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.

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