Archives January 2011

Internet Of Things

Antenna Length

Antenna Length

Antenna length is important, 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 to radiate from the antenna.”

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.

It also reduces transmission range, which is crucial in systems such as LPWA (Low Power Wide Area), such as LoraWAN, that are used in IOT (Internet of Things).

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.

Helical Antennas

Helical antennas are those short rubber covered antennas, typically used on portable handheld radio equipment.

You can have 1/4 wave helical antennas, but they are shorter than a quarter wave ‘solid metal rod’ antenna.

The reason they are shorter, is because they use wound copper wire. The copper wire is wound around a solid ‘former’.

The job of the central ‘former’ is to keep the antenna rigid, and provide something for the copper wire to be wound around.

Dipole & Isotropic Antennas

We provide training to companies and individuals, on all aspects of RF communications, including dipole and isotropic antennas.

Our courses can be specifically tailored to your individual or company needs, and can be delivered on-site , at your premises, at our premises, or online.

We also offer a range of courses, that are ‘off the shelf’, and ready to be delivered.

Why not contact us, to discuss your rf training requirements on: +44  (01522) 740818

Twitter: @yeswaylimited     Email: Craig.miles [at] yesway.co.uk

(c) 2011-2020 Craig Miles

    Taxi system with multiple faults

    Taxi Radio Faults

    Have been to a taxi firm in Gainsborough today to look at their radio system which had stopped working, and had taxi radio faults.

    The firm had reported that the car could not talk to the base and vice versa. I Found numerous faults including an open circuit magnetic mount aerial system on the car, which  was  the cause of the car not being able to talk to the base radio.

    The base station was also not transmitting, even if the car was parked almost next to the base station aerial system. Even with the car aerial centre wire detached at the aerial end, you should have received a signal, as my frequency meter was reading that the base transmitter was transmitting a frequency.

    The base station transmission could also be heard on my handheld scanner held near the radio. What it turned out to be was the base station power supply, which was a cheap CB type one. A few weeks previously the owners had decided to put the base radio in one of the cars for some reason. This was doing what they wanted, but they had also used one of the mobile radios as the new base station.

    The original base station was set to 5W RF output which the cheap power supply could cope with.  The mobiles however are set to a the higher legal output that they can use. Hence the cheap power supply was under strain, and had partially failed, i.e, the radio lit up, but could not cope with the current being drawn by the higher powered taxi radio transceiver.

    ATEX Intrinsically Safe radios

    Superyacht ATEX

    Superyacht ATEX equipment, is designed to prevent explosions.

    Superyachts and other Marine vessels, are at potential risk of explosion.

    Explosions are caused by electrical sparks being produced by equipment, being used in areas with potentially explosive atmospheres.

    An example of a potentially explosive environment, is a ships battery room.

    In a battery room, potentially explosive gases are released by the emergency batteries, which charging.

    The batteries are there to power key marine system, such as navigation, steering and communications, in the event of main system failure.

    Other potential explosion risk areas, include fuel tanks.

    Whilst its true that most Superyachts currently use diesel power as the Prime Mover , there are potentially other fuels onboard.

    We can help ensure that your superyacht is using the correct radio equipment, in the correct environment, for maximum safety.

    +44 (0)1522 740818, and speak to Craig about superyacht ATEX survey audits, and product solutions.

    Craig first worked in marine electronics aged 17, and has taught ATEX systems to trainee Marine Cadets & ETO’s, at South Shields Marine School.