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I need to buy a 6-7m long SMA to rp-SMA cable to connect a Wifi radio unit (2.4 GHz and U.FL connector) and an external antenna (2.4 GHz and SMA connector). The longest ones on Farnell and Digikey are 300 mm long, but Google shows some suppliers. As I have never dealt with high speed cables and connectors, I would very appreciate if someone who knows these could advise me how I should select a cable that would cause less distortion on the signal: if I should pay attention to any parameter on the datasheet or should I avoid a cable that is made of a specific material, or what else?

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All WiFi systems I've seen use a 50 ohm impedance and apart from that you want the cable with the lowest attenuation at 2.4GHz. Wikipedia has a fairly nice table to get you started: Coaxial cable standards. The table lists the attenuation at 750MHz but the comments area has attenuation at 2.4GHz for many types of cable. Some entries aren't complete but it'll give you the most common types so you can find datasheets where you can check the atennuation / loss at 2.4GHz.

The other thing to look for is the size and cost of the cable. For example RG-58/U has an outer diameter of 5mm, suitable SMA connectors and the cable are widely available at reasonable cost but attenuation is shown as 1.056 dB/m @ 2.4 GHz. The 6-7dB loss for your length of cable is quite high and will probably halve the range.

LMR-1700 coax has a loss of 0.056 dB/meter @ 2.4 GHz however its outer diameter is a rather large 42mm making it hard to terminate properly onto SMA connectors. Maybe something like LMR-195 would be a good compromise.

Another thing you could do is use a larger / lower-loss cable with a pair on N connectors at each end and a pair of N to SMA adapters, I've seen many commercial installations that use that technique. If you go that way be sure to buy good quality adapters that specify the insertion loss at 2.4GHz.

The other and probably the best technique if it's possible in your case would be to extend the data and power connections to the access point so it can be placed nearby the antenna. You may find that an IP67 enclosure so you can put it all outside is actualy cheaper than the higher grades of cable and connectors.

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