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Antenna Length

Antenna Length

The size / length of the Antenna matters. This is because of  something known as ‘resonant frequency’.

An antenna will operate at its most efficient, when resonant, therefore allowing the maximum power to radiate from the antenna.

An antenna will operate at its most efficient, when resonant, therefore allowing the maximum power… Click To Tweet

If the antenna is not at the resonant length, then not all of the available RF Power will be able to be transmitted .

What will happen instead is that some of the RF Power will be ‘reflected’ back down the coaxial cable towards the transmitter.

This reflected power can potentially cause damage to the transmitter, and therefore should be avoided.

A piece of test equipment called an SWR (Standing Wave ratio) Meter is used to measure both Forward Power (Power going from the transmitter to the antenna), and Reflected Power (Power being reflected back towards the transmitter).

The aim should be for as little Reflected Power as possible, and maximum power directed forwards and out of the antenna.

Antennas therefore need to be on the resonant frequency. This therefore requires that antennas are the correct length or size.

If we consider a typical antenna that you often see on a taxi, it usually is about 30 Centimetres / 12 inches long, and made from a single thin piece of metal rod (known as a whip).

The reason that it is roughly that particular length are as follows:

Typically the taxi antenna is cut to a quarter of the ‘Wavelength’. This is known commonly as a quarter wave antenna.

The higher the radio frequency, the shorter the WaveLength. The majority of taxi companies have their radio systems operating in the VHF (Very High Frequency) band, roughly around 160-170 Mhz (Mega Hertz).

These frequencies mean that the taxi antenna length is roughly 30 Centimetres long at 1/4 WaveLength.

What is meant by quarter wavelength, is exactly how it sounds, namely that the length of the antenna is only a quarter of the actual length of the transmitted radio wave.

You could double the length of the antenna, so that a 30 Centimetre antenna becomes a 60 Centimetre length antenna. This would then be known as a ‘Half Wave’ (1/2 Wave) antenna, and would also be at ‘resonant’ frequency.

You could also have a full wavelength antenna if you wished, which would be at resonant frequency.

The main reason that most taxi companies and other mobile radio users use quarter waves, is that they are fairly short. This is particularly important if the antenna is on top of a high vehicle, such as a truck.

Quarter waves and half wave antennas send and receive their signals from all directions, and do not have gain (which will be explained later down the article).

Quarter wave antennas are however not the most efficient option available.

Of course antennas come in many shapes and sizes, which are worth mentioning.

Yagi Antennas

The yagi antenna is named after the japanese inventor of it.

The Yagi is a directional antenna with high Forward Gain (the way its pointing), and low backward gain (the backwards facing end).

The most common place that you will see a Yagi antenna is on the roofs of houses, as the common tv antenna is a type of Yagi.

Whilst tv antennas only receive signals, Yagis are used to transmit radio signals as well.

Being directional, the Yagi antenna is useful in situations where you only want to transmit and recieve in a certain direction.#

Firstly lets look at the recieve only Yagi  tv antenna. The reason that the Yagi antenna is so commonly used to receive television signals is that it has ‘High Gain’, as well as being directional.

What having high gain does is make the received signal stronger. Television signals are transmitted by high powered UHF transmitters, however they can be many miles away. Having a high gain and directional antenna helps receive the signal over long distances at adequate signal strength levels.

Similar Yagi antenna designs are used for transmitting radio signals too. Typically the same single antenna would be used for both transmitting and receiving radio signals.

An example of where a yagi might be employed in Two-Way radio communications, is to send and receive communication from a fixed offshore wind farm, back to the base on land.

Being directional can help screen out unwanted signals that may be using the same frequency, as you can point the Yagi towards the station that you wish to communicate with.

Of course the Yagi is not suitable for communicating with mobile radios such as handheld Walkie talkies and vehicle radio installations, as they move.

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