Antenna Gain and Directivity.

[lord loh.]

What is the difference between the two antenna parameters - Gain and Directivity

Which is practical (not considering the ideal case of isotropic radiators)?

 

[Nigel Goodwin]

What is the difference between the two antenna parameters - Gain and Directivity

Which is practical (not considering the ideal case of isotropic radiators)?

Gain gets higher as the directivity gets higher - there's nothing 'magical' about gain in an aerial, you're basically focusing the aerial more accurately on where you want.

The standard to compare against is a 1/4 wave dipole - this (when vertically polarised) receives in a circular pattern so has no directivity at all, and has the lowest gain.

 

[JimB]

Gain and directivity are linked to each other.

Starting with the isotropic radiator (sorry!), that has a uniform spherical radiation pattern, lets say that it has a gain of 1 (0dB).
The basic practical antenna is the half wave dipole. Its radiation pattern is "doughnut" shaped. In directions at right angles to the dipole it some gain (wrt the isotropic), about 2.15dB if memory serves correctly. In the direction along the line of the dipole, there is a very deep null in the radiation pattern, the gain is very low, in practical terms about -20dB.
So, a dipole has a broad "beamwidth" at a wild guess about 120 degrees between the -3dB points. Its "Directivity" is poor.

Now, consider a 10 element Yagi type antenna, (here I am looking at data in a book).
It has a gain of 13dB (reference not stated but I am guessing a dipole), the beamwidth of the major lobe is 40 degrees between the -3dB points. Its directivity is good.

In practice, antenna performance is specified by the forward gain, front to back ratio and beamwidth. There are other parameters, but lets keep it simple for now.

To get an idea of some practical antennas, look here:


JimB

 

[JimB]

The standard to compare against is a 1/4 wave dipole.

Half wave Nigel, each "leg" of the dipole is 1/4 wave, the overall length is half wave.

JimB

 

[Nigel Goodwin]

The standard to compare against is a 1/4 wave dipole.

Half wave Nigel, each "leg" of the dipole is 1/4 wave, the overall length is half wave.

Sorry, I was thinking of a 1/4 wave whip, where the other 1/4 (to make the dipole) is reflected in the ground plane!.

 

[stevez]

Note that proximity to the earth or other conductive objects can profoundly affect antenna characteristics. A fair assumption for published data on antenna gain is that it is high enough above the ground and far enough away from objects. For VHF and higher the height above ground (in terms of wavelengths) is usually easy to overcome. At lower frequencies (longer wavelengths) the height above ground can affect the antenna performance significantly. A quarter wave whip might heavily depend on being close to a metal surface (it's ground plane). All of this is outside your question but is helpful to know. A small directional antenna at UHF might or might not depend on proximity to ground.

 

[RadioRon]

Directivity is the propensity of the antenna to focus its radiated power in some directions at the expense of others. So, Directivity is the ratio (usually in dB) between the power radiated in a particular direction divided by the total radiated power. Gain, on the other hand, takes into account the power losses in the antenna itself and in the impedance mismatch at the antenna power feed point.

So, if you are working in dB:

Gain=Directivity + Antenna Internal Power Losses + Feedpoint impedance mismatch losses.

As you can see by this, Gain and Directivity, when plotted in a 2D or 3D pattern have the same shape, but Gain is always smaller because you always have some losses.

In practice, Gain is a much more practical and measurable term and is more often used. An antenna designer will be interested in both, but probably more the Gain.

 

[mstechca]

While we are on antennas, Isn't it true that if the length of the antenna is equal to the wavelength of the frequency to be received, maximum gain (or sensitivity) is achieved? or is wavelength defined for some other reason?

For example, If an antenna is 2 meters as opposed to 1 meter, and the frequency of interest is 144Mhz, is maximum gain (or sensitivity) achieved?

 

[Nigel Goodwin]

While we are on antennas, Isn't it true that if the length of the antenna is equal to the wavelength of the frequency to be received, maximum gain (or sensitivity) is achieved? or is wavelength defined for some other reason?

For example, If an antenna is 2 meters as opposed to 1 meter, and the frequency of interest is 144Mhz, is maximum gain (or sensitivity) achieved?

No, not really.

The aerial has to be tuned to give the correct impedance to match the transmitter/receiver - full wave aerial are fairly useless for this.

A 1/2 wave dipole (or a 1/4 wave whip) has a convenient impedance around 50 or 75 ohms, which by a happy 'coincidence' are the two values commonly used for such purposes. A folded dipole gives around 300 ohms, and is usually fed via balanced 300 ohm feeder or a balun (balanced/unbalanced transformer), which matches the balanced 300 ohm to the unbalanced 75 ohm.

 

[RadioRon]

Agreeing with Nigel, a maximum gain is not achieved when the antenna is one wavelength long. In fact, gain will continue to increase as length increases, but in a complicated (ie. not convenient and often not useful) way. The pattern breaks up into a series of lobes which are too difficult to describe here, but I'll try anyway. As the length of your wire increases beyond one wavelength, the radiation pattern changes from squished ball to a sort of a peanut shape. And as you go longer still, the peanut turns into a couple of lumpy cones joined at their pointy ends. These cones become narrower and narrower as your antenna increases in length. But even these descriptions depend on the location of your feedpoint with the simplest being in the center of the antenna. But as Nigel points out, this is the worst place to feed an antenna that is exactly an integer multiple of one wavelength since the impedance is so very very high that arranging an impedance matching network to affect maximum power transfer from feedline to antenna becomes unworkable.

The principle that the pattern gets narrower and has higher gain in very specific directions is behind various long wire antennas including the Rhombic which is usually many wavelengths long, but is arranged so that the cones of maximum gain of two longwire sections combine constructively along the axis of the rhombic to from a very high gain antenna indeed but one with a very narrow beamwidth.

This concept of the directivity breaking into cone shapes is hard to explain here, so it may be time to crack open a text book or try it out on a simulator if you want to see a picture of the patterns. Such beamforming using longer antennas is often very inconvenient, but justified in some cases. The trend in the industry in recent years has been in the opposite direction, that is towards getting good performance from electrically small antennas of less than half wavelength, rather than going larger. This is driven mainly by the demand for ever smaller portable devices with integral wireless functions that the industry is dealing with now.

The best place to see examples of electrically large antennas, like multiple wavelength wires, remains in the services using LF, MF, HF and low VHF frequencies, as once you go above 100 Mhz or so, most people do their beamforming with driven or parastic arrays instead.

The term wavelength is useful in many ways, not just in describing the length of wire antennas.