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Monday, 27 May 2013

How repeaters work

This is a basic introduction to how standard radio repeaters work.
Repeaters are used to extend the range of a radio. They are usually put somewhere high up, so that there are no objects between the antenna of the repeater and the users it wants to serve. This allows the repeater to hear and transmit much further than a person on the ground. Using a repeater you could expect as much as 10 times the range or more compared to using handheld to handheld.

You may hear about offsets when repeaters are mentioned, this is the difference between the transmitting frequency and the receiving frequency. (called TX and RX for short)
The repeater listens for transmissions on its RX frequency (or input), and then when it hears a transmission on that frequency it transmits it back out on the TX frequency (output). The frequencies must be a certain distance away from each other, otherwise when the repeater starts transmitting, it will desensitize the receiver and will no longer be able to hear the user talking.
Radios will be programmed to listen on the output of the repeater, and to transmit on the input. This means they will always hear any transmission that the repeater makes, and the repeater will hear them whenever they transmit. The radio hears other users via the repeater, even if the other user is right next to them. If they are out of range of the repeater they will not be able to hear anything even if another user is transmitting right next to them, because they will be listening for the repeater to transmit that message.

Generally when scanning you will hear two types of repeaters. Amateur repeaters and business radio repeaters. Amateur repeaters are open to access by anyone with an amateur radio license, which can be got from ofcom by sitting a simple test to show you have an interest and a basic knowledge of radio. Business repeaters are closed to the public, and are only for use by employees of the business or people they allow to use the repeater. You will hear businesses like taxis, shopwatch schemes, local authorities, parking wardens who will usually have their own repeaters.
Amateur repeaters have fixed offsets in each band, for example the 2m band, which goes from 144-146MHZ, has an offset of +600KHz. This means that for all repeaters in that band will have their input frequency exactly 600KHz above their transmit frequency. For example: If you hear a repeater transmitting on 145MHz, to transmit into the repeater and have the message repeated back out, you would have to transmit exactly 600KHz higher, which is 0.6MHz. So you would have to transmit on 145.600MHz. (Which you would of course only do if you had a license to do so!)
Business repeaters will have varying offsets, but they are often the same in certain ranges set by Ofcom. The frequencies of repeaters can be found on the Ofcom WTR (Wireless Telegraphy Register).

Repeaters will also usually have an extra requirement to satisfy to be able to use them. This is a CTCSS (Continuous Tone Coded Squelch System) or DCS (Digitally Coded Squelch). People often refer to these as a tone, code or PL, which you can assume means CTCSS or DCS.
To put it simply, these are sub-audible noises that are added to your transmission and then picked up in receiving radios/the repeater and decoded. If you transmit with the wrong CTCSS/DCS, or without one, the repeater will completely ignore your transmission. Once you have the correct one set, the repeater will hear you and transmit your message out on the TX frequency, often with the same tone/code.
Using the same CTCSS or DCS on the repeaters transmission is useful because it stops the receiving radios from picking up and other users who may be on the frequency (unless the also have the same tone set), or interference caused by electronic devices being close to the radio.
Scanners also sometimes have CTCSS and DSC which is used to filter out different groups, or interference on a channel, and only hear the group you want to hear who will be using the same code.

Digital repeaters will not use CTCSS or DCS, but they will use other software parameters that are set in the programming software. In DMR this would be the colour code. DMR repeaters work in a very similar way to analogue repeaters, but they listen on the input frequency and then receive and demodulate the data. Then they will modulate the data again and transmit it on the output frequency, often adding data on one timeslot, such as "slot empty" messages when only one timeslot is in use.
Because users transmit on the same input frequency, but on different timeslots, the repeater can repeat two voice messages at the same time by simply listening to both timeslots on the frequency. The output of the repeater would repeat both timeslots and receiving radios can listen to either of the timeslots depending on which group is set to use which timeslot. So if one group of users is programmed to use slot1, another group of users who have their radios programmed to use slot 2 will not hear transmissions by the first group because their radios are listening on the wrong timeslot.

Sunday, 26 May 2013

VHF and UHF digital voice types

This post is aimed at new users of radio, who are interested in digital voice on UHF and VHF. Digital is used mostly by commercial users at this stage, and there are a limited number of digital products on sale for private individuals without getting a business radio license. I hope this comprehensive and extensive article will clear up any confusion for newbies to the field, and also give out new knowledge to the same group.

One product that can be used without a license is the Icom IC-F4029SDR. This is a digital handheld that works on the PMR446 frequencies and on the digital allocation which is just above the normal analogue allocation. These radios currently cost in the range of £200 each, so as you can see, it is an expensive option for communications where analogue radios which would achieve similar range can be bought for £40.
This Icom radio uses 6.25KHz wide channels, and adheres to the dPMR446 standard.(more info here)

Terms you will need to know:
Timeslot: When the channel is divided into segments by time. Data is transmitted in bursts. The switching is done at a rate that it is fast enough not to disrupt the users communications - many times each second.
An example:
A user wants to transmit their digital voice to someone else. They need a certain amount of spectrum to transmit the constant stream of data which is produced by the vocoder. The amount of data per second they need to send is constant but there are different methods of sending it:
They could use 6.25KHz and occupy all of the time on the channel.
They could use 12.5KHz and send the data in half the time, but they need the data sent constantly (because it is a stream of live voice), so the channel is split up into 2 timeslots and the radio transmits the data at twice the rate it would when using 6.25KHz, but the quick bursts that only occupy half of the total time.(1/2 timeslots)
They could also use 25KHz, like tetra, and use only 1 timeslot out of 4. All of these methods allow the same amount of data to be sent, but it different ways. If you still don't understand timeslots, have a look at the article on wikipedia about TDMA (Time division multiple access)
Bandwidth: How wide a channel is. The wider a channel is, the more spectrum it takes, up, and the more data it can be used to transfer. Also license fees increase for wider channels. For comparison, 12.5KHz is a standard analogue FM voice channel; 200KHz is approximately what an FM broadcast station would use; and TV channels are about 8MHz wide (1MHz=1000Hkz);  GSM (mobile phone standard) channels are 200KHz wide and have 8 timeslots.

For commercial users there are quite a few options:
DMR - The most popular mode by far, equipment is sold by Motorola as MOTOTRBO, and also by Hytera and other manufacturers. This standard uses 12.5KHz wide channels and fits 2 timeslots in each channel.(This is called TDMA)  A single repeater can repeat 2 voice conversations (1 way, not duplex) at the same time on the same channel by the timeslots. The vocoder used here is AMBE+2.

TETRA- This is the second most common option. Tetra radios are usually more expensive than the other 2 types discussed in this section, but also have better features. Tetra radios are definitely more advanced than the other two, since they provide functions such as duplex calls (listening and talking at the same time, like a mobile phone); call handover, which is where individual radios are transferred between base stations without losing connectivity so they can continue transmitting or receiving while moving; and DMO gateways, which is where one radio can repeat a signal from the base station to allow another radio that is further away and out of range to connect. Tetra also uses a different vocoder - ACELP. This vocoder in my opinion sounds a lot better than AMBE. Tetra does not often have a range as good as the other two, but the advanced features make up for it. 25KHz channels are used, but in terms of bandwidth per voice channel, it is just as efficient, because 4 timeslots are used. Tetra is usually only used by larger companies or organisations, most likely because of the cost of the base stations, which are far more than DMR or dPMR.
The emergency services (Police, Fire, Ambulance) throughout the UK use tetra on the airwave network which they lease off a private company. This network is fully encrypted and closed to the public.
Manufacturers include Sepura, Motorola, EADS.

dPMR - Equipment sold by Icom and other manufacturers. This standard also uses the same AMBE+2 vocoder as DMR, but this time uses half the bandwidth (6.25KHz) and does not use timeslots.(This is called FDMA) The consequence of this is that each repeater can only repeat one voice at a time, but this also means you do not need a whole 12.5KHz channel and can license a 6.25KHz channel instead, which is cheaper.
The range of DMR and dPMR are similar, but dPMR probably has just a tiny bitof an advantage because it does not use the timeslots and uses only 6.25KHz which makes the radio transmit the same amount of power but in a smaller space.
There is also a very similar standard which is not interoperable with this one, called NXDN. This is manufactured by Icom and Kenwood. Icom make radios that work with both NXDN and dPMR.

Other modes you may hear of:
P25 - Used by the police in America. Scanners are available that can listen to this mode, unlike all of the other modes mentioned in this post. There are two types available: Phase 1, which uses the older and inferior IMBE vocoder and 12.5KHz bandwidth with no time division, and Phase 2 which uses AMBE, and 12.5KHz 2 timeslot division.(similar to DMR)

D-STAR - This mode is used by amateur radio operators. It is omptimised for amateur radio and would be very difficult to adapt to any other type of user. Repeaters require callsigns to be used and sound quality is not that great even though it uses the AMBE+ vocoder. Equipment is kept expensive by the fact that there is no competition to the only manufacturer of the standard - Icom.
It is also unpopular with some users due to the fact that it uses the AMBE+ vocoder which is strictly protected by patents and copyrights, stopping amateur users from experimenting with the vocoder.

Why use digital?
Digital is popular with commercial users who are updating their radio systems because of a few factors:
  • It gives consistent sound quality right up to the fringe of communication range. This means no hiss and noise on the radio, but the downside is that it makes it harder to tell how good reception is, and whether you are about to go out of range. When someone is about to go out of range, digital errors will be present, and this will cause the vocoder to make strange noises, sometimes compared to the little robot, R2D2! This would only happen when analogue voice would be too quiet and noisy to understand.
  • Digital can send voice and data. Users can have their ID show up on other radios when they talk; status messages can be sent; text messages can be sent; emergency priority can be turned on which speaks over other users in an emergency.
  • Because digital uses a vocoder instead of sending the voice using analogue, even if you speak into the radio very quietly or from far away, the message will not be overcome by hiss on the receiving end because there is none!
  • Digital voice is much easier to secure than analogue. Many digital technologies come with basic security and the digital stream can simply be encrypted, which is far easier than for analogue, where it would have to be inverted which is not very secure. Scrambling on analogue loses voice quality, whereas it does not on digital.

Saturday, 25 May 2013

First post, introduction of the blog and what it's for

Hello and welcome to my new blog!
This blog will be closely linked with my YouTube account and will feature similar themes. I aim to cover topics to do with radio including the use of scanners; software defined radio; PMR446; UHF and VHF radio; amateur radio; shortwave listening; digital modes such as DMR, dPMR, NXDN, TETRA, maybe even D-STAR, and decoding some of these modes.

Sometimes I will have text only posts; at other times I will have posts with pictures and sometimes I will have posts with video from my YouTube account, along with further or more detailed text info to go with the video.

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