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By Peter Cochrane, 18 July 2007 15:00

COMMENT

Written at my hotel in Paris, polished and dispatched via a free wi-fi service from a conference in Barcelona a couple of weeks later

For over 20 years I have lived and worked in a 'five-bar' home and office (that's the number of bars I see for signal strength on my mobile phone). With direct line of sight to the nearest mobile phone mast, plus at least one other some distance away, I have enjoyed a really good connection with a strong signal and few dropped calls.

However, I have just moved home and office to a new location that is in the zero to one bar (some of the time) class! The physical situation responsible for this condition is depicted, in the non-scale pic, below.

In reality the geography is not as severe as indicated, and there are at least two mobile masts serving the area. But there is no line of sight at ground level; trees and scrub provide additional attenuation; and we are also some considerable distance from all the towers serving the area.

It turns out that climbing on the roof gives four to five bars but it is a bit windy and unsafe up there. So I needed a solution that would give a reasonable signal location in all rooms including the office. At this point it is important to recognize this is an energy gathering and redirection problem - and something we can easily understand in optical terms. It is therefore an easy step to conceptually see what is necessary in the microwave and UHF region as shown below.

Of course the human race has been overcoming this class of problem for thousands of years, and in the last 100 the use of passive reflectors (as illustrated below) has been a feature of microwave communications in some mountainous countries. Extending the principles to mobile communication is therefore an obvious step, and one I found myself having to engineer.

In my case the basic trick is to find a means of gathering sufficient mobile phone signal energy at roof level and delivering it to ground level rooms. And all at a reasonable cost of course! My recently engineered solution is depicted below.

I elected to use hi-gain TV antennas specified up to 860MHz to collect and redirect the mobile phone energy at around 950MHz. Of course this involves a loss of gain from the optimum realized for TV use but for those enthusiastic enough, it is an easy job to trim down and reposition the elements according to the approximate shortening ratio 850/950 depending on the specific 2.5G channels.

This is a really low cost solution as TV antennas are mass produced and available from most hardware stores whilst 2.5G mobile antennas are not, and are thus more expensive.

Does it actually work? Yep! Without going into the complex propagation, loss and gain, and radiation equations involved I can report that at ground level I get a stable two to three bars everywhere, which is sufficient to provide reasonable quality.

Because the system is passive it is also reciprocal and ensures the signal from mobile phones at ground level can find a direct path to the cellular antenna site. Of course the inclusion of electronic amplifiers would improve the system dramatically but in some countries, especially those with densely packed populations in the EU, such solutions are, unfortunately, illegal.

Over the past decade or so quite a few people suffering the same problem have cornered me at conferences, and having been faced with the problem first hand, I was at last prompted to write it up. But I must point out that the detailed engineering is complex, and at worst people might have to plug the antenna cable directly into their mobile device, or at least, stand real close to the antenna.