Wireless Waffle - A whole spectrum of radio related rubbish
Martian radio amateurs appeal spectrum allocation decisionsignal strength
Thursday 22 January, 2015, 10:48 - Amateur Radio, Broadcasting, Licensed, Radio Randomness, Spectrum Management
Posted by Administrator
Radio amateurs with designs on operating from the planet Mars are appealing against a decision by the Consultative Committee for Space Data Systems (CCSDS) to allocate the 70 cm amateur band (430 - 440 MHz +/-) for communications between satellites in orbit around the red planet and the numerous rovers that criss-cross its surface.

In a statement, released by the Mars United People for Planetary and Earth Transmissions (MUPPETs), tea-drinking general secretary Arthur Dent said,
MUPPETs have been planning a DX-pedition to Mars for some time. To discover that our officially allocated radio frequencies are already in use is just not fair. It constrains our ability to talk about radio stuff to each other and means other radio amateurs around the solar-system will be denied extra points in the forthcoming 'talking about radio stuff with other radio nuts' contest.

Responding to the accusations, Prostetnic Vogon Jeltz of the CCSDS commented,
prostetnic jeltzThe 70cm frequency band has been used for communications on and off Mars since the Viking lander first set foot on the planet back in 1976. The MUPPETs have had plenty of time to comment. The plans for frequency use on Mars have been available at the local planning office on Alpha Century for fifty of your Earth years, so they've had plenty of time to lodge any formal complaints and it's far too late to start making a fuss about it now. I'm sorry but if they can't be bothered to take an interest in local affairs that's their own regard.

Appallingly obvious references to the Hitch-Hikers Guide to the Galaxy aside, it may surprise many people to learn that there is, indeed, a frequency plan for Mars. And that there are already 5 communication satellites in orbit around the planet! For communication from the rovers on the surface to the orbiting satellites, frequencies in the range 390 to 405 MHz are used. For the link down from the orbiters to the rovers, the frequency range 435 - 450 MHz is used, which falls inside the amateur radio 70cm band.

The choice of the particular frequencies in use (on Mars) is designed to try and stop anyone deliberately causing interference from the Earth, whilst retaining ease of use on Mars (i.e. the ability to use omni-directional antennas). The various satellites orbiting Mars typically get no nearer than around 400 km from the surface and communication with rovers typically takes place when the satellites make their closest pass. The shortest distance between the Earth and Mars is typically around 60 million km. The table below shows the path-loss at 415 MHz of these distances.

Route Distance Path Loss
Satellite to Mars surface 400 km 137 dB
Earth to Mars 60,000,000 km 240 dB

So the difference in path loss is just over 100 dB. For a transmitter to cause interference from the Earth to communication on Mars, it would therefore have to have a radiated transmitter power 100 dB higher than the signals passing between the rovers and the satellites.

mars uhfA very good description of the communications with Mars is provided by Steven Gordon (from whom the diagram on the left is shamelessly plagiarised). The transmitter power used on Mars is 5 Watts (7 dBW), so in order to cause interference from Earth, a transmitter power of around 107 dBW, or 50,000,000,000 Watts (a.k.a. 50 GigaWatts) would be required. Would it be possible to generate such a signal?

Firstly, it ought to be possible to generate at least 100,000 Watts (100 kiloWatts or 50 dBW) of power at the necessary frequencies as television transmitters for the UHF band that reach this level are available. So what is then required is an antenna with a gain of 57 dB. This requires a dish with a diameter of around 150 metres. The largest dish antenna in the world is the radio telescope at Arecibo, Puerto Rico, which is 305 metres in diameter.

muppets cq dxIf a high powered television transmitter was therefore connected up to the Arecibo radio telescope antenna, it ought to be more than possible to jam the transmissions between the Mars rovers and the orbiting satellites during periods where the Earth and Mars were closely aligned. Of course this kind of power level is way beyond the normal licensing conditions of a typical radio amateur and the right conditions would occur roughly every 2 to 3 years when the Earth and Mars come closer together. Nonetheless, commenting on this finding, Arthur Dent of the MUPPETs jeered,
Safe from interference, eh? Who looks silly now then Jeltz!

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Ofcom proposes closure of stable doorsignal strength
Monday 19 January, 2015, 16:25 - Radio Randomness, Spectrum Management
Posted by Administrator
comtrend pltOn several past occasions, dating back to 2008, Wireless Waffle has reported on how several users of the short wave spectrum including radio amateurs, broadcasters, air traffic controllers and NATO, have raised concerns about interference caused by power-line telecommunications (PLT) devices (such as the Comtrend unit pictured on the right). PLT devices allow the use of home electrical wiring to carry computer data by injecting radio signals over the wires. As electrical wiring is designed to carry signals with a frequency of 50 Hz and not 5 MHz, the injected radio signals have a tendency to leak out everywhere and cause radio interference over a wide area.

Groups such as UKQRM and Ban PLT have long campaigned that PLT devices (also known as Power Line Adapters) should be taken off the market as they do not comply with the relevant emissions standards.

Over all of this period, the UK spectrum regulator Ofcom, has staunchly refused to accept that these devices contravene any regulations, though they have taken action in a number of cases where the interference they cause has exceeded even their expectations. Over the same timeframe, a number of other devices have also been found to cause high levels of radio interference, particularly cheap electrical devices, often imported on the grey market from China. These include things such as laptop power supplies, LED lighting and solar panel electrical controllers. Yet Ofcom continued to refuse to accept that anything needed doing.

It is therefore somewhat of a bolt from the blue that on January 15th, Ofcom released a consultation document, entitled 'Notice of proposals to make The Wireless Telegraphy (Control of Interference from Apparatus) Regulations 2015' in which it wishes to implement new controls over these devices by making it a criminal offence to operate them if they are causing interference to wireless telegraphy (e.g. radio services). Part of this seems to be driven by the fact that Ofcom were unable to deal with many interference complaints under existing regulations and that this could be regarded as a potential safety-of-life threat where the interference was caused to, for example, aeronautical services.

According to the consultation paper, there were 114 cases of interference reported in 2014, 'where undue interference was caused ... and capable of resolution'. Of those 114 cases, only 3 could be cured quickly using existing legislation and in the other cases it required voluntary action by the user of the equipment to bring about a solution. Under the proposed changes, all of these cases could be dealt with by law, meaning that instead of volunteering to fix the problem, users could be prosecuted if they didn't.

devil slop 2The big question has to be whether such a change would make any difference. Would those selling the devices stop doing so? Presumably not, as there is no law against selling them, just using them. Would they warn buyers of the new law? Not if it damaged sales. And what will happen if Ofcom threaten prosecution to someone who believes they have been using equipment quite legally, having purchased it legitimately, and having seen the various markings on the box that said it complied with the necessary standards? Will Ofcom then look silly for allowing such devices to have been sold in the first place? Sadly, the UK legislation on such things is still somewhat muddy. It is possible to sell, for example, FM transmitters, mobile signal boosters and even GPS jammers. It is just not legal to plug them in and use them.

ofcom horse boltingBan PLT are recommending that as many people as possible respond to the consultation, encouraging the changes to be implemented. No doubt organisations which manufacture or sell the devices will be arguing against the changes and so it is important that those who use the radio spectrum, especially the short wave spectrum, respond to show the strength of feeling.

There is an old saying in England, 'closing the barn door after the horse has bolted'. Effectively it means trying to solve a problem, after it has happened. The new powers proposed by Ofcom may have some effect in allowing Ofcom to convince users to turn off whichever device it is that is causing the problem, but it is by no means certain that the threat of 'turn it off or we will sue' is going to win anyone any friends, Ofcom, suppliers, BT and radio listeners alike.
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Echos Of Signals Pastsignal strength
Sunday 30 November, 2014, 11:02 - Radio Randomness
Posted by Administrator
It's easy to forget that the radio spectrum has been in use for over 100 years, but that then (as now!) communications has not always been wireless.

During World War I, teams of 'signals' engineers risked life and limb to lay down wires to allow communication between the front line and the headquarters.

"Signals From The Great War" takes the reader back to these times through a series of mémoires written by Archibald Gordon MacGregor RE MC combined to tell the story of a young signals officer's experiences on the front line in Belgium and France from 1917 - 1919.

Though the mémoires were written in 1968, almost 50 years after the events they are describing, the level of detail and clarity are impressive and the book makes for a fascinating read for anyone interested in finding out more about military communications before radio became the default means of information distribution.

It is gratifying to note that Lieutenant MacGregor received the military cross for his bravery.
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Going Underground...signal strength
Sunday 20 October, 2013, 13:49 - Radio Randomness
Posted by Administrator
In major cities around the World it is now unusual to find yourself in a place without a WiFi connection or two going for free. Coffee shops, bars, department stores and many other places now offer free WiFi to their customers. Normally all that is required is to enter an e-mail address and accept a set of terms and conditions and you are surfing gratis. For anyone roaming and facing extortionate fees for their data connection, these WiFi connections can be a godsend.

But there are still some places around the world, even in major cities, where WiFi connections are few and far between and top of the list are the various underground metro systems. To make matters worse, many metros are not only WiFi free zones but are lacking mobile phone coverage too. Thankfully this situation is becoming less frequent as operators (whether transport or mobile) decide to provide the infrastructure necessary to enable mobile coverage underground. Covering underground tunnels and stations is not difficult, but it can be expensive. It requires the use of special antennas and in many cases the laying of leaky feeder - a coax cable specially designed to 'leak' signals along its length thereby acting as a long antenna.

The London underground has long been an area devoid of any kind of broadband internet connectivity, but for the 2012 Olympic Games many stations (not the tunnels connecting them) were fitted with free WiFi by internet service provider Virgin Media. Since the games, the WiFi has no longer been free (except for existing Virgin Media customers) but coverage is slowly being extended.

If you want to know which stations have coverage, you could visit Virgin Media's station guide, or alternatively 'there's an app for that'... The Station Masterstation master app app (unfortunately only for the iPhone at the moment) will let you know exactly which stations have WiFi connectivity and is updated regularly with information from people using the underground network and so often it shows stations that have got WiFi but which have not made it to the official station guide yet. As if to prove a point, at the moment Station Master is showing 123 stations with a WiFi service whereas the Virgin Media web-page only shows 120!

london underground wifi map

Not only does it show you where there's WiFi on the London underground but it also has 3D plans of nearly all of the central London stations (so you know which stairwell is quickest and easiest for getting from one line to another) and also will tell you exactly which door to stand at to get on the train so that you are in the right place to get to that stairwell or walkway when you get off the other end. For anyone who uses the London tube to get around, the app is a must have (even at GBP1.99). Of course you'll have to have a WiFi or mobile connection first to be able to download it in the first place!
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