Antenna length plays an important role in maximising radio performance in a vehicle based installation.
This video explains why in more detail.
Video Practical Demo On How To Replace the Antenna Cover on a Kenwood Walkie-Talkie
How To Replace the Antenna Cover on a Kenwood Walkie-Talkie
Full Instructions for kenwood Pro Talk TK3301:
- Remove battery from back of radio, by lifting up clip on bottom of radio.
- Remove the two knobs from top of radio, by pulling upwards.
- Using cross-head screwdriver, remove the two screws that hold the ‘belt clip’ to the radio (located towards top of the back of the radio).Then remove belt clip assembly.
- Remove the two smaller screws located at the bottom of the rear battery compartment (rear of unit).
- Carefully prise the metal chassis apart from the plastic case, from the bottom of the unit. With the front (speaker side) of the unit flat on a surface, the bottom part of the case can be separated in an upwards direction. Caution, the plastic case is fragile and easily broken, so avoid using force. If possible get your fingernail between the chassis and case, rather than using a metallic screwdriver, which could cause damage.
- Once you have prised the chassis and plastic case apart, do not separate it more than about 2 centimetres. This is because both the metal part of the antenna, and the front speaker are attached, and could be damaged.
- Now carefully withdraw the metal chassis away from the top of the plastic case. Imagine that the metal chassis is falling out the bottom of the plastic case, which is the direction it should go in.
- The speaker wires are quite short, and care should be taken not to stretch of break them when separating the chassis from the case.
- As you withdraw the chassis in a downwards direction (in relation to the case), the metal antenna part (coiled wire), which is covered by the antenna cover, will be revealed.
- Once the coiled antenna wire is totally withdrawn, push the plastic antenna cover from the top of the plastic case, in a downwards direction. The plastic antenna cover should come out of the plastic case.
- Fitting a new antenna cover is a reversal of the removal procedure, taking care to ensure that the shaped base of the antenna cover locates in the recess inside the plastic case. This should be obvious when trying to fit.
- Make sure that the rubber grommet that is located on the top of the plastic case, at the base of the antenna cover, is pushed down fully. This sometimes pops out when fitting a new antenna cover (as it did on the video above).
- The rest of the reassembly process is a reversal of the dismantling instructions, however care needs to be taken to ensure that the speaker wires are positioned so that they are not trapped between the plastic case and chassis, when refitting them together. Its easily done, and can result in having to get out the soldering iron, to repair a ‘pinched’ open circuit connection.
- Once you have the plastic casing and metal chassis back together, then fit the battery and test that the radio works. This can save time, if you have accidentally broken the speaker wire, as you do not have to remove the screws to repair.
- Once happy that you have audio from the speaker, then refit screws, belt clip, battery.
- Congratulate yourself, and celebrate your achievement :-)
Disclaimer: No liability if you break something or injure yourself. If in doubt, get in touch for help. This is for information only.
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
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