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:

Materials:

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.

Connector:

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.

Isolation-removal:

Now cut of the isolation and remove the metall shield.

Lambda/4 cut:

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

Shrink-tubing:

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.

Closing words:

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!