Wireless Waffle - A whole spectrum of radio related rubbish
Noisy Neighbourssignal strength
Thursday 30 October, 2008, 22:23 - Amateur Radio
Posted by Administrator
It may appear to have been quiet here at Wireless Waffle over the past couple of months, but that's because a number of things have piqued the interest and we've been doing a bit of experimentation and investigation. The first of them is the issue of Power Line Telecommunications (PLT) also known as Broadband over Power Line (BPL) and in particular the problems being experienced by many UK short-wave radio listeners with it. One of these devices could recently be heard, albeit quietly, across the HF spectrum (oddly it has since disappeared) at Waffle HQ but it raised the question as to how many more there were in the area. To find out, I fitted and HF antenna to my car and connected it to an HF receiver and then drove around a nearby housing estate to see what could be heard.

The antenna used resonated in the 18 MHz (17 metre) amateur band but received perfectly well in the 17 MHz (16 metre) broadcast band. Tuned to a clear channel around 17460 kHz, the car was driven around the area under test. Over the area covered by a small local estate, three devices were detected. Two were almost certainly the 'Comtrend' device, sounding just as the ones that UKQRM has demonstrated on YouTube. The third (shown in red on the map) emitted a more continuous tone, interrupted by occasional blips; the range of this device was somewhat less than the other two. Their range at other frequencies was not tested but anecdotal evidence driving round the area using other antennas and the same receiver suggests that the coverage on frequencies around 27 MHz is similar.

plt bpl interference rangeI've plotted the approximate location of the three devices identified together with the area over which they were clearly receiveable on the map on the right (the map covers approximately a 500 metre by 500 metre area). The blue device had the largest interference range and within the areas marked it is unlikely that any short-wave reception would be possible.

The density of these devices means that over the whole estate, short-wave reception would be virtually impossible. A cursory glance around the area indicated no amateur radio antennas so it is likely the devices are going undetected (or just unreported!) Not a positive result and unless the powers that be do something to halt the spread of these devices, it would be easy to foresee a situation where HF reception could be pretty much impossible over whole towns and cities, in residential areas for certain.

One piece of positive news is that Ofcom have set up a special team to deal with PLT interference and appear to have begun taking the problem seriously. Let's hope that this is more than just paying lip service to the problem before the whole HF spectrum is lost to the laziness of those who can't be bothered to use WiFi or put a piece of wire between various bits of equipment in their home. These devices are a disgrace and a menace and before they wipe out all short-wave reception and neighbour-on-neighbour war breaks out, serious action by the authorities is absolutely necessary.
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Wire-More LAN (Part VI) - Maximise WiFi coverage and reliabilitysignal strength
Thursday 28 August, 2008, 17:12 - Radio Randomness
Posted by Administrator
Like all radio systems, wireless LANs can never be 100% reliable. Many factors affect radio transmission and even in situations where something might be expected to work well, problems can often occur. Wireless LANs suffer from a number of particular problems, not least the frequencies they use (around 2.4 GHz) are easily absorbed and reflected and can suffer from high levels of interference. But with a little care, their coverage and the reliability of the connection can be improved quite easily. Here are the Wireless Waffle top 5 tips for improving WiFi signals.

1. Choose the right channel

Interference from other wireless LANS is the most common cause of network quality degradation. The various surveys that Wireless Waffle has conducted have generally shown that channels 1 and 13 are the least likely to suffer from interference from other wireless LANs. Further, channels 10 upwards are the least likely to suffer interference from other users of the 2.4 GHz band. So generally speaking, in an area with a low likelihood of finding other WiFi users, channels 11, 12 or 13 are likely to be best. In congested areas (i.e. where interference from other WiFi systems is likely to be worse than from other users in the band), channel 1 is the best choice.

2. Position your WiFi hub centrally

Radio coverage from most transmitters radiates in all directions from the point of the transmitter, and this is certainly the case for wireless LAN equipment. It therefore makes total sense to position your central WiFi hub as close as possible to the middle of the area you wish to cover. Further, radio signals travel best if there is a clear path between both ends of the link. If you put the central unit in a cupboard or behind a bookshelf, where the path is already obstructed, coverage will reduce. Place the unit in an place with a clear view of the area you wish to cover, the higher the better (look at hubs installed in public places such as coffee shops and hotels - they are almost always mounted on the roof!)

bluetooth side effects3. Keep your wireless equipment away from other wireless equipment

There are many other sources of radio signals in the typical home or office environment and many of these are on frequencies similar to (or even the same as) WiFi. Placing these other radio sources near to wireless LAN equipment can result in reduced performance from the LAN as it struggles to cope with the interference caused by the other equipment. It's especially important to make sure that equipment such as digital cordless phones, satellite TV downleads, bluetooth and zigbee devices, mobile phones and digital baby alarms are not used in close proximity to a wireless LAN if service quality is not to suffer.

4. Use equipment with proper aerials

Devices such as USB WiFi adapters are generally quite small and this means that the aerial inside them is also small. Smaller antennas pick up less signal and thus range and the quality of the connection will be adversely affected. If you are installing a wireless network for your home computer, use a proper PCI wireless card with an external antenna, rather than a USB wireless 'dongle'. This step alone can easily double or more the range you achieve from your wireless network and such cards are often cheaper than their dongle sidekicks.

5. Upgrade your antenna

It's easy to extend the range of a WiFi network by using higher gain antennas on hubs and on PC cards. Changing the antenna on the hub will affect all of the network, whereas changing it on a remote device will affect that device only. Most standard antennas have 2dB gain but 5, 7 or even 9 dB gain antennas can be purchased relatively cheaply. Changing from 2 to 7dB gain antennas will virtually double the range of your network. And if you replace the antennas on both the hub and the remote units with 7dB gain antennas, range will nearly quadruple.
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A Noisy Noise Annoys (Part 3)signal strength
Sunday 27 July, 2008, 09:12 - Amateur Radio
Posted by Administrator
The European Telecommunications Standard Institute (ETSI) is currently in the process of finalising a standard for in-home Power Line Telecommunications (PLT). This is equipment that allows internal electrical wiring in a house to be used as a local area network, to carry computer and other data signals around without having to re-wire your house to do so. Whilst wireless networks do much the same, the advantage of PLT is that it could be built into, for example, a television, to allow it to interconnect with a video server or even a set top box, without the need for additional leads and connectors.

comtrend pltThese devices (for example those sold by Homeplug) work by sending signals around the mains wiring at frequencies between around 3 and 30 MHz. Now the wiring in a typical house is designed to carry signals at 50 or 60 Hz, depending which country you are in, and although it is possible, with sufficient brute force and ignorance, to get signals in the short-wave (high frequency or HF) frequency range from one socket to another, the network is very (very) leaky. Much of the signal leaves the wiring in the house with the potential to cause interference to any HF user in the vicinity (say, within 100 metres or so).

Much of the work being done by ETSI is to try and find ways of stopping these PLT devices from causing interference. To start with, the specification requires that there be notches in the transmitted spectrum in all the amateur radio bands to try and reduce interferece to radio amateurs. This is a good start but does not solve the problem for short wave listeners (e.g. those who like to listen to international short wave broadcasts).

short waveThe most recent studies that have been done have been to address specifically this issue and the result is a specification which attempts to monitor the various short-wave bands and if it senses activity in a band, will not use frequencies in that range. So if the device detects a signal on, say, 5.9 MHz, it will not use frequencies in the 49 metre band. There is a little more intelligence than this included, such that if only signals on low frequency ranges can be detected, the device will assume it is night-time (when few high frequencies propagate) and vice versa, and from this information will make further informed decisions about which frequencies might be clear to use.

This is fine, as long as: (a) the detection algorithms work properly and (b) the frequencies allocted to the short-wave broadcast bands remain as is and don't get expanded or moved around. Of course such a system will not protect out-of-band broadcasters or other services which use HF frequencies such as the military, maritime and aeronautical communications. Nonetheless, it at least shows a willingness to take account of domestic short-wave radio usage where the impact of the devices will be greatest.

Step up to the podium, then, BT Vision who have begun using PLT technology to connect their set top boxes into televisions (the two usually being in different rooms in the house) through the electrical wiring. However the devices they are using were developed prior to the completion of the ETSI standards and do not have the sensing mechanisms in place. One device in particular, made by Comtrend (Model DH-10 PFUK to be specific). It provides a 200 Mbps connection between sockets and can be bought independent of BT from various suppliers.

noisy noiseThis device (and some others like it) have started to cause enormous headaches to domestic HF reception, both amateur and broadcast. You will find various videos on YouTube demonstrating the problem and a Yahoo! group (UKQRM) has been set up for users to share experiences with these devices. What is encouraging is that Ofcom is taking the situation seriously. One radio amateur reports that after having reported the problems to Ofcom, action was taken and the offending devices replaced with alternative ones which seemed to cure the problem.

Driving around the town where I live, I have noted three of these devices, each blocking out reception right the way across the HF frequency bands over a range of around 100 metres. Thankfully none of them are in the immediate vicinity of my own station but the threat exists. If you are suffering this kind of interference, I would urge to you take a look at the UK QRM site or contact Ofcom.

Apparently BT are aware of this problem, with the specific units in question, and will replace them if contacted by their subscriber. This, however, requires anyone who is affected to determine where the interfering signal is coming from and then speak to the person whose house the equipment is installed it. If this is a friendly neighbour, you are fine. If it is not, then your only recourse is through Ofcom. Bon chance!
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Hang Fire a Minutesignal strength
Saturday 26 July, 2008, 16:01 - Radio Randomness
Posted by Administrator
Watching a number of hot air balloons float gracefully past recently, the question arose as to whether or not they used radio to communicate with the ground and, indeed, whether they used radios to communicate with air traffic controllers. A bit of digging around revealed that they do indeed carry aeronautical radios with them, and what's more they have a special 'balloon to ground' frequency. Not just that, but the same is also true of gliders, parachutists, microlights and hang- and para-gliders.

Below are all the commomn frequencies (in MHz) which are set aside, in the UK, exclusively for these special purposes, including a frequency set aside for fire services at airfields:

120.900, 130.100, 130.125 and 130.400 Gliders
129.975 Gliders (to mobile field units)
122.475 Hot Air Balloons
130.525 and 129.900 Parachutes
129.825 Microlights
118.675 Hang/Paragliders
121.600 Ground Fire Services

hang glider interferenceThe very observant amongst you may note that the frequency for Hang/Paragliders is just 25 kHz away from the Heathrow Airport (arrivals) tower frequency of 118.700 MHz. Typically, frequencies this close are not used in the same area so that any Hangglider flying near to Heathrow may cause disruption to the tower communications (and indeed the governing body for aero frequencies, ICAO, does not allow such clashes). The exceptionally observant amongst you will realise that if a hangglider got close enough to Heathrow airport to cause interference, it would have greater things to worry about than radio! So by simple spatial collision avoidance, interference is likewise stopped.
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