Wednesday, September 05, 2012

Amplifier/Antenna Impedance: 50ohms vs 75ohms

the SignalBoost DB Pro is one of the few 75ohm amplifiers offered by Wilson Electronics

If you've looked at the technical specifications for a cellular amplifier or antenna, you may have noticed a listing for "impedance," with a rating of either 50ohms or 75ohms. Or if you own an antenna and you've called a store like 3Gstore looking for an amplifier to use with it (or vice versa), you may have been told that you couldn't use certain components with one another because one item was 50ohms while the other was 75ohms. So what is impedance, and why does it matter whether your equipment is 50ohms or 75ohms?

Electrical impedance is a measure of the opposition of electrical current. If you're interested in the engineering science behind impedance there are plenty of resources online (like this Wikipedia article, to start), but with regard to cellular signal boosting equipment, there are just a couple key things you should know about impedance:
  1. Antennas/amplifiers/cables have impedance of either 50ohms or 75ohms.
  2. Most cellular equipment is 50ohms, including antennas that are used with an adapter cable to connect to an aircard, as well as most amplifiers and repeaters.
  3. RG6 and RG59 cable, the types of cable used in cable and satellite TV installation, are 75ohms, making these cables incompatible with most cellular antennas and amplifiers.
  4. 50ohm cabling (like RG58 and LMR400) has less loss than 75ohm cabling, which is why most cellular equipment is rated at 50ohms (so it can work with the lower-loss 50ohm cabling). 75ohm antennas and amplifiers exist so that users who must use RG6 cable (for example, offices where RG6 cabling is pre-installed in the walls and they cannot install new cable) have a solution.
In most cases, you won't ever have to worry about impedance — regardless of the manufacturer, repeater kits are always bundled with matching/compatible cables, antennas, etc; for antennas and amplifiers, 3Gstore will guide you to compatible equipment (for example, we prompt you to select cabling for all of our antennas, and we only show compatible cabling).

If you are buying equipment "a la carte" — for example, if you purchased an antenna from one website and are looking for an amplifier from a different source or if you are hoping to use cabling you already have along with a new antenna or amplifier — you need to check the impedance of all the equipment you're planning to use. Combining 50ohm and 75ohm equipment is not recommended. It can be done by stringing together a variety of connectors/adapters, but there will be significant signal loss. It is much better to use compatible equipment!