last updated 16 May 2006Range - they may advertise these radios as having a 3km range ('up to' may be slightly more honest) but the simple truth is that the range depends upon what is in the way. You could get 100km from a mountain top at one extreme, or you could stand right above a cave and not be able to hear someone from inside.
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Really trying (!) : miles Theoretical maximum 667 approx! High altitude, line of sight 125 or more Freak conditions past the horizon 100 or more? Normal use, ground level : Outside, clear flat terrain 1.5 - 3 Suburban neighborhoods 1 - 1.5 Urban areas & car to car 0.5 - 1 Inside buildings 0.5 or 5 floors Between buildings 0.12- 0.5 (1/8 - 1/2) Woodlands, level, moderate vegetation 1 - 1.5 Woodlands, hilly, thick vegetation 0.25- 1 |
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Unfortunately, asking "How far will my radio go?" is as vague a question as "How far can I
flash a message with my torch?". On a large flat piece of land (on a clear night of course)
your torch can be seen for miles. From a high vantage point on a distant (visible) mountain
you could detect a good torch being flashed right at you from tens of miles away, even if a telescope is required.
Then again, within a house you may only see a torch from the next room if the door is open.
Obviously RADIO gets through things a lot more than LIGHT does, but they are essentially the same thing
(electromagnetic waves), only the frequency/wavelength is vastly different.
The key to range then, is how much 'stuff' is in the way, with the additional possibilties of
reflections too - even if the door is closed, the house across the road may have a nice
big window in just the right place for your torch beam!
The main thing that will stop your signal reaching another distant radio is the ground itself between you - is there higher ground in the way? If there's a hill between you then it's going to be tricky, no matter what (this also includes the problem of the curvature of the Earth). If you're on a hill and can actually see the area you're trying to reach, things will generally work VERY well, even at considerable distance. The other possibility is generally level conditions - this is when the amount of buildings and vegetation between you is the main limiting factor. Even low power radios like PMR446 have the potential to work at staggering distances of hundreds of miles - *IF* there's absolutely NOTHING in the way. This ought to show you how it's the 'amount of stuff in the way' that really limits the range. There's a section on the Technical Page titled 'More power won't help as much as some say!' which explains how the range is limited by this 'amount of stuff in the way' PER UNIT OF DISTANCE (i.e. so much loss per 100m, etc), and also how the curvature of the Earth limits the distance beyond a certain limit (fairly small at ground level!) anyway. That section attempts to show how boosting the power 10 times (from 500mW to 5W) will NOT give you ten times the range. At most it would IN THEORY (only in space!) increase your range by 3.16 times - in the same way as trying to flash a torch up at the space shuttle at night bears little relation to what happens down here. It would simply give you another 10dB of power, which may not help as much as you think - if you're struggling to manage with a loss of 140dB over just one mile then that extra 10dB will give you what? About another 100 yards to play with?! If comparing radio with light, bear in mind that a lot of things that we cannot see through are effectively made of clear perspex to UHF radio waves, or are cloudier (opaque), or shiny and mirrored. Imagine torch flashing on a very dark night in an odd and ethereal ice world that is very bright by day, where most objects are either transparent, frosted or very shiny. That is how I imagine UHF works around OUR world - even if you cannot see the direct beam (or receive a direct freespace radio wave) you can still detect a signal in other ways. Even if you cannot see the torch itself, you can see if it has lit up another object or not, or whether it's lighting the haze of the moisture content in the sky above. In the torch example there may be some areas where there are brightly lit spots amongst dark shadows, and similarly with UHF if you're relying upon reflected paths you may find reception varies wildly as you move. (see the section "get the best out of handheld radios" on the 'Calling' page) The 70cm range (400-470MHz) fairs quite well in cities and is well used for mobile applications - at this frequency many buildings are effectively reflective or partly transparent. This is less noticeable on lower bands (VHF). Out in the countryside however, UHF is not so useful. Vegetation is the culprit, or in other words trees. We know water conducts electricity, right? A tree is a whole load of moisture sticking up in the air with quite an effective connection to ground! To radio waves, a load of trees is like a grey hazy cloud - some waves may get through but nothing will be reflected - dark grey to UHF and somewhat lighter to lower frequencys where the longer wavelengths penetrate more easily. A great article worth reading www.thetravelinsider.info/2003/0627.htm Range Testing : Even on your own, you can test your range. Placing your 446 up high in your loft, you can walk/cycle/drive around you local area and see how good your home coverage is. This is usually very educational! Base Receive method : One of the most simple ways to test your range, is to rig up a recording device at home, and drive around making test transmissions stating your location (this is the legal method for radio amateurs too - we can make test transmissions not aimed at any other stations in particular, and you may be able to kill two birds with one stone by making test calls on different bands from each location). A cheap VOX operated tape recorder will do nicely. If you can connect to your PC, there are Voice operated recording programs that you can run - when you return home you can easily listen to each transmission and judge the signal quality by the level of background noise. If you've got a PC controlled scanner that records signal strengths that's even better. The only disadvantage of all this is that you have to wait until you get back home to get your results, and you don't get such a 'feel' for the coverage out and about. Plus, if someone else starts using the same channel it'll mess things up a bit! Base Transmit methods : Radio amateurs aren't supposed to make unattended test transmissions, but we can on 446. You could engineer a way to transmit constantly (complicated equipment like rubber bands holding the PTT down) but your batteries won't last long. Even if you hook up the radio to a mains power supply, you may rightly be nervous about the radio overheating if you're going to be having a good drive around for an hour or more. Here's how to transmit intermittantly - a blast for 5 seconds every minute won't melt your radio, and will still allow you to assess your coverage while you're out mobile. You will need to use VOX, and find a way to feed in some audio for a few seconds followed by a minute's silence, repeating until you get back home. One way is to make a suitable track on the PC, and set the WAV/MP3 player (i.e. WinAmp) to repeat the track over and over. The line out may be too loud to connect to the radio's vox mic socket, but it should be possible to lower the output level to the point where the sound is ok and the silence doesn't trigger the vox. Perhaps use an audio attenuator, like the eQSO crowd do (variable resistor to lower the levels). If you can't connect the PC easily, you may be able to use a portable CD player, or Minidisc etc. - anything that can repeat tracks. If you make a MD or CD, you might want to record several tracks - each one a different call tone followed by a different length of silence. Put the player in repeat random mode, and you'll never know what's coming next or when - it'll be much harder for anyone else to figure out what's going on! (make sure there are no other full length audio tracks on the same disc!) A 446 and portable CD player fits nicely into a small box, and therefore it's portable enough that you can fun with it in other locations too... test the range from your mate's house, or office, etc., maybe even a parked car up on a high hill while you go for a walk. Perhaps record a beacon message for us all to listen for, giving details of a throw-away email address that people could report reception to. Transponder method : Make your radio answer you back! There are many toys and novelties that play a tune or audio sample when a loud noise triggers them. It should be possible to rig up a vox mic/earpiece headset. If you can find where the toy's microphone is, place the vox earpiece next to it. When you transmit a whistle from down the road, the audio received by the radio will trigger the toy into playing its tune. The noise this makes then triggers the radio to transmit for as long as the toy does its thing! It might drive you mad listening to a cheesy Christmas tune over and over, but it does the job :o) The alternative solution involves setting up a simplex repeater. This is a device which records incoming messages, and plays them back afterwards. You transmit a message, and when you stop transmitting you hear it echo the whole message back at you. These are available as standalone units, but it may be easier to use a PC to do it. Try http://perso.club-internet.fr/f1orl/simplexg.htm or http://www.synergenics.com/sc/ You'll get more instant results with the methods that transmit from home, and you'll see just how reception is affected by distance and the amount of buildings/trees/earth in the way. If you've a distant hill that you can see from home, you'll appreciate how much a line of sight path helps, when you can hear your radio well from miles away. If your mobile receiver has an accurate signal meter, this can help you assess different radio positions at home, the improvement that height makes, coupled antennas, etc. All good clean fun! You may be pleasantly surprised by the range and coverage you'd get from up high in your loft to other high positions. Looking back at my CB days, many breakers were using poor antennas on their cars in the driveway, or other lousy twigs poorly located with dubious coax, low down in built up or hilly areas. CB was capable of decent range, but in practice most of the breakers I knew were within a mile of me, about a quarter of them from further afar but hardly ever much more than 3 miles (my location wasn't great). There were so many breakers, so close, that more distant signals were walked over anyway. In many areas, the sheer level of activity on all channels from all distant directions meant that there was a constant S5 mush, which limited range anyway. Given the low range on CB for these reasons, I'm convinced that a 446 in the old loft would make the trip to many of them if they did the same thing - in the area mostly at the same height as my old gaff (although contacts 'over the hill' wouldn't work as 'well' as 27MHz did). If you must compare 27 and 446, at least put the antennas in similar places. The unfavourable comparisons are usually due to CB being mostly from almost decent mobile antennas or highly placed homebases, whereas 446s are handportable. Even things out a bit with placements and 446 looks a lot better. Please take a look at the DX and Records page!
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