Five Eighths Wavelength Vertical Antenna

The five eighths wavelength vertical antenna (5/8 λ) provides gain over the quarter wave vertical for little extra complexity & cost.

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Vertical antennas includes:
Types of vertical antenna Quarter wavelength vertical 5λ/8 vertical J pole

Although the quarter wavelength vertical antenna provides a good level of performance, it is easy obtain some additional gain by extending the radiating element of the vertical antenna to five eighths of a wavelength, 5λ/8.

As a result the five eighths wavelength vertical has gained popularity in many instances and is widely used at VHF and UHF as well as sometimes on the higher HF bands.

The five eights wavelength vertical is used as it retains all the advantages of the quarter wavelength vertical antenna for various two way radio communications applications where it provides an omnidirectional radiation pattern as a result of its vertical element. However it also provides a little additional gain for low angle signals.

For very little additional cost in terms of manufacture, the additional gain is often seen as an ideal solution for many radio communications situations.

Five eighths wavelength antenna concept

The concept for the five eighths wavelength vertical antenna starts with the quarter wavelength antenna.

The quarter wavelength vertical antenna works well when it is loaded against a ground or ground plane.

The five eighths wavelength antenna is often used in mobile situations where it uses the metal of a vehicle as its ground plane.

In these mobile situations a quarter wavelength antenna works well, offering a good level of radiation at an angle close to parallel to the ground, thereby giving very good local coverage, which is typically what is wanted.

Quarter wave ground mounted vertical polar diagram and radiation pattern. — Quarter wave vertical polar radiation pattern

Although the quarter wave vertical puts out much of its radiation at a relatively low angle, there are opportunities to focus more of the energy close to the ground.

It is well known that when a dipole antenna is extended beyond a half wavelength, the radiation pattern changes. Initially more energy is focussed at right angles to the axis of the conductors of the antenna, but then a point is reached where side lobes start to form and the energy at right angles to the conductors starts to fall.

It is found that the optimum point for the maximum radiation at right angles to the conductors occurs at a point where the dipole is about 1.2 times a wavelength.

Rather than using a dipole, if a vertical using a ground plane for the second element of the antenna, then it equates to a length of about 5 / 8 wavelength, λ.

Antenna impedance & matching

For most applications, it is necessary to ensure that the antenna provides a good match to 50Ω coaxial cable. This is very important if power is not to be reflected back to the transmitter and then be absorbed.

If the level of reflected power is too high the protection circuitry in the transmitter may start to act and reduce the transmitter power. This would likely nullify any gains the actual radiating element may provide.

It is found that a 3/4 wavelength vertical element provides a good match, and therefore one solution for providing a good impedance match for the 5/8 wavelength vertical antenna is to make it appear as if it was a 5/8 radiator but have the electrical length of a 3/4 element.

Outline of the 5/8 wavelength vertical antenna concept

In this way it radiates like a 5λ/8 radiator, but has the impedance of a 3λ/4 antenna.

This is achieved by placing a small loading coil at the base of the antenna to increase its electrical length. Additionally a small capacitor is also added to cancel out any remaining reactive elements.

The match for the antenna is typically provided by the antenna itself, because the 5λ/8 vertical is generally used at VHF or UHF where separate antenna tuning units are rarely used.

5λ/8 vertical antenna gain

In an ideal world, it is found that a five eighths wavelength vertical antenna could exhibit a gain of close to 4 dBd, i.e. 4 dB gain over a quarter wave vertical.

To achieve this gain the antenna must be constructed of the right materials so that losses are reduced to the absolute minimum and the overall performance is maintained, otherwise much of the advantage of using the additional length will be lost.

A further reason for the theoretical gain not being realised is that the radiation pattern is somewhat raised when compared to the quarter wave vertical, and therefore not all the power is radiated where it is really needed.

That said, figures for the gain of these slightly extended verticals is often optimistically quoted as 4 dB.

Also be aware that many of the gain figures that are quoted are relative to an isotropic source and not a dipole. The gain relative to an isotropic source will be 2.1dB higher than those quoted relative to a dipole. Gain relative to an isotropic source will be quoted as dBi.

An isotropic source is one that radiates equally in all directions. It has the advantage that it is a a standard against which many other antennas are quoted against. It also ahs the advantage that the gain relative to an isotropic source looks better than if it is quoted against a dipole by 2.1dB.

Mechanical considerations

Five eighths wavelength vertical antennas are often used on automobiles. They provide a useful level of gain, they can be made mechanically robust for travelling at speed and they are very convenient to fit.

Many of these antennas are manufactured with a magnetic mount, 'mag mount', base. This enables them to be very easily placed into the vehicle chassis without the need to drilling holes, etc. which can impair the integrity of the vehicle body and also reduce its resale cost.

However, one of the main constraints for these antennas is the coil in the base. If this is not rigidly mounted and fixed it can bend and vibrate. For some cheaper mobile antennas, the coils can move and this changes the impedance matching to the feeder.

This can cause signal variations and under extreme circumstances it may even cause the SWR detection circuitry to reduce the output power. Sometimes, as this occurs according to the vibration of the vehicle, it can appear as very severe flutter on the signal.

When selecting an antenna for VHF / UHF mobile operation it pays to select a good antenna type, and preferably a type that has been recommended by a colleague, friend, etc.

The five eighths wavelength vertical antenna is mainly used at VHF or even UHF where its length means that it can easily be accommodated without the need for its size becoming too large. Here it can be easily accommodated on vehicles for applications like point to point two way radio communications including professional radio communications and amateur radio. It provides a useful amount of gain for an increase in size that can often be accommodated with no issues.

This antenna format can also be sued for HF as well. The same concepts apply, but the antenna would tend to be used for base station use rather than for mobile radio communications. One of the advantages of the 5/8λ antenna for HF is that the low angle of radiation can be especially useful for long distance ionospheric communications where low angles of radiation are needed to attain the large distance.

Overall, the 5/8λ antenna is a very useful form of vertical antenna to consider for many radio communications applications.