Saturday 26 February, 2011, 02:18 - Amateur RadioOn a number of previous occasions, Wireless Waffle has commented on actions being taken by various regulatory authorities which seem to be attacking the use of the 70 centimetre band by radio amateurs. But in the USA, things have just gotten a whole lot worse, with the tabling of a bill which suggests that two thirds of the (admittedly large) US 70cm allocation be given over to broadband services for first responders (or the emergency or blue light services as we call them over here).
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The bill, snappily entitled 'A bill to enhance public safety by making more spectrum available to public safety agencies, to facilitate the development of a wireless public safety broadband network, to provide standards for the spectrum needs of public safety agencies, and for other purposes.' or 'Broadband for First Responders Act of 2011' for short (herinafter referred to as 'the six million dollar bill') states, in section 207, subsection (d):
Not later than 10 years after the date of enactment of this Act, the paired electromagnetic spectrum bands of 420–440 megahertz and 450–470 megahertz recovered as a result of the report and order required under subsection (c) shall be auctioned off by the Federal Communications Commission through a system of competitive bidding meeting the requirements of section 309 of the Communications Act of 1934.
The spectrum referred to in the aforementioned subsection (c) and in section 207, subsection (a) appears to refer to that which is freed by the cessation of use by public sector users who should migrate to the 700 and 800 MHz bands in the interim. However, there is little, if any, use of the frequency range 420 to 440 MHz by these users in the first place, as it is part of the amateur band. Indeed, according to the United Stated Frequency Allocations the band is only to be used by either radio amateurs (on a secondary basis), or by Government radiolocation services (eg the PAVE PAWS radar installations at the Clear, Beale and Cape Cod air-force bases). So, if taken literally, the frequencies in the range 420 to 440 MHz which would become available are - none, because none of them are used and thus none have been cleared! However, things are never that straightforward and it could be argued conversely that the fact that the first responders are not using that range of frequencies is a clear indication that they have cleared out of the band!
Either way, this assault on the 70cms band might be time for US amateurs to think carefully and propose something in their mutual interest. North of the border in Canada, the 70cm band stretches only from 430 to 450 MHz and in most of the rest of the world it is only 430 to 440 MHz (and as indicated before, there were moves afoot in Europe to further constrict this to 432 to 438 MHz). It may be, therefore, that agreeing a reduction in the US 70cm band to be in line with Canada at 430 to 450 MHz whilst in return guaranteeing some certainty of tenure would be a good way ahead. In Europe, for example, a reduction to 432 to 438 MHz in return for clearing out all those annoying low power devices around 433 and 434 MHz and also guaranteeing primary status for amateurs in the band would be a fair compromise. Or what about making the 70cm band 430-433 and 435-440 MHz, thereby leaving the low power devices to wallow in their own crapulence, sure this would require the re-tuning of many repeaters in the UK and elsewhere, but this is already having to be done in many cases due to the incoming and outgoing interference problems caused by the self-same low power devices.
In many countries around the world, the frequency range 410 to 430 MHz is used for digital mobile radio systems which support blue-light activities. In the UK some spectrum in this range is available for use by the Ambulance service, for example. This means there is equipment available. Equally the band 450 to 470 MHz is similarly used. Therefore, giving US first responders the ranges 410 to 430 and 450 to 470 MHz makes a lot of sense and for once, means that the US frequency usage would be in harmony with that of most of the rest of the world and harmonisation leads to economies of scale and cost reductions and so forth, as its protagonisist are always keen to argue.
Of course, any change in usage or allocation in a band leads to the need to re-plan, re-tune and re-think, but if the rest of the world's radio amateurs can cope with just 10 MHz of 70cm spectrum, and Canada can cope with 20 MHz, perhaps it is time for American amateurs to relinquish part of the band in the common good and figure out what they would like in return?
Thursday 13 January, 2011, 04:22 - Spectrum ManagementPreviously on Wireless Waffle we have discussed ways of checking and even gaining some knowledge of the state of propagation of the short-wave bands. But for truly advanced users, there is a way to find out the actual state of propagation for a particular location in real time. Scattered around the world are a series of ionosondes. These ionosondes are rather like radars in that they transmit a signal to the ionosphere and measure the time taken to get a response. They do this across a range of short-wave frequencies.
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The result is a chart called an ionogram. An ionogram is effectively a radar picture of the height of the ionosphere at the location immediately above the ionosonde as well as providing an indication of its refractivity, over a range of frequencies. An example ionogram taken from the ionosonde in Dourbes, Belgium, is shown below.
The ionogram is the ultimate way of assessing short-wave propagation. It tells us exactly what is going on. To help interpret the ionogram, there are also a useful set of figures provided in the diagram which give us some very useful information. So... how do we interpret the ionogram to help understand HF propagation?
In the ionogram above, the strong red/pink line extending from just below 3 MHz to just above 6 MHz shows that the ionosphere above Belgium was refracting radio signals in that frequency range straight back down again (ie at an angle of 180 degrees) - it was acting like a mirror for radio frequencies in this range. As the frequency goes above 6 MHz, the line bends upwards until eventually it goes off the top of the chart. This is the point at which the ionosphere stops refracting signals back down (at 180 degrees), however it will continue to refract signals at higher frequencies which hit it at lower angles (less than 180 degrees).
From this simple data, together with the height of the ionosphere (the scale up the left hand side of the chart) it is possible to calculate a number of very useful figures, and this is done for us.
Firstly, we have the maximum usable frequency (MUF). This is shown amongst the figures to the top right of the chart (in this case 27.62 MHz) and is also repeated at the bottom of the chart (under the label 3000 km). The MUF is the highest frequency which the ionosphere will reliably reflect radio signals. It is also the one which has the lowest refraction angle. What this means is that signals at this frequency will be refracted by the ionosphere (above Belgium in this case) but only where the path between the ends of the link hits it at a low angle, which equates to a path length of around 3000 km. Two stations, each 1500 km away from Belgium, the centre of whose path is above Belgium, will therefore be able to communicate at a frequency of 27.6 MHz. So a station in Western Ireland and one in Romania are likely to be able to communicate on this frequency. Equally one in Spain and one in Sweden might too.
The second useful frequency shown is the one shown as 'foF2' in the diagram (top right). In this example foF2 is 7.15 MHz. foF2 is the highest frequency at which the ionosphere above Belgium will refract signals at an angle of 180 degrees, ie straight back down. If you therefore want to communicate from somewhere in Belgium, to the same place in Belgium, using the ionosphere, this is the highest frequency I can use. How useful! But the best bit is the interpolations between foF2 and the MUF. These are the figures shown at the bottom of the chart under the various distances (from 100 km to 3000 km). These are the maximum frequencies I can use to communicate over the distance shown.
In this example, if my path length is 100 km, the highest frequency I can use is 7.9 MHz. If my path length is 1000 km, the highest frequency I can use is 11.7 MHz. Now this is really useful. If I want to communicate from London to Stuttgart, a distance of approximately 800 km, of which Belgium is roughly half way (in the centre of the path) the highest frequency I could use, in this instance, is 10.2 MHz.
What is the lowest frequency I could use? That is more difficult. What the diagram does tell us, however, is that for short paths, (ie from Belgium to Belgium) the ionosphere was successfully refracting signals at frequencies as low as 3 MHz. How do we know this? There is a nice red/pink reflection on the chart at this frequency. Below it, the picture becomes rather scattered indicating that the refracted signal was not reliable.
So, what can we ascertain:
- The highest frequency being refracted by the ionosphere above Belgium is around 27.6 MHz. This is the highest frequency at which two stations separated by 3000 km for whom Belgium is in the centre of their path, will be able to communicate - the MUF.
- The highest frequency which can be used to communicate from one location to the same location (in Belgium) using the ionosphere is 7.15 MHz - foF2.
- For a range of distances, we can work out the maximum frequency which can be used.
- For short paths, we can find out the lowest possible frequency being refracted by the ionosphere (around 3 MHz in this case) and thus the lowest frequency which can be used.
We can also take a stab at assessing how strongly the ionosphere is refracting. The phantom reflections shown at around 450 km height are signals which were refracted from the ionosphere, then reflected by the earth and then refracted again by the ionosphere. These phantom reflections would tend to suggest that the strength of refracted signals is particularly good, as it has been strong enough to rebound from the earth and refract again! Sometimes, three or even four phantoms can be seen, indicating very strong refractions which would suggest that short wave signals would be very strong.
The ionosondes in Europe include:
- Dourbes, Belgium
- Juliusruh, Germany
- Chilton, United Kingdom (registration is required but is free)
- Warsaw, Poland
- Rome, Italy
Saturday 25 December, 2010, 15:04 - Radio RandomnessFollowing the immense success of the SuSi as a piece of apparel for improving short-wave reception, it is an honour to have received an e-mail from Nicolas Sant of Malta who brings news of an increadible new development in the field. Nicolas takes up the story:
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Here in Malta, the summer months are very hot and the beaches are crowded with tourists. Having walked around several beaches it seems that the 'SuSi' is very popular as I have seen many women using them, as always receiving a good reception from their boyfriends and husbands and also from passers by.
The great thing about living in Malta is that it remains warm at Christmas time and it's quite possible to spend a day on the beach even in December. Me and my friends thought that it would be interesting to try and make a more festive version of the SuSi to be worn around the Christmas period. The result is what we call the Malta Universal Frequency Telereception Instrument (or Mufti for short). I hope you like it.
Thank you for this piece of festive sunshine from the Mediterranean Nicolas.
Your Mufti looks very promising, though the close proximity of the vertical struts will serve to reduce it's effective bandwidth. The positioning of the supporting structure (particularly in the upper region), covering the Mufti will also dampen it's effectiveness as the structure tends to absorb many frequencies. We suggest that the Mufti should be clear of all obstacles, and be fully visible. Do not allow the supporting structure to be allowed to cover the Mufti up in this way.
The purpose of the furry adornment on the top of the supporting structure is unclear. Is this some kind of pre-amplifier? If so, a simple repositioning of the supporting structure should increase the signal such that no additional amplification is necessary.
Finally, we do give a solid thumbs-up to the connector that you have used. It is clearly free of any oxidisation being nice and shiny and silver. This will ensure a good contact is made.
Anyhow, good work Nicolas. Do keep those pictures of your efforts at the SuSi coming in...
Friday 29 October, 2010, 16:06 - Radio RandomnessEver wondered what goes on in those increadibly high frequencies that might almost be called 'nanowave' instead of 'microwave'? Well other than a bit of use for looking at the earth from satellites (a.k.a. earth observation) the main uses tend to be military. This is partly because it becomes quite difficult (and thus expensive) to generate any kind of power at these frequencies but also because even if you do, it doesn't tend to go very far because of the poor propagation characteristics. At these frequencies, signals do not penetrate very far inside solid objects such that even the thinnest membrane will stop them dead in their tracks. Even the thin blue line of atmosphere that surrounds our fragile planet is enough to nobble extra high frequencies.
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But those clever military people realised that this ability of signals to not penetrate anything very deeply might have an application other than for radio communication, navigation or any of the other uses you normally associate with the spectrum. They realised that a microwave oven at a frequency of, say, 95 GHz, would only cook the very outside of anything that was put in it (at a depth of no more than a half a milimetre) and leave the insides untouched. So you could use it to char the skin of a red pepper (or capsicum as they are known in lesser countries) whilst leaving the flesh crispy and fresh. Or you could char a peach, leaving the juicy bit inside uncooked. Or you could fire a beam at a human and make them feel as if they were on fire without actually burning them. No, seriously, not only could you do this, but this is exactly what a new line of devices being used by the military (and some other governmental bodies) are actually doing.
Known as the 'Active Denial System' (or ADS for short), these devices were initially designed to use for dispersing unwanted crowds gathering at military establishments, enclaves, camps or hide-outs. By blasting protestors, marauders and other such types with several hundred watts of high frequency 95 GHz microwaves in a tightly focussed beam, you can make them feel as if they are on-fire by heating up the nerve endings near the surface of the skin without heating the skin itself. This makes for a pretty good deterent and they soon move away, out of the beam.
Prisons soon realised the potential of the ADS to 'gently direct' prisoners away from certain areas too. Los Angeles County prison has installed one of these devices and according to the prison chief officer, "we likes 'em California char grilled", though it is unclear whether he was referring to his prisoners or to his burgers.
Sadly, attempts to use the device to produce instant suntans failed, partly due to the excrutiating pain involved in standing in front of the beam but also because it's completely the wrong type of radiation! Silliness aside, if such a device could be reduced in size to become handheld, it might be possible to generate enough oomph to produce a 'heat ray' beam to temporarily disable miscreants in your immediate vicinity. Now we have moved from HG Wells' martian heat rays to Gene Roddenberry's phaser guns. What with Star Trek communication devices having been introduced in the 1990s, and Star Trek style tri-corder being oh-so similar to iPhones, the time is nigh for someone to develop a real-life warp engine to propel humanity into the future. By the way, whilst you're there, could you check whether our Oidar is working and send us a message backwards through time to let us know. Ta muchly.