How Do You Determine What Type of Antenna You Need?

How do you determine what type of antenna you need for an application? The answer depends, of course, on the requirements.

Antennas Next to Large Metal Objects

Will the antenna be mounted against a large plate of metal?

The solution is to use an antenna, such as a monopole, that will use the piece of metal as a ground plane. Something like a dipole, which does not need a ground plane, will probably behave in an undesirable way when mounted close to a large metal object.

Antennas for Portable Devices

Are you designing an antenna for a portable device?

You will need an antenna with low directivity. The low directivity means that the antenna can “see” in more directions than a highly directive antenna.

For a portable application, it is impossible to know exactly where the user will be relative to a communications tower or receiving station at a given moment.

One thing to note is that the lower the directivity, the lower the gain of an antenna. Monopoles, inverted-L, and inverted-F antennas are common examples of low-directivity antennas.

On the other hand, if it is important that the antenna is directional, the Yagi seems to be a favorite for this application, though various horns and dish antennas are highly directional, as well.

Broadband Antennas

Is bandwidth a concern? Some antennas have fairly broad bandwidths and others are narrow in bandwidth.

Many communication devices require a broadband antenna. This bandwidth plays a role in how much data can be sent in a given amount of time. If the bandwidth is too narrow, the data rate will be restricted.

Many devices use a barrage of frequencies with only one or two antennas to handle them. While there are antennas like the spiral or log-periodic that can neatly handle a wide array of frequencies, a common fix is to broaden the bandwidth of a more narrow-band antenna. This may involve multi-resonance antennas or adding loss to make the bandwidth “appear” broader.

Antenna Gain

High gain is almost always desirable — but what is considered a “good gain?”

Gain is increased as the directivity of the antenna is increased, but as we showed above, sometimes a directional antenna is not desirable.

The perfect, lossless, truly omnidirectional antenna has a gain of 0 dBi. From that number we can infer that for many portable devices we don’t want a gain more than 1 or (maybe) 2 dBi. The extra gain, in fact, just implies that the antenna is concentrating power in certain directions at the expense of other directions.

As it happens, however, we often find that the gain number is usually less than 0 dBi. If our antenna must be small (and who doesn’t want the antenna to be small these days), the antenna will likely be too small to radiate efficiently, creating loss that will need to be dealt with. To make matters worse, if we try to broaden the bandwidth of the antenna, we may very well add even more loss. In the end, antenna gains are usually rather lower than the optimum, as the gain must suffer to meet size, bandwidth, and other constraints.

So what type of antenna do we use for high gain?

If a broad area of radiation is required, we must not use a highly directive antenna, so the gain will be lower. Beyond that, it may not matter so much what antenna is used. Directivity aside, the gain of an antenna, regardless of type, is dependent on obtaining a certain minimum size requirement dependent on the lowest frequency covered. If the allotted room for an antenna is too small, we can expect low gains.

The Compromise

As you might have noticed by now, we find the paradox of antenna design: many desirable features come at the expense of other desirable features.

So if we want a high antenna gain, we can expect a tradeoff between the antenna becoming more directive and the antenna taking up more room.

Bandwidth? We might lose gain to increase the bandwidth.

And so it goes. We can certainly select better antenna types to meet the needs. For example, you will not likely use a Yagi for a mobile device. However, the type of antenna may not matter as much as how it is used. Frequently, antennas for very small devices start looking rather unlike their basic, standard configuration. A too-small monopole may end up looking like a zigzag trace on a PC board.

Of course, some antennas are simply easier to use than others. The inverted-F, for instance, allows for much easier impedance matching than the inverted-L. This is because the inverted-F has an adjustable feed point.

The bottom line, however, is that the antenna type is determined by the requirements, and some requirements (such as small size) may more or less determine the performance regardless of what type of antenna is used.