it was cheap and I wanted to try it out vs the dipole. It's supposed to have and upward direction so not as much wasted power. There is still a Null zone at top though.

2) dipole just fits in the nosecone!
at 3k feet I pointed the null zone at the transmitter and it droped only one NEMA sentance. I put the other dipole on and pointed its null zone at it and lost all signal. The dipole was chose because it 1 fits in the nozecone and 2 puts out a relative 360deg pattern.

Like most things in life, and engineering, this is a trade off. There is a loss of -3dB when using a circular polarized antenna to receive a linearly polarized signal. On the other hand a crossed polarized antenna, vertically polarized receiving a horizontally polarized signal, there is as much as a -25 dB loss. Since we can't control the relative polarity of the antenna in the rocket we can choose to give up some signal all the time to avoid those times, however brief, where the signal drops by 25 dB.

I. Flight, the polarity isn't much of an issue, you have vertical polarization in most cases. Upon landing, you have a mixed bag. On nice flat ground where things are perfect, you now have horizontal polarization. Hit a ditch, large weeds, a tree, etc you have some other orientation.

But...you need not actually have a continuous signal in many cases. In many cases once you get to the last known location, you will be back in range if you have a reasonable range to start with. Even if not, if you take a moment, you have a start place, and a track. Which is plenty unless you fly over a hill that changes the wind direction.

I have no idea about the xBee modules, but the Hope modules used in the Egg Finder give an easy half mile range on a relatively flat field.

the ground thing won't really be an issue as long as I'm getting data close to the ground. Even 50' off the ground is still 'line of site' and will still get you REALLY close with GPS coordinates.
I'm still waiting for the first launch of the season though since this month was scrubbed due to bad weather.
With my snazzy Marshal Transmitter and my functioning but not tested GPS module I'm confident in recovery
If all is successful with the GPS I'll start designing a nice, tiny surface mounted components version.
Just curious if anybody would be interested in such a thing?
If I spend the copious amount of time in designing it, I can send off a production order for the boards and sell them if anybody is interested.
Otherwise I may just run off a couple prototypes for myself.

AndyC said:

Cool! I'm working on my own project as well, same xbees, 3dB dipole antennas, ublox max7 GPSs, some other telemetry bits, and Teensy 3.1 microprocs. I haven't tested range yet, but hoping to get at least a few miles in the air without going to a yagi.

A question - why are you using a circularly polarized antenna on your receiver, and a linear dipole on the transmitter? I had wondered about the circular antennas (HobbyKing) - but one web page I found said you loose some range (used on both trans & rec), but gain because there is no required antenna orientation - good for things like planes, copters, etc that dont go as far. I didn't try it because the circular pol antenna was not going to fit in my rocket. Do you know how the combination you are using effects signal strength, and how much immunity does it really give you from polarization if the transmitter is still linear? I'm asking because I know little about antennas and honestly have no idea.

Click to expand...

Andy, I think it has to do with the radiation pattern as some pointed out here:

https://beta.ivc.no/wiki/index.php/Cloverleaf_FPV_antenna

https://www.cisco.com/c/en/us/produ...ssories/prod_white_paper0900aecd806a1a3e.html

You want the receiver antenna to "cover" most of reception without having gaps and loss of signal.

John Ritz