Is This the Best Portable Multiband HF Antenna?06/12/2022
The End Fed Half Wave (EFHW) Antenna is a highly desirable alternative for several antenna installations, including those for commercial radio transmission and multiband ham radio operation.
The EFHW antennas are a popular choice among radio amateurs due to their ability to allow multiband operation without the need of traps or stubs, while consuming little space and providing a minimally unpleasant aesthetic impression.
Typically, a broad-band matching network is required to convert the high impedance of the EFHW feed point to 50Ω, which is compatible with conventional coax cable.
It is possible to operate the EFHW antenna with a tiny counterpoise, and in certain cases, no counterpoise is employed at all, although this is not always advisable. Alternately, the lower end of the antenna may be grounded to decrease static noise.
Being a single wire and end fed, it is very simple to set up, frequently requiring only a few minutes, making it excellent for both portable ham radio operation and base station use.
Additionally, the EFHW antenna is appropriate for most commercial radio communication applications.
The EFHW Antenna Basics
Everyone is acquainted with a center-fed half-wave dipole antenna. In free space, this presents a 72Ω impedance to the feeder, making it a pretty excellent match to 50Ω coax cable, particularly when the closeness of the ground and other objects tends to diminish the feed impedance.
When the feeder is linked to the end of a half-wave antenna, the circumstance changes drastically. Here, the feed impedance is far greater than typical, and a matching device is necessary to achieve a good match to the commonly used 50Ω coax; otherwise, there will be a discontinuity and power will be reflected.
Due to the fact that the antenna feed point occurs at a high voltage point on the radiating element, these antennas are also known as voltage fed antennas.
Typically, a transformer is utilised at this stage to provide the required impedance change for the match. Typically, the matching transformer changes the impedance from around 4000Ω or 5000Ω at the EFHW antenna feed point to 50Ω at the coaxial feeder.
Additionally, the EFHW antenna is capable of multiband operation. It is usable on all odd and even harmonics of the fundamental frequency, exhibiting the same high impedance at all frequencies.
The EFHW Antenna for Amateur Radio Bands
The End Fed Half Wave antenna functions at both odd and even multiples of a half wavelength, which is one of its benefits. There is a voltage point at all odd and even half wavelengths. This is not the case for a half-wave dipole, which is supplied at a maximum current only in the dipole’s centre for odd multiples of a half wavelength.
|Harmonic of Fundamental||Frequency||Comments|
|1||03.55 MHz||Fundamental operation at bottom end of 80 metres band|
|2||07.10 MHz||Between CW and phone sections of 40 metres band|
|3||10.65 MHz||Would need an antenna tuner to operate between 10.1 and 10.15 MHz|
|5||17.75 MHz||Would need an antenna tuner to operate between 18.068 and 18.168 MHz|
|6||21.30 MHz||Within the SSB portion of 15 metres band|
|7||24.85 MHz||Not far from 24.89 and 24.990 MHz of the 12 metres band|
|8||28.40 MHz||Within the SSB section of 10 metres band|
Length Calculation of the EFHW Antenna
On the lowest frequency range, the EFHW antenna is half a wavelength long, as its name suggests. However, the physical length differs somewhat from that of a free-space half wavelength. This is the outcome of a number of issues related to the fact that the current flows through a wire of finite length. The word ‘end effect’ refers to a crucial component.
For the purpose of calculating the length of the half-wave antenna, a variety of numbers are utilised. The parameters employed in the following calculations account for the antenna wire end effect and the wire’s thickness, assuming a standard wire thickness.
The EFHW antenna length in metres equal to 141/F or in feet equal to 463/F, where F = MHz.
Selecting the optimal fundamental frequency that is compatible with higher frequency bands requires some math. This will depend on which bands are most important, as well as which portion of the bands will be utilised. This decision is a matter of individual preference. This is likely to be a compromise, as the optimal length for one band may not fit so readily with another. However, the differences can be resolved by utilising an antenna tuner on the transceiver to lower any excessive SWR levels. If coaxial cable with a low loss at the operating frequency is utilised, then the losses caused by an SWR that is greater than anticipated will be minimal.
After determining the fundamental operating centre frequency, the length may be computed.
As previously stated, lengths vary slightly depending on the precise structure of the antenna. Cutting the length slightly longer than necessary and then trimming to get optimal performance is always the correct method for antenna installation.
Radiation Pattern of the EFHW Antenna
One thing to keep in mind about the EFHW antenna is that its radiation pattern will vary depending on the frequency band it runs on and, consequently, the number of half wavelengths it employs.
As a half-wave radiator, its performance is identical to that of a half-wave dipole with the typical figure-of-eight radiation pattern, with maximum radiation occurring at ninety degrees to the axis of the wire.
As the length of the wire expands, the radiation pattern changes, with more lobes emerging and the highest points of radiation shifting from being at right angles to the axis of the wire to being aligned with it.
EFHW Transformers and Ununs
To link the end-fed half-wave antenna to the coaxial feeder, a matching network or transmission line transformer is required. The impedance must be matched, as the coaxial feeder impedance is likely to be 50Ω and the antenna impedance is perhaps about 4000Ω or 5000Ω.
Typically, a transformer is utilised to achieve this match. As it matches an unbalanced load to an unbalanced load, these transformers are commonly known as ununs as opposed to the more common balun, which matches a balanced to an unbalanced line.
The transformation of impedance is roughly 50:5000 or 1:100. As the impedance at this point is not well-defined, many individuals utilise a 1:9 transformer, even though it only corresponds to a 450Ω impedance. Utilizing a 1:49 or 1:64 ratio is significantly superior.
There are several electrical performance criteria for these transformers. As with other transformers, they require enough primary inductance at low frequencies.
The EFHW antennas have gained popularity as a low-visibility antenna that is simple to install for both home and portable use. It is quick and inexpensive to construct. Additionally, other choices are available for purchase. With some care, the EFHW antenna is a good choice for many HAM radio stations and other types of radio communication.
In this video, Matthew Miller, MØDQW demonstrates the practical application of a very basic, inexpensive EFHW antenna.