Antennas

Last Updated on April 18, 2026 by John Berry

This site would be incomplete if I didn’t give some basic coverage of antenna principles – at least on characterising antennas so that those data can be use on other pages.

In essence we find two antenna applications in ham radio – antennas that are used in near-free space, and antennas that are used so close to the ground that the antenna and ground are one. The HF, VHF or UHF Yagi is a good example of the first category, while the HF ground plane and the doublet are examples of the second.

Wave launch

Antenna function is complex. It’s Ampere’s law. A current passing in a wire creates a field around that wire. If the current is alternating with frequency f, the field builds and is then destroyed and rebuilt as the current repeatedly reverses. That alternating field launches an electromagnetic wave at that frequency.

The nature of the wire above varies hugely.

Free space antennas

Let’s take the free space style first.

The structure centres on a driven element to which we would connect the coax or other feeder. This might be a dipole, or some other basic structure, often complemented by something to reflect the wave generated and create a focussing in one direction. The reflector might be a parasitic element or a solid structure like a dish.

There are two key characteristics of this antenna type – directivity, and relative gain – both from the phenomenon of focussing. Both are expressed relative to a theoretical isotropic (or point) antenna which radiates equally in all directions. Because an isotropic radiator has no focussing, it has no relative gain.

A third basic characteristic is the polarisation – the antenna orientation expressed relative to the horizon. When horizontally polarised, the drive element lies parallel to the horizon. When vertically polarised, the driven element is orthogonal to the horizon. The electric field lies parallel to the driven element. There are also other more complicated mixed and rotating polarisations.

Directivity

We can express the directivity completely in 3-D. But typically, the directivity is summarised by two simpler responses – one in the H-plane (in the plane of the horizon) and one in the V-plane (orthogonal to the horizon). These characteristics apply under any polarisation. Example H-plane and V-plane responses and the corresponding 3D visualisation are shown below.

Technically, we can read off the response at any elevation and any azimuth relative to the bore sight – the axis of maximum radiation. Some engineers abbreviate the H and V plane responses to HRP and VRP.

Antenna gain

The response at each azimuth/elevation point can be quoted in decibels (dB) relative to an isotropic (point) antenna, or relative to the bore sight response. The response along the bore sight is called the forward gain. Other discrete responses are sometimes given, like the response to the rear called the front-to-back ratio. The angular width (to the -3dB points) or beamwidth in both H-plane and V-plane of the bore sight is also often quoted. This width is a measure of the antenna focussing.

Engineers can optimise antennas using numerical techniques in computer programmes. In the case of a Yagi, optimisation focusses on element size, number and layout. It’s important, for example, that an antenna for EME minimises responses towards ground and environmental sources of noise.

A forward gain of 12dBi is typical of a VHF 6-element Yagi operating over 144-145MHz. The unit dBi signifies gain in decibels relative to an isotropic antenna. The unit dBd is sometimes used to relate the gain to a dipole. A dipole has a gain of 2.15dB over an isotrope, so 12dBi when quoted relative to a dipole would be about 10dBd.

Now let’s look at antennas close to the ground.

Ground coupled antennas

As the antenna comes closer to the ground, we find that the ground (and ground characteristics) influences the antenna characteristics. In the case of a VHF Yagi, the V-plane response tilts upwards when the antenna comes close to the ground. It can only be described as free space (and relatively free of ground effects) when it’s higher than about two wavelengths from the ground.

Another example is a 66-metre-long HF doublet sitting just one eighth wavelength above the ground at its lowest operating frequency.

Let’s look at these two examples.

Monopole responses

Unless the radial base is unevenly distributed, the monopole radiates equally in all horizontal directions. But its V-plane response is totally dependent on the electrical length (the length in fractions of wavelengths of the operating frequency). See below for examples of some of the response patterns generated by typical antenna electrical lengths.

Again, we’d quote the gain of the main lobe, even though that lobe may be at some huge elevation relative to the horizon, and there may be more than one of them.

In these diagrams, we see the effect of real ground. The dotted lines indicate radiation when the antenna is used above perfect ground (very salty and wet). The solid lines indicate responses over real ground (very dry).

Doublet responses

The doublet at two wavelengths in the air behaves almost like a free space antenna. As the height decreases, the ground effects increase. The V-plane response is elevated until, for an antenna near the ground, maximum response is almost straight up in the sky. Some users favour this case. It enables what’s termed near-vertical-incidence skywave, minimising interference to and from stations toward the horizon.

The H-plane response is some variation of the classic dumbbell, with low response off the ends and most radiation broadside to the driven element.

I’ve shown typical V-plane responses for the doublet below.

The doublet discussion above applies for most horizontal wires including the popular half-wave end fed (HWEF).

Gain and launch angle

I’ve deliberately kept things simple here. The emphasis is on two main parameters:

• The antenna launch angle (elevation of the principal V-plane response); and
• The antenna gain (quoted in dBi or dBd).

The launch angle is key in analysing the path geometry, and the gain is key in calculating the likelihood of communications in the path budget. Note that in the above responses (as in the real world) there may be more than one major lobe. Hence there may be more than one ‘bore sight’, and and hence more than one ‘launch angle’.

For more on antennas, probably the best book available is the ARRL’s Antenna Book. The diagrams above were extracted from that book. The citation for it is in the bibliography.