RF Antenna Cable Loss and Amplifier Budget

Coax loss is quoted per hundred feet at a stated frequency and rises with frequency, so a run that is fine for a 200 MHz intercom is lossy for a 600 MHz microphone. Every decibel lost between the antenna and the receiver comes straight off the system's range and cannot be recovered downstream, because an amplifier at the receiver end amplifies the noise the cable added along with the signal. The target is UNITY gain, not maximum gain: an inline amplifier is there to replace the cable's loss, not to exceed it, and a net meaningfully above zero pushes the receiver front end toward overload and intermodulation, whose symptom looks exactly like a weak signal. Aim for a net between about -3 and +3 dB, and put the amplifier at the antenna end where it amplifies signal before the cable degrades it. A 150 ft run on RG-8X-class coax at 600 MHz, about 8.8 dB per 100 ft, loses 13.2 dB, so a 12 dB inline amplifier lands at -1.2 dB net and the system will work. Change one thing -- the same 150 ft on LMR-400-class coax at about 3.9 dB per 100 ft -- and the loss is 5.85 dB, close enough to unity that no amplifier is needed at all and there is one less active device in the path. Better cable is almost always the better answer. A budget estimate; the cable manufacturer's published loss at the operating frequency and a measured RF level govern.

cable_loss_db = length_ft / 100 x loss_per_100ft_db; connector_loss_db = connectors x loss_per_connector_db; total_loss_db = cable_loss_db + connector_loss_db + splitter_loss_db; net_gain_db = amplifier_gain_db - total_loss_db.

RF antenna feedline budget from the cable manufacturer's published loss per 100 ft at the operating frequency, plus connector and splitter insertion loss, against inline amplifier gain, by name; the unity-gain target (a net between about -3 and +3 dB) is the wireless-microphone manufacturers' published antenna-distribution practice. A measured RF level governs.

The budget is addition and subtraction in decibels. The per-100-ft loss figure is the cable manufacturer's published value for the specific coax at the operating frequency, entered by the user rather than bundled.

Estimate. AHJ and licensed professional govern.

Field names used by the API: length_ft, loss_per_100ft_db, connectors, loss_per_connector_db, splitter_loss_db, amplifier_gain_db, cable_loss_db, total_loss_db, net_gain_db

• Unity target aim for a net between about -3 and +3 dB; more gain than loss overloads the receiver front endwireless-microphone antenna-distribution practice
• Amplifier placement at the antenna end, so it amplifies signal before the cable degrades itRF distribution practice
• Loss figure the cable manufacturer's published loss per 100 ft at the OPERATING frequency; loss rises with frequencycoax manufacturer data