GPRS antenna?

The first step was the figure out what the extent of the problem was. I located my nearest T-Mobile base station using the government??™s Sitefinder service. This also confirmed the frequency that the transmitter used - 2100Mhz. This is the standard frequency for UMTS (i.e. 3G) services in Europe.

By drawing a line between the transmitter??™s location and my building in Google Earth, I was able to confirm the approximate distance and angle of the signal I needed to catch.

Buying a directional antenna wasn??™t really an option - for a start, they are expensive - and anyway I couldn??™t be sure that such an antenna would actually help. If it didn??™t, I??™d have wasted ??60-??100.

However, in an incredibly geeky flash of inspiration, I realised that there really isn??™t much difference in operating frequency between WiFi (around 2.4Ghz) and UMTS (2.1Ghz). And there are loads of different clandestine WiFi antenna ideas floating around the Internet. If I could find an easy-to-build directional WiFi antenna, perhaps I could reverse-engineer its dimensions and adapt it for 2100Mhz use.

So I set about the task. I decided on the biquad antenna type, as it??™s fairly compact and easy to build, yet provides decent (10-14dB) gain and is quite directional. My primary sources of information were the many WiFi biquad and double bi-quad antenna tutorials and blog entries, such as: Engadget??™s; Trevor Marshall??™s tutorials. More can be found on my del.icio.us page for the tag ???antenna??™.

Both WiFi and UMTS operate in microwave frequencies - however, there??™s a substantial difference between the middle WiFi channel (around 2.4Ghz - what people usually tune their WiFi antennas to in order to give a good amplification factor across the channel range) and UMTS??™ 2.1Ghz. To my knowledge no-one has built a homebrew biquad UMTS antenna before, so there wasn??™t much to go on. What also didn??™t help was that most WiFi biquad tutorials just give you the measurements verbatim - not the calculations of formulae.

Having done no physics since school, my expertise in antenna building is poor to say the least. Still I did realise a few things about most of the designs floating around the Web: all of the dimensions were multiples of the wavelength at 2.44Ghz (122mm or 0.122m). So then, I just needed to figure out the multiplication factors in each case and I was sorted.

My list is as follows: (?» = wavelength)
Emitter wire total length: 2?»
Emitter ??™square??™ side length: 0.25?»
Emitter offset from reflector: 0.125?»
Reflector width/height: 1?»
Reflector ???lips??™ height: 0.25?»

So, at 2.1Ghz (2,100,000,000Hz - ?» = 142.8mm),these dimensions are:
Emitter wire total length: 285.6mm
Emitter ??™square??™ side length: 35.7mm
Emitter offset from reflector: 17.85mm
Reflector width/height: 142.8mm
Reflector ???lips??™ height: 35.7mm

I made the reflector out of galvanised steel mesh and mounted an N-type connector to the centre. I made an N-type coaxial to FME coaxial cable to hook up the phone to the antenna. The emitter itself is made from the copper centre conductor taken from a length of high-quality satellite TV coaxial cable that I had left over. I used some scrap wires to connect the ends of the biquad ???bow-tie??™ back to the reflector, and placed some of the original dielectric insulation from the satellite cable back on the ends of the bow-tie??™ shape to prevent the antenna from ??™shorting??™ (in an RF sense). The emitter is then soldered into the N-type connector in a most slapdash style.

I didn??™t have enough mesh to make the ???lips??™ of the antenna??™s reflector match the measurements I??™d planned, but made them the longest equal lengths that I could. In other respects, I managed to get the dimensions down to within a couple of millimeters of my target measurements.

High-quality cable is a must - I only used 50cm or so of RG-58 type cable to go between the phone and antenna, and unless you??™re using something very high-grade (like LMR-400), I wouldn??™t go too much further than that.

Presently the antenna is fixed and aimed in a pretty shoddy way - it??™s fixed onto a set of ???3rd hands??™ - and there??™s a Post-It note there to provide (some) insulation between the stand and the reflector??¦ I plan to investigate more permanent mounting options at some other date.

The biggest problem with the antenna is aiming it - but having said that once it does catch a signal, the phone holds on to it very well. I??™m aiming it over the rootfops of the building, hoping to catch some of the signal??™s diffraction.

I have no idea how much gain the antenna produces. When aimed correctly (which is very tricky), it gives me a consistent 1-bar UMTS connection, or a 2 bar GSM signal. It works better at night, holding on to a signal for many hours.





... or...

this could be another alternate...but I dont think it would be work yet :( but for sure for 2.4Ghz will work perfectly,dont know for the GSM GPRS nor EDGE even the 3G ...???





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or the most easy way was doing like this...perhaps help a bit ...