Test Traffic Measurements - FAQ

How can we check the cable and antenna?

A simple check is measuring the electrical resistance of antenna and cable with a multi-meter. Put the positive side of your multi-meter on the core of the cable, the negative side on the mantle. You should measure a resistance of approximately 70ohms, a little bit more for longer cables. If you measure 100 ohms or more, check the connectors.

How can we see if the antenna is positioned correctly?

Please refer to:
http://www.ripe.net/test-traffic/General/access.html

After a while (10 minutes to a few hours, you should see several satellites. If you don't then the signals are too weak or noisy.

We have placed the GPS receiver behind a window and no satellites are seen?

The GPS receiver might see a handful of satellites but the signals could be too weak or noisy to determine its position and thus the local time and your coordinates. If you can open the window, then it might be useful to put the antenna outside the window for a couple of hours. This often helps: the receiver gets a good position fix and can then survive with a noisier signal once the antenna is put back inside.

How long can the antenna cable be? Can I extend it?

Depending on the quality of the cable and the signal strength at your site, up to approximately 40m.

In general, the more that the antenna can see of the sky, the stronger the signal and thus the longer the cable can be.

One also loses signal if the antenna is behind a window, or even worse, inside a re-inforced concrete building.

The longer the cable, the more signal you lose. Regular RG-58 coax has a high loss per meter, in 15 or so meters, you can easily lose all signal even with a good spot for the antenna. The thick "Aircell 7" extension cable that was included in the box, has a far lower loss per meter and we have succussfully installed boxes with up to 40m using that cable. Aircell7 is readily available from radio amateur shops and such. There are other cables on the market too, look for low-loss, 50 Ohm impedance, coaxial cable. Do not use 75 Ohm cable, it simply won't work. If you have a problem finding a cable, or aren't sure if the cable will work, then please contact us again and we'll have another extension cable made.

BTW.  The Aircell cable may look very think and robust, but in practice it isn't. Try to avoid sharp bends in the cable.

Can we share the antenna with another application?

No, this is not possible. The Motorola antenna is an active antenna, with signals flowing in both directions on the cable. It is not possible to share this antenna with a second receiver.