Wireless Waffle - A whole spectrum of radio related rubbish
Wire-More LAN (Part V)signal strength
Thursday 26 June, 2008, 06:51 - Radio Randomness
Posted by Administrator
Another long train journey, another opportunity to run Netstumbler to find out what is going on in the world of WiFi. On this journey 425 WiFi (802.11b and 802.11g) access points were identified. No 802.11a (5 GHz) access points were spotted. Of these, 24% were not encrypted or protected in any way (however, some of these are public access points such as 'BT OpenZone' and so would not be expected to be encrypted). This means that the proportion of home WiFi nodes that are now protected has gone up significantly since the last time this was tested.

The graph below shows, in orange, the number of times that each channel was found to be in use. As expected, channels 1, 6 and 11 were the most commonly used, with channel 1 winning out over channel 11 in terms of usage for a change.

wifi channel usage

What the graph also shows is the combined interference effect of all the nodes across the whole WiFi frequency band. This has been done by adding the use of a channel with (5-n)/5 times the use of the frequency n channels away. So a channel 3 frequencies away from the channel in question would add 2/5ths of its interference to the total combined effect. This takes account of the OFDM nature of WiFi and thus the fact that the less two channels overlap, the lower the interference problem.

What's interesting about this result is that interfernece remains fairly constant across the whole band, with the exception of channels 12 and 13. This is perhaps not surprising as, channel 9, for example, will suffer 3/5ths of the interference in busy channel 11, and 2/5 of the interference in busy channel 6. Only when we get above channel 11 does interference fall as only one busy channel (11) is now adding to the combined interference 'mush'.

The frequencies above channel 13 (i.e. above 2483.5 MHz) are not used for WiFi and although no account has been taken of usage above (or below) the band, if usage is not widespread (at present it is reserved for an as yet non operational satellite system), then the overall level of interference in channels 12 and 13 will, indeed, be lower.
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Frequency Sharpenersignal strength
Tuesday 1 April, 2008, 10:24 - Radio Randomness
Posted by Administrator
frequency sharpenerYou've no doubt heard of pencil sharpeners and knife sharpeners, both designed to ensure the maximum of performance from the devices on which they operate. Well, on April 1 this year, researchers at the University of Al Tayr, in Tripoli, Libya released details of the results of a 5 year study that they have been conducting in which they believe they have discovered the world's first 'frequency sharpener'. Details are somewhat scant but if the device can deliver the kind of results that have been quoted in the research, the frequency sharpener could be being included in many new digital radios of the future as soon April of next year.

In the University's press release, Prof. Ali Lo says of the frequency sharpener:
With weak signals, it is like a thousand camels standing through your receiver to blunt the sound. The frequency sharpener acts between these blunt and dull edges using a special digital encoded Saif Al Nisan algorithm that swipes away and cuts off the noise to leave only clear oasis of surrounding signal.

The release is rather thin on technical details of how the sharpener functions but goes on to claim:
Tests in laboratory of testing have shown, masha'allah, that 12 times improvement in frequency sharpness can be made to receive signal with almost no hump blockage in background or foreground. Works of programme suggest that remaining disturbance no worse than size of golden sand rabbits of similar proportion. By development, frequency sharpener will deliver 20 to 25 times decrease in annoyance of radio camel noise for new design of radio receiver in Arabian region. Better results with Saif Al Nisan algorithm than traditional threshold extension or synchronous detection methods employed before this discovery.

desert radioWe've sent an e-mail to the professor to see whether he could provide more information on how the frequency sharpener works, however from reading between the lines of the press release, we have been able to piece together how we think the device works:
  • The sharpener first stores ambient noise received when the wanted signal is gone, or from areas around the wanted signal.
  • When required, this noise can be metabolised to act as a source of 'anti-noise' (similar to noise cancelling headphones) which cancels out the incoming 'live' received noise.
  • This yields both a reduction in noise and, as a bi-product, generates spare energy that can be used to boost the wanted signal.
  • The process of noise metabolisation generates a net reduction in both the stored and received noise, as well as amplifying the wanted signal.
  • The system though, if it works as we suspect, requires the noise store to be 'topped up' from time-to-time. Once the noise store is empty (for example, if there is no longer noise on the received signal), the device returns to the normal state with no noise reduction taking place.
This fascinating development is something we, at Wireless Waffle are keen to keep an eye on and will bring you updates as we get them.
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Spot the HF Antennasignal strength
Tuesday 11 September, 2007, 14:07 - Radio Randomness
Posted by Administrator
OK, so it's not as snappy as 'spot the ball' but the idea is the same. Look at the picture below and see if you can spot the HF antenna. Clue: Look for the 'x's.

admiralty

A bit silly perhaps, but the thing of interest is that this building is the Admiralty Building as seen from Horseguards Parade in Central London and that those really are HF antennas (see the expanded picture below). Whilst it's easy to dismiss good old fashioned short wave communications as outdated, especially in the age of satellites and mobile phones, it's very satisfying to see that those who need some assuredness of communication think it worthwhile to defile historic buildings in the centre of an area of tranquility and beauty with whopping great big, ugly HF aerials.

admiralty antennas

It doesn't necessarily follow, but it might be fair to presume that the people who did this (let's call them the 'military' for want of a better word) will be keen to ensure continued, low-interference access, to HF spectrum for some time to come. Which has to be good news for short-wave listeners and radio amateurs alike.

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Capture Area - Take 1signal strength
Tuesday 31 July, 2007, 15:26 - Radio Randomness
Anyone who really knows their stuff when it comes to radio propagation and stuff like that will tell you that any aerial (or an antenna for that matter) has a certain capture area.
capture areaIts capture area defines, in effect, how much of the signal that is in the ether it can capture and thus present to an attached receiver. The bigger the aerial, the greater its capture area and the stronger the received signal. Obvious really.

For some antennas, such as satellite dishes, the capture area is relatively obvious. A satellite dish which covers an area of 1 square metre has a capture area of roughly 1 square metre (ignoring edge effects). For linear aerials, however, the capture area is less obvious. Which has the greatest gain - a 4 element yagi or a 4 wavelength long colinear? Not so easy is it. However, it seemed to me that there ought to be a simple rule-of-thumb which allowed simple comparison between antennas. I therefore hypothesise that:

* The gain of a linear aerial ought to be somehow related to the amount (length) of metal it represents. Thus if you add up the length of the elements on a yagi and stretch them out into a colinear, the two should have the same gain.
* A doubling of the amount (length) of metal should double the overall gain (i.e. increase it by 3dB).

For now, I'll leave my first hypothesis and concentrate on trying to test the second one. Taking the quoted gain figures for over 20 amateur radio colinear antennas I've plotted their length in multiples of half a wavelength against their gain (in dBi). The length is done logarithmically, so '1' represents a length of half a wavelength, '2' represents one wavelength, '3' represents 2 wavelengths, '4' represents 4 wavelengths, '5' represents 8 wavelengths and so on, doubling in length for each increase in the index. Some of the antennas are multi-band (and thus might represent a compromise of gain balanced across more than one band), some are single-band.

antennagain

The dotted line represents my hypothesis - i.e. that the gain rises by 3dB for each doubling of length. The solid line represents a 'best fit' line. As the antenna gets long (in terms of wavelengths) there is a noticable drop in gain with respect to my hypothesis. It is probable that as the antennas become long, the losses in the 'metal' itself cause a fall away from the my theoretical figure. Alternatively my hypothesis is incorrect. Either way, the original premise isn't that far off!

Next job is to see whether my hypothesis about the total length of elements of a yagi and the length of 'wire' in a a colinear holds any water.
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