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VK3KHB

Voltage Standing Wave Ratio
to Loss


.. .. . .

VSWR is a measurement of energy sent along a transmission line and reflected back to the transmitter and expressed as a ratio. A ratio of 1:1 means all power injected into the transmission line (coax / flat ribbon / wet string) is either absorbed as heat or radiated out through the antenna and / or transmission line. Values greater than 1:1 mean some loss or mismatch is occurring and the energy is reflected back to the transmitter. Absorbed by the transmitting electronics (Power Amplifier / RF final stage) as heat or reflected back and forth along the transmission line in a decaying wave.

All energy injected should be radiated by the antenna, however transmission lines have an inherent loss as do the connections and the radiator material. Any losses can be detected by VSWR measurement (at the load / antenna) which can be shown in dB on transmission lines with VSWR greater than 1:

The below figures are based on a perfectly matched transmission line with the coax loss figure measured in dB per 30m (100ft).


VSWR
Coax loss range
dB / 30m
Additional loss
to signal dB

notes
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 heat or radiated energy in the cable or as reflected energy back to the Power Amplifier (final stage) and absorbed as heat by the electronics.

A VSWR meter is usually placed at the junction of transmitter and transmission line, where a measurement of the energy is taken.

A typical meter setting is: CAL (for calibration) and you adjusted a knob for full scale deflection of the meter indicator. which is either a needle or led bar graph. At that 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

73
Gary, VK3KHB


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