Fun and Destruction with Antennas

30 08 2012

Sorry about the slowness of my posts. I’m back at university now so my time is slightly limited. As promised I will now update you all on my adventures into the world of range optimisation for my transmitter/receiver circuit. I am just going to start rambling now so I hope it all makes sense. If not, then just hit me up in the comments.

Since I had a working connection between the transmitter and receiver, and was happily sending out “hello world!” for everybody to see, it was time to figure out if it is possible to get the required range out of these little modules for the distances that a HAB require. My initial set up had a half wave whip antenna on both the transmitter side and the receiver side, this allowed me to get to the green dot on the picture below

Fig 1 This is a map of how far I could get reception with different antenna combinations. The red dot is where the transmitter was set up. It was on the lounge room floor inside my house. The other dots are the different distances I achieved with the receiver before I lost reception completely.

After my initial trial run I found out that I should be using a quarter wave antenna on the transmitter end, not a half wave one. I’m not sure why this is but I will be doing some research to find out why in the future. So, after I replaced the transmitter antenna with a quarter wave whip, I managed to get reception up until the blue dot.

At this point, it was time to get a little bit fancy. I decided I was going to attempt to construct a Yagi-Udon antenna for the receiver. I understood enough about antennas that I was going to be connecting a balanced antenna onto an unbalanced cable back to the receiver module. This means that I was going to require a balun to connect the antenna to the cable. I followed the directions here on how to make a ferrite core 1:1 voltage balun and ended up with the following half wave dipole test antenna

Fig 2 Test balun with two pieces of solid core CAT5 cable strands acting as a simple dipole antenna.

At this point, I was beginning to run out of time (I had to pick up one of my boys from school). Also, I had a bit of an idea (yes they are dangerous words). I thought, I have one of these 43 element UHF television antennas sitting in the garage, maybe I can use that instead of going through the hassle of making one of my own since they operate on the 470 to 862MHz band and I was operating on 433MHz. I understood that the gain of the antenna would be severely diminished at that frequency, but I was willing to give it a try because I wouldn’t cost me much due to the fact that I already had the antenna. So, I put aside my home made balun/antenna and picked up my son from school. On the way I bought a PCB mount F59 socket to solder onto the receiver shield. This way I could easily attach a 1.5m RG6 cable between the antenna and shield. After we got home, I soldered on the connector and off we went for a quick range test. I left the transmitter set up at the same place as last time (red dot on picture below) and took the antenna, a camera tripod, receiver and pc to the test site (blue dot).

Fig 3 Red dot is where the transmitter was left at home. Blue dot is where I set up the UHF antenna attached to the receiver and pc. Purple dot is the unforeseen anomaly that will soon be discussed.

I clamped the antenna to the extending neck of the tripod so that I could have a convenient, and stable, way of pointing it to where I wanted without having to hold it by hand. I then started up the pc and the software, as well as connected the cable to the receiver shield. I then pointed the antenna in the rough direction that I thought was home. But when I plugged the arduino into the pc I thought I heard a pop come from the receiver. I couldn’t be sure though as there was a truck driving past at the time. Not thinking much of it, I opened up the serial monitor for the arduino and was delighted to be receiving some of the data that was being sent from the transmitter, although not very much. I thought, well that is good and bad. It was cool that I could get a cheap transmitter and receiver pair to work over that distance, but I was a little let down that I couldn’t get more out of it. I did a quick test to see what the effective “viewing” angle of the antenna was by rotating the antenna till I couldn’t receive any more data then sweeping it back through the reception area till I reached the other extreme of where I couldn’t get any more data. I found the  effective viewing angle to be approximately 30 degrees, which was a lot bigger than I was expecting. We then packed up and headed home.

A couple of days went by before I could do some more tests. But when I got a chance, I decided to test out my home made balun/antenna. I took it out to where I lost reception with the half wave whip antenna and started up the serial monitor. I was a bit disappointed though when I was only receiving a very limited amount of data, “quiet oddly close to the amount I was getting from the television antenna” I thought. Even so, I had tested the other antenna combinations, except for the television antenna, to the maximum of there range, i.e. to the point where I couldn’t receive any data at all. So, I continued to walk down the alley way I was using for testing and into the park a block over before I finally lost all reception (see purple dot on Fig 1). On the walk back I decided to leave the serial monitor on to test at what point I could receive all of the packets. Strangely, I got all the way back inside my house and I was still getting the same amount of data that I was up the alley. Curiously, I disconnected my home made dipole antenna and reconnected the original half wave whip antenna. To my surprise, I was still getting very little data, even when the transmitter and receiver were about 1m apart.

At this point it clicked. That pop that I thought I heard when I was testing the television antenna was actually a pop  and it must have destroyed the module on my receiver shield. I was absolutely astounded at this thought. I was thinking “there is no way this antenna could get that much power from a little 3dBm transmitter module to be able to pop the receiver module”. I then started to think of other ways that it may have picked up enough power from something to be able to destroy it the way it did. It was then that I realised what I had most probably done. I have lived in my area all my life, so I know the ins and outs of all the streets pretty well. It was this knowledge that gave me the break through. A few blocks over from my house (the purple dot on Fig 3) sits the mobile phone tower for my area, this tower transmits on the ~900MHz band that the mobile phone providers use for their GSM networks. Since this is the only high powered transmitter anywhere in the area, my television antenna must have had enough gain at that frequency to create a huge power spike to be able to blow out the receiver module when I pointed it at the rough direction of what I thought was home. That is the only explanation that I can come up with that makes sense to me (feel free to correct me if you are better informed than I).

After all of this adventuring, I have managed to solder a new module into the shield but it didn’t work straight after installation and I haven’t had time to debug it since. Mainly due to me going back to university and also making a start on my other project (the recumbent bicycle). Anyway that’s it for now, but I will inform you all if I manage to get any time to work on it before the end of semester.


Transmitter/Receiver Design

5 08 2012

Before I tackle the design of the payload, I am wanting to design a transmitter and receiver set up that will work over the distances that will be occurring during the flight. I’ve had some fun with this part of the build. It’s been good to see what works and what doesn’t.

I’ve noticed that most people that send up HABs, that transmit their data on the 433MHz band, usually use the Radiometrix modules. I am going to try my luck at the cheaper modules I got from my work. This is a rough circuit diagram of the set up I am using:

Circuit Diagram for transmitter. Note the buffer IC being used between the Arduino and the transmitter module.

Since the Arduinos digitial I/O pins operate at TTL logic levels, and the transmitter module works on 3.3V logic levels, I had to use a buffer IC to act as logic level converter so that they could operate together safely. The 4050 chip has input protection built in so that the inputs can be safely driven higher then its supply voltage. Because of this, all that is required is that both the module and the 4050 are connected to the 3.3V power output from the Arduino for safe logic conversion between them. I am using the Freetronics Eleven as my ‘duino of choice for this test. The main advantage of this is that is has a beefed up 3.3V power rail, allowing for 200mA of current draw. This means that the logic IC and the module can run off it without a problem and also means I don’t have to design a separate power supply to run them. Oh, by the way, all of my designs are done in KiCAD, since I am an open source kind of guy. In the actual flight, I will most probably swap to either an Arduino Fio, or just load the arduino bootloader onto a 3.3V Atmel chip with a custom board. This way I will be able to reduce the weight of the payload.

Test transmitter circuit set up on a breadboard with a quarter wave whip antenna.

For the receiver, I am using this Freetronics 433MHz receiver shield I got from work on a Arduino Duemilanove.

433MHz receiver shield on an Arduino Duemilanove with a half wave whip antenna

Next up I will talking about all of the “fun” adventures I have been having with testing different antenna combinations.

Glue Tests

5 08 2012

I have been doing a bit of experimenting with what type of glue to use for the envelopes. I purchased a glue that is suppose to be flexible once set and also can handle extreme temperatures. This sounded really good at the time so I picked some up to give a try. This is a picture of what I bought.

Don’t use this glue. Doesn’t set properly

I fear that I jumped the gun a little with this one. It did not occur to me at the time to think about how this stuff sets. So after the 12 hours that the packaging says to wait for before the glue sets on non-porous materials, the glue still wasn’t dry. I got carried away with life for the next few days and when I came back, most of it still wasn’t dry and the parts that were dry just pealed away.

I did a little bit more research at this point and found that the water in the air is actually the substance that causes super glues to set. Since space blankets are water proof, there was no way that the moisture in the atmosphere was ever going to get inside properly to create a strong join. I also found a reference, lost that one sorry, that said that PE needs to be surface treated before it will take a proper bond when gluing it together. For the next experiment I am going to test some plastic glue I have laying around that comes with a marker that you prime the surface with before applying it. If this fails to work then I am going to try and find a two part epoxy kind of glue that is flexible enough for a balloon envelope but also does not rely on molecules in the atmosphere to set.

For now though, I will go through the process I used to test the glue;

The first thing to do is find out which side of the blanket is the aluminium and which side is the PE. To do this, it is a simple matter of using a sharp knife to scratch a either side of the blanket. The side that the metal scrapes off of is obviously the aluminium side. I then labelled each side for future reference.

Find the metallised side by scratching at each side with a knife, shown here in red. Once found, mark each side with a permanent marker for future reference.

Next, I cut off  two small pieces and labelled both sides of each piece so that I knew which side was the metallised side. It is important to glue the two PE sides together and not the metallised sides, or one of each. Otherwise, the join will just peal away due to the weak bond between the PE and the metallised layers.

I then ran a small bead of glue down one side of a PE side of a piece of the blanket and laid the PE side of the other piece on top of that.

Make sure to glue the two PE sides together and not the metallised sides, or even one of each.

After the two pieces were glued together, I sandwiched them between a bunch of books to promote a strong join. After the 12 hour period, when I found out that it hadn’t set, I removed the books and let it sit out for another few days.

Test piece was sandwiched underneath a bunch of book for the initial 12 hour setting time. But, after it didn’t set, the books were removed and it was left for a few days.

Balloon Envelope Research

5 08 2012

I have been looking into the construction of Mylar envelopes and have found out a fair few things. The first is that some people refer to Mylar as the stuff that metallic party balloons are made out of. This stuff is actually a layer of polyester coated with nylon on one side and metallised on the other. This allows them to be heat sealed due to the nylon coating but is not the same as proper Mylar, which is BoPET (Biaxially-oriented polyethylene terephthalate). I did, however, come across this forum thread about people making hobby blimps out of Mylar. This got me thinking about ways that I could do a similar thing. I am a bit sceptical as to whether or not the glue they are using will work at high altitude though. Also, I couldn’t find the glue that they refer to at any of my local hardware stores. As a result, I have been doing some research into suitable alternatives that could be found locally. More on this a little later.

Since there is a little bit of confusion when referring to the Mylar trade name, I am not really wanting to go and buy a roll of “Mylar” from ebay or the like just to find out that it is the wrong stuff. I then went on a bit of a hunt to find a suitable alternative that is locally available and cheap. Through further study online, I found out that space blankets, a.k.a. emergency blankets, are actually metallised PE. These are both locally available and cheap, meaning they hit both of my requirements. There are two, still to be answered, problems though. The first is that they may be the proper material, but are they biaxially oriented? This is a requirement because the envelope won’t handle the pressure at high altitude if it is not. The second problem is with the way that they are packaged. Due to them being folded for easy of selling/transporting/etc., the blankets have creases from the folds all over them. I am not sure if this will pose a problem when it comes time to gluing the seals and if they will cause points of weakness that will result in premature eruption of the envelope as the balloon gains altitude.

Time will tell I suppose.

Balloon Progress Catch-up

5 08 2012

I have split the research for the High Altitude Balloon (HAB for short) into two parts. The first is researching balloon envelopes and the second the payload that will be sent up.

I have decided to try and make my own balloon envelopes out of mylar instead of using latex weather balloons. This is due to the fact that they will have to be made this way as the payloads are scaled up. Trying to find a 50 – 100 metre latex balloon would be quite costly, if not impossible. With this in mind, I thought the first flight would be used to see how effective a home-made Mylar envelope is and whether or not it would handle the pressures and temperatures at near space altitudes.

The payload for the first flight will be a basic GPS transmitter that will sent back its coordinates on the 433MHz band. It will also monitor its own altitude and be responsible for destroying the envelope and/or deploying the parachute in the event of the balloon reaching a stable altitude and not bursting. This should prevent the balloon being carried away out of recovery range. I am undecided as to whether or not I will send up a camera on the first launch or save that for a later date.

More details to come.

High Altitude Balloon – The Concept

1 08 2012

The idea behind the whole high altitude balloon launch platform is to see if it is viable to launch small payloads into orbit by first getting them past the majority of the atmosphere. To accomplish this, I have broken the project into a number of steps.
The first is to actually make a balloon, and then launch it, to develop the skills and equipment to make a successful rocket launch possible.
The next is to test a small rocket, maybe a rocket that takes C size engines, so that I can obtain a realistic comparison between identical rockets fired from the ground and then from high altitude.
This will then give me an idea as to whether or not atmospheric friction is a significant contributor to the fuel required to launch a rocket into orbit. If at this point I find that it is the cause of a lot of fuel usage, then I will scale everything up progressively and. hopefully, end up successfully launching a rocket into space.