Multiband Half wave end fed antenna
The end fed half wave antenna is an attractive option for many radio amateurs as it is able to provide multiband operation without the use of traps or stubs whilst occupying a minimum amount of space. A broad-band matching network is typically used to transform the high impedance of the EFHW feed point to 50Ω suitable for standard coax cable. The end fed half wave antenna can be operated with a small counterpoise, and in some instances, no counterpoise is used at all, although this is not necessarily wise. Being a single wire, and end fed it is very easy to set up, often taking only minutes to do and this makes it ideal for ham radio portable operation, as well as for base station usage.
We are all familiar with a centre fed half wave dipole antenna. In free space this presents and impedance of 72Ω to the feeder and this means that it provides a relatively good match to 50Ω coax cable, especially when the proximity pf the ground and other objects will tend to reduce the feed impedance. When the feeder is connected to the end of a half-wave antenna, the situation is quite different. Here the feed impedance is very much higher, and to enable a good match to be made to 50Ω coax which is normally used a matching device is required, otherwise there will be a discontinuity and power will be reflected.
The end fed half wave antenna is also able to provide multi-band operation. It can be used on all odd and even harmonics of the fundamental frequency, presenting the same high impedance at these frequencies. The end fed half wave antenna is also able to provide multi-band operation. It can be used on all odd and even harmonics of the fundamental frequency, presenting the same high impedance at these frequencies.
EFHW Length
As the name indicates the EFHW antenna is half a wavelength long on the lowest band of operation.However the physical length is slightly different to that of a half wavelength in free space. This results from a variety of factors associated with the fact that the current is flowing in a wire of finite length. One key factor is what is termed the end effect.
EFHW transformers & Ununs
In order to match the end fed half wave antenna to the coaxial feeder, it is necessary to have a matching network or transmission line transformer. The coaxial feeder is likely to be 50Ω and the antenna impedance is possibly around 4000 or 5000Ω, the impedance needs to be matched. Often a transformer is used to perform this match. As it matches an unbalanced load to an unbalanced load, these transformers are often referred to as “ununs” as opposed to the more familiar balun that matches a balanced to unbalanced line.
The impedance transformation is approximately 50:5000 or 1:100. As the impedance at this point is not well defined, many people use a 1:9 transformer, although this only matches to an impedance of 450Ω. It is far preferable to use a 1:49 or 1:64 ratio. There are several requirements for these transformers in terms of electrical performance. At low frequencies, like all transformers, they require adequate primary inductance at lowest frequency.