- The Cable Antenna:
The Cable Antenna:¶
The antenna that has been shipped with the BS11 is a so called sector-antenna. That means it is designed not to cover only a part of the area around the BTS. And it has a gain. The gain comes from the signal that is bundeled and sent into the sector. The advantage: You can use several BTSs to cover a cell (Each cell sector has its own BTS ==> More calls) and you can cover more distance with less power.
What is good if you want to run a mobile network is bad for experimental proposes. For us an antenna that covers the area around the BTS with a nondirected signal (like a donut) is perfect - so lets build one.
Note: I have Adaptors that adapt the BS11 N-Connectors to BNC. Because BNC is my favourite connector type so the most of my homebrew radio equipment / cables have BNC-Connectors. That is the reason why this howto relates to BNC-Connectors. Just use your favorite connector type or N-Connectors for the BS11
Step by step¶
The antenna is made out of a pice of old coax-cable (about 15-20cm length) and a BNC connector. To make it all looking good we add a pice of shrink tubing at the end.
The first task to do is to connect the connector to the cable. I used a solderable BNC-Connector. If you have an old network cable you can cut off the and. The result is the same.
Now cut of the isolation and remove the metall shield.
Now the most important step has to be done. We need to part where the shield was removed to lambda/4 length. To calculate the length you can use the matlab/octave script listed below. It is very important that you know the scale factor for the cable that you use. I used H155 cable which has a scale factor of 0.79.
%Parameters: c = 299792458%m/s (Speed of light in vacuum) v = 0.79 %Cable scale factor (The wave moves slower in the cable/metall) f = 900000000 %Hz (GSM900) %Calculation disp('Wavelength [m] is:'); l=(c*v)/f disp('Antenna length [m] (Lambda/4) is:'); l4=l/4
In my case lambda/4 is about 6.57cm
Now simply add some shrinktubing to the cable. That makes it save to handle and industry grade looking. I recommend to do a final test with a multimeter to ensure that your antenna has no short circuits.
I have tested the antenna with 2 BTS as RX-Antenna and TX-Antenna. It worked fine and we could not see any harmful effects to the BTS so far. But i must warn. Use it on your own risk!