All energy injected should be radiated by the antenna, however transmission lines have an inheirant loss as do the connections and the radiator material. Any losses can be detected by VSWR measurment (at the load / antenna) which can be shown in dB on transmission lines with VSWR greater than 1:
The below figures are based on the transmission line being perfectly matched, with the coax loss figure measured in dB per 30m (100ft).
| VSWR | Coax loss range dB / 30m |
Additional loss to signal dB |
notes on equipment |
| 1.5:1 | 2.0 - 10 | 0.1 - 0.2 | typical |
| 2.0:1 | 0.4 - 10 | 0.1 - 0.5 | |
| 3.0:1 | 0.2 - 10 | 0.1 - 1.2 | max safe level |
| 4.0:1 | 0.2 - 10 | 0.2 - 2.0 | |
| 5.0:1 | 0.2 - 10 | 0.3 - 2.5 | Too High |
| 7.0:1 | 0.2 - 10 | 0.5 - 4.0 | |
| 10:1 | 0.2 - 10 | 0.8 - 5.0 | Danger! |
| 15:1 | 0.2 - 10 | 1.0 - 6.0 | |
| 20:1 | 0.2 - 10 | 1.5 - 7.0 | Damage? |
Note: For every 3dB, you've lost half of your power as either or both heat in cable or reflected energy back to the Power Amplifier (final stage) and then absorbed as heat by the electronics.
A VSWR meter is usaully placed at the junction of transmitter and transmission line, where a measurement of energy sent up to the antenna is taken. A typical setting is: CAL and you adjusted a knob for full scale deflection of the meter indicator, which is either a needle or bar graph. At the same junction point a measurement of reflected power (REF) is also taken. Forward power (FWD) and reflected power (REF) are usually shown in watts, reflected power should be much less than forward.
By placing the VSWR meter at the transmitter / transmission line junction and recording the readings, then placing the VSWR meter at the transmission line / antenna junction, the energy loss of transmission line can be directly measured and compared to expected loss figures. Also the amount of energy presented to the antenna is now known and other calculations for signal strength by antenna gain can be done.
Typical VSWR meter