1/2 wave antenna

1/2 Wave Antennas

The 1/2 wave antenna can be either horizontal or vertical and be fed at any point along the length the formula below may be used to determin a 1/2 wave in free space.

1/2 - wavelength ( feet ) = 468

f (MHz)

A word of caution the above formula is a good starting point to find half wavelength at a given frequency it will not will not take into account when building antennas hight above ground, diameter of material used and surrounding objects that effect all antennas in the real world.

1/2 Wave Vertical Antenna

Vertical antennas provide good omnidirectional low angle vertical radiation making them ideal for the upper HF and VHF bands the basic formula above can be used to calculate the length.

End Fed Half Wave ( Zep )

Unfortunately we are unable to directly feed the 1/2 wave end antenna with coaxial cable because this type of antenna is voltage fed and presents a very high impeadance of around 2500 + Ohms at it's extremities.A simple matching device can be used, sometimes referred to as a base tuner to match the high impeadance to our low impeadance 50 Ohm transmission line.

Picture of a 4m End fed vertical half wave matching coil 6 turns are wound on a 30mm former tapping point is 2.5 turns from base.

The base tuner simply consist of a coil of wire tapped at a suitable point along the lenghth of the coil for impedance matching.The diagram shows the basic schismatic layout of the of the base tuner.

The unfortunate downside to this simple arrangement is it can be quite tricky to find the tapping point,once a resonable point has been found the 1/2 wave element may be fine tuned for minimum swr, it is not impossible but if you have access to an antenna analyser it will prevent hair loss !!

The plot above shows what we can expect from a 28mhz 1/2 wave end fed vertical element with the lowest point 10mtr above ground, the plot predicts a peak gain of 3.51 dbi at 8.9 deg.

Centre Fed Half Wave (Hertz)

The centre fed di-pole is the most common form of antenna used for fixed base station applications,it can be either horizontal or vertical in polarization,the di-pole is current fed in the centre with an impedance of around 72 Ohms in free space.

The plot above shows what we can expect from a 28mhz 1/2 wave centre fed element with the lowest point 10mtr above ground, as you can see both antennas provide similar performance and radation pattern,the plot predicts a peak gain of 2.88 dbi at 10 deg.