Wireless Waffle - A whole spectrum of radio related rubbish
Make your ADSL connection more reliable!signal strength
Saturday 11 February, 2012, 12:52 - Amateur Radio, Radio Randomness
Posted by Administrator
A while ago on Wireless Waffle, I commented on the daily loss of service that was occuring on my home broadband (ADSL) connection. It seems that this struck a chord with a number of people and remains one of the most commented-on articles on the site.

The question is, 'can anything be done to fix this problem'? The answer seems to be, that for a few pounds (a few and a bit Euros, or a few and a half Dollars) you can easily make a device which will provide additional filtering on your ADSL line and help with the problem of interference on the line. This solution is not a 100% guarantee of an improvement but does help and has resulted in an enormous (almost complete) reduction in drop-outs and, to boot, an increase in connection speed on the Wireless Waffle line!

The increase in broadband connection speed is perhaps an odd outcome but this is a result of a more stable connection on the line. This is why... The equipment at the exchange sets a target 'signal to noise (s/n) threshold' for each line depending on how reliable the line is. If the line continually drops out, the target s/n is increased to provide an additional 'safety margin' to try and stop future drop outs. If the line remains stable for a while (typically a couple of weeks), the exchange notices the difference and will lower the target s/n threshold. Each change in threshold is typically 3dB (a doubling or halving of power) and results in an approximately 500 kBps change in line speed. The Wireless Waffle line was originally synchronised with an s/n threshold of 12dB and a speed of just over 2 Mbps. As a result of applying the solution about to be described below, over a month or so, the target s/n was reduced (by the exchange) to just 6dB and the connection speed is just over 3.5 Mbps! Even with only 6dB s/n the line has remained completely stable for over two months and has not dropped out again once.

So how does this miracle solution work. Well, the first step is to gather the teeth of some bats at full moon when wolves are howling. Add to this, three drops of lizard spittle and some onyx droppings. Only kidding! It's not magic it's just simple electronics!

The solution comprises two simple pieces:
  • A common mode choke which serves to remove any non-common mode signals from the line
  • A low pass filter to get rid of any noise on the line which is outside the range of the ADSL signals
Signals on an ADSL line should, in theory, be balanced, meaning that of the two wires used for the connection, the signal on one wire should be the direct opposite of the signal on the other. But there are a number of wiring and other issues which can lead to this not being the case, and in particular can lead to interference becoming present on both wires, but in an unbalanced way, or even just on one wire. Any unbalanced signal which is on both wires [u]should[/u] be filtered out by your ADSL modem but it seems that giving this an additional bit of assistance with an external 'common mode' choke can make a helpful difference.

The low pass filter just stops any signals arriving on the line which are not needed for ADSL from getting into your modem and causing havoc. This could be strong short or medium wave broadcast signals which appear after dark, or could be strong local transmissions from a nearby radio amateur. Three different filter designs are presented which have differing degrees of effectiveness and you can choose which is the best for your situation.

To make the Wireless Waffle Anti-Daily Service Loss (or WWADSL) device, you will need:

1. Two pieces of (single core) wire, around 30 cm long
2. A ferrite toroid (eg FT50-43)
3. Two capacitors (either 1nF or 470pF - see below)
4. Two inductors (either 3.3uH, 4.7uH or 6.8uH - see below)
5. Six pieces (three pairs) of 'chock block'
6. An old phone socket to ADSL modem lead

No soldering is required and all the parts are reasonably readily available.

wire twistedLet's start with the choke. This simply consists of two pieces of (single core) wire, twisted together and then threaded through a small ferrite toroid. The exact number of turns is not critical, but the type of toroid is important. You need a ferrite toroid which are usually black in colour, rather than any other sort which are distinguishable by the fact that they are generally painted a different colour (eg red or yellow). Ferrite toroids usually have names beginning 'FT', such as FT37-43 or FT50-43. The last number (eg 43) is the type of material which is the important bit. The first number is the diameter of the toroid in hundredths of an inch. The one used here is an FT50-43 which is about the right size. An FT37-43 is a bit small. Ferrite cores ending 43, 72 or 77 are ideal for this project.

Take the two pieces of wire and twist them together, all the way along their length. Wind these around the toroid to leave yourself with about 2 inches of wire (5 cm) free at each end. The result should look something like the pictures below.

bifilar toriod 1bifilar toriod 2bifilar toriod 3

toroid chock blockConnect one end of the wires into one side of a double piece of chock block, and the other into another.

Now you need to take a decision... If your line synchronises at over 4 Mbps, the chances are you might be using ADSL2+. ADSL2+ differs in that it uses frequencies on the telephone line up to 2.2 MHz, whereas ADSL and ADSL2 only use frequencies up to 1.1 MHz. There are three choices of filter:
  • The 'mild' filter will allow all possible ADSL signals through including ADSL2+ and is suitable for all lines.
  • The 'medium' filter will still allow ADSL2+ through but might cause some loss of connection speed (at the expense of greater reliability of course).
  • The 'strong' filter will not allow ADSL2+ fully through as its frequency response begins to roll-off below 2 MHz and is therefore only suited if your line is running at speeds below 4 Mbps to start with.
None of these filters will affect ADSL or ADSL2 connections. You can see the frequency response of these filters in the diagram below: mild is the green curve, medium is blue and the pink curve is the response of the strong filter.

low pass filters

capacitors and inductorsEach filter is made of two capacitors and two inductors. The table below shows which values you require, depending on your choice of filter.


Next take two of the inductors and a capacitor. On the opposite side of one of the chock blocks that has your choke connected, insert each of the inductors into one of the remaining holes and put the capacitor between the two holes. The other ends of the two inductors should now go into the final piece of chock block, and the remaining capacitor across the same chock block.

completed filterThe results should look something like the picture on the right (if you haven't spotted it yet, you can click on the pictures to see a larger one).

Finally, take the 'phone socket to ADSL box lead' that you have and cut it about 30cm from the end which goes into the ADSL modem. Strip back the shielding on both pieces, which will reveal some wires. If you have two wires, no more action is taken. If you find you have four wires, identify which are the middle two and cut off the outer ones (these aren't used). In the wire shown below the red and green wires are the inner two and the black and yellow wires can be disguarded.

Take one piece of the cable and insert the two wires into one end of your chock block construction - it doesn't matter which end. Then do the same with the remaining piece of wire, at the other end of your construction.

adsl plugadsl cablecompleted adsl filter

Bingo! You've finished. At this point it might be worth checking that there are no short-circuits. If you have a volt meter with a setting to measure resitance, check that:
  • there is a connection between the two ends of the filter; and
  • there are no short circuits across the chock blocks.
Replace the cable connecting your ADSL modem to your phone line with the new one you just made and Bob's your uncle.

It's probably worth stating, just to be certain, that this is for ADSL connections (ie that which comes into your property on traditional copper telephone lines). It won't work with mobile broadband, cable broadband or satellite broadband!

Let us know how you get on and whether the WWADSL filter helps. Perhaps you might also like to share your construction experiences or tips with others too.
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Harmonics - Putting The Record Straightsignal strength
Monday 23 January, 2012, 04:09 - Spectrum Management
Posted by Administrator
Over the years, Wireless Waffle has tried to explain and demystify many of the more esoteric technical terms and concepts used in the wireless world such as OFDM, intermodulation and even interpreting ionograms. There is one very straightforward technical concept that is so often misused that it's time the record is set straight. That concept is harmonics.

Harmonics should be the easiest concept to understand. Passing any radio (or for that matter audio) signal through anything that is not perfectly linear (and the only things that are perfectly linear are pieces of wire) will produce differing degrees of harmonics. The non-linear device will produce other things as well (such as the aforementioned intermodulation) but harmonics are probably the number one resultant.

A harmonic is simply a copy of the original signal but with it's frequency multiplied by an integer. The second harmonic is therefore the original signal but with all it's frequencies doubled.
  • The second harmonic of 1 MHz is at 2 MHz;
  • the second harmonic of 10 MHz is at 20 MHz;
  • the second harmonic of 150 MHz is 300 MHz;
you can clearly see the pattern emerging. harmonics of deep purpleMusicians will recognise the second harmonic as being an octive. The third harmonic is simliary the original frequency, but tripled. The third harmonic of 1 MHz is 3 MHz, and so forth. The n-th harmonic is the original frequency multiplied by 'n' so the 273rd harmonic of 2 MHz is... 546 MHz. You can even play the game with light (just about) as the second harmonic of deep red (almost infra-red) is deep purple (ultra-violet) which probably explains why so many wannabe rock groups use devices such as 'harmonisers' and 'harmonic sweetners'. The only rule is that 'n' has to be an integer. There is no such thing as the 'second and a halfth' harmonic, and this seems to be where the confusion arises.

As harmonics are so common, much effort is made to ensure that transmitters are filtered to remove them. A low pass filter is one which allows lower frequencies through but attenuates higher ones and is almost universally tacked onto the output of any transmitter. You would not want a high power TV transmitter on 534 MHz (UHF channel 29 in Europe) radiating strong signals at its second harmonic frequency of 1068 MHz, in the middle of the aeronautical safety band, any more than you would want an aeronautical system at 1068 MHz radiating at 2136 MHz and causing interference to 3G base stations!

mistress harmonicSo often, you will see spurious emissions from a transmitter being called 'harmonics'. Unless those emissions are on direct multiples of the main transmitter frequency they are not harmonics, but will either be intermodulation or could be caused by the transmitter squegging. Either way, the term harmonics seems to have been awarded a new meaning to encompass all spurious emissions from a transmitter. As a Wireless Waffle reader, now that you know different, any violations of use will be punished strictly and severely.

For completeness, it is worth pointing out that there are (very rarely) such things as sub-harmonics. These occur on frequencies that are integer multiples of integer fractions of the original frequency. As an example, a problem was reported by the operator of a private mobile radio system on 72.45 MHz of breakthrough from a co-sited FM broadcast station on 96.6 MHz. 72.45 is precisely three-quarters of 96.6. This rare problem was caused by the synthesiser in the transmitter which had an oscillator on 96.6 MHz which was fed into a pre-scalar that divided the signal by 4, producing an output at 24.15 MHz. This signal was rich in harmonics and due to the shoddy design of the transmitter, the third harmonic of this signal was being fed into the transmit amplifiers and appearing at the antenna output - nasty! An additional filter on the output of the FM transmitter cured the problem. It's perhaps no surprise that the company that made the transmitter in question (who won't be identified) is no-longer manufacturing them!
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Russia is the Tsar of Piratessignal strength
Thursday 19 January, 2012, 14:30 - Amateur Radio, Pirate/Clandestine
Posted by Administrator
Wireless Waffle has talked extensively about pirate radio in the past, from short-wave music stations, to Brazilian sat-jackers. But it seems that, of all the nations on the planet, the Russians hold the baton for being the biggest pirates of them all.

This story begins when reading the latest intruder report from the IARU Region 1 Monitoring System. The report indicated that there had been an intrusion into the 80 metre amateur band between 3.5 and 3.6 MHz by Russian pirate stations running AM. Now historically the Voice of Korea (the North Korean broadcaster) has been transmitting in the 80 metre band (or the 75 metre band as it's called in in North America) on 3560 kHz in AM and the immediate assumption was that these new signals couldn't possibly be Russian pirates, but must be the Voice of Korea and perhaps a few other stations trying to jam it. The IARU report, however, says that the carriers are very unstable and that the modulation is voices in Russian.

blue soldier red squareSo the only thing to do to verify this story is to turn on a receiver and have a listen. Having done this, there were no obvious signals in the 80 metre amateur band. Having previous heard pirates just below the band at around 3450 kHz, the tuning dial was slowly rotated to ever lower frequencies. Nothing. And then, at 3175 kHz, something. A weak carrier... no, two carriers alternating... both rather unstable in frequency. Switching the receiver to AM yielded weak modulation. A bit more tuning, to 3125 kHz and a much stronger AM signal with a Russian voice and a wobbly carrier. Hey presto!

But what are these odd signals? Are they military operators in a private net (if so, why AM and why unencrypted)? Are they some kind of harmonics or intermodulation? Googling didn't bring much until a page on Sparky's Web Blog was found. It seems that these are effectively the equivalent of Russian CBers but presumably using much lower frequencies given the large distances between Russian cities. The band is known as the тройка band ('troika' in English which has several meanings from 'three of a kind' to a sledge or fairground ride). The band runs from approximately 2900 to 3200 kHz which are internationally allocated to the Aeronautical Mobile and Mobile services.

red square blue squareThere are aeronautical frequency assignments in the band (2872, 2899, 2921, 2962 and 3016 are frequencies assigned to North Atlantic traffic for example), but these lower frequencies are less often used unless propagation makes it totally necessary. Oddly, the various frequency lists for the band show very little aeronautical use in Russia (other than Irkutsk on 3016 kHz) - a coincidence? Probably the pirates know this and therefore feel free to mess about in the aviation bands, knowing that the Russian authorities are likely to be little interested in their activities.

If you're in Europe, when it gets dark (and lower frequency propagation opens up over the continent), why not give them a listen. It's fun to chase the carriers up and down in frequency. If you speak Russian, perhaps you could provide some translation as to what on earth they are talking about!

P.S. You might also want to take a listen to 2920 kHz USB as this seems to be a common calling channel for the more technically adept Russian pirates.
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BBBBCC Rraaddiioo Ssccoottllaannddsignal strength
Friday 9 December, 2011, 15:17
Posted by Administrator
It seems that it's not just Rraaddiioo Nneeddeerrllaanndd that suffer from an echo on transmissions. It was a dreich day and being concerned for relatives who live in Northern Scotland, last night the Wireless Waffle receiver was tuned to BBC Radio Scotland on 810 kHz.

jenny bbc radio scotlandThe 810 kHz service is transmitted from three stations, two high powered (Burghead and Westerglen) at 100 kW a piece, and a lower powered fill-in at Redmoss at 5 kW. From the south of England, only the two higher powered stations are audible and during the evening are pretty much at similar strengths and lo! and behold, there was an annoying echo echo on the signal.

Where multiple transmitters operate on the same frequency, it is as important that the audio feeds are synchronised as it is that the transmission frequencies are. Where both can be received at similar signal strengths, any difference in frequency between the two causes an audible heterodyne (a.k.a. whistle) to be heard. If the audio is not synchronised, an echo can be heard. Any difference in audio delay between the sites should typically be kept below a few tens of milliSeconds if the resulting echo isn't to cause a loss of intelligibility.

There are many reasons why transmitters may have audio feeds that are not synchronised. For short-wave services, the feeds may be on completely different routes. This became a bigger problem for international broadcasters when short-wave feeder transmitters were replaced by satellites. For many years, the BBC World Service maintained a 24 hour English language service on 6195 kHz (these days it is used for a variety of different languages and is shared with other broadcasters too). In those halcyon days, different transmitters around the world were used to maintain the frequency and typically sites in the UK, Middle East and Singapore would share the honours. At times, however, some of these transmitters may be on at the same time. It was not uncommon for Singapore and one of the UK sites (eg Skelton, Woofferton or Rampisham) to be transmitting English language programmes at the same time. But Singapore was fed via satellite and Rampisham by direct land-line. The result - 'The news the news read by Damien read by Damien Trellis Trellis' - over most of the world inbetween the two sites. This was eventually resolved by adding in a delay to the UK feed at Bush House to bring the networks back into alignment.

It's a shame, therefore, that BBC Scotland haven't learnt the lessons of their sassenach colleagues and got their own delays sorted out. Not least, the national DAB network has to be synchronised to within microSeconds so sorting the medium wave network out ought to be child's play.

As they say in Scotland, 'Whit's fur ye'll no go by ye!'
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