T4B: Electrical wiring, dangerous voltages and currents; SWR meaning and measurements

The heart can be fatally affected by a very small amount of electrical current.

The minimum voltage that is usually dangerous to humans is 30 volts.

As little as 1/10 of an ampere of electrical current flowing through the human body could be fatal.

The green wire in a three-wire AC line cord should be connected to the chassis in a power supply.


An SWR meter is an instrument that is used to measure the relative impedance match between an antenna and its feed line.

An SWR reading of 1:1 means the best impedance match has been attained.

An SWR reading of less than 1.5:1 is a fairly good impedance match.

A very high SWR reading means the antenna is the wrong length, or there may be an open or shorted connection somewhere in the feed line.

An SWR readings for your 1/2-wavelength dipole antenna at the low frequency end of an amateur band is 2.5:1, increasing to 5:1 at the high frequency end of the same band would tell you that the antenna is too long for operation on the band.

An SWR readings for your 1/2-wavelength dipole antenna at the low frequency end of an amateur band is 5:1, decreasing to 2.5:1 at the high frequency end of the same band would tell you that the antenna is too short for operation on the band.


If you use an SWR meter designed to operate on 3-30 MHz for VHF measurements, it may still be accurate if it is properly calibrated to full scale in the set position.