Wireless Waffle - A whole spectrum of radio related rubbish
WiMo, WiMo, It's off to work (DX) we gosignal strength
Wednesday 14 June, 2006, 08:22 - Amateur Radio
For a while now, I've been considering how best to go about installing a decent multi-band HF antenna that would be unobtrusive but still work. I first tried a long inverted-L made of thin wire running down the garden using the house-hold central heating system as an earth (yes, yes, I know this is a no, no, but it's all I could get my hands on in the area available). It wasn't totally invisible but was largely unobtrusive. It resonated at about 2.5 MHz and with a simple ATU I could tune it to get a low SWR on all bands from 160m (the top end only) to 6m. Of course it's difficult to measure the effectiveness of such an antenna but I did manage one or two true 'DX' contacts with Hong Kong (5976 miles), Tokyo (5915 miles) and Sao Paulo (5984 miles) on bands ranging from 17 to 10 metres. The downside to the antenna was that it received rather a lot of background noise, probably because the vertical portion of the 'L' ran close to lots of IT equipment. I also discovered that next door's television was a major source of RF interference on 20 metres too!

trapped dipoleHowever, I couldn't help but feel that performance was probably not even as effective as a straightforward dipole. Having measured my loft, I realised that I only had about 9 metres of space to play with - enough for a 17 metre dipole, but not much more. After a bit of digging around I found an antenna made by WiMO of Germany, which used the fact that, below their resonant frequency, tuned 'L-C' traps become inductors and as such have the effect of electrically shortening an antenna. With clever positioning of two sets of traps, they have produced a trapped-dipole that covers 20, 15 and 10 metres in an overall, occupied space of only 8 metres. This sounded like just the thing for my loft so I ordered one. A week later it arrived.

antenna wireInstallation was a cinch, mounting the centre balun transformer from a hook at the apex of the loft, and dangling the two antenna wires over the rafters to sit as close to the roof (and thus as high) as possible. I quickly ran to the shack to see whether or not such an antenna could cope with being in such a confined space (where, I should add, there are electrical cables feeding loft lights as well as a television antenna, splitter and various down-leads and a loft ladder, all of which could de-tune the antenna). A quick tune around and I have to say I was impressed: with no tweaking at all, the antenna provided a perfect 1:1 match at 28.0, 21.05 and 14.15 MHz - a little low in frequency for SSB working - but the match is relatively wide and a little intervention from my MFL-902 'Travel Tuner' ATU quickly solved that problem (oddly, the setting to raise the resonant frequency was the same on all 3 bands...)

How does the antenna perform? At the moment it's difficult to say. I haven't really had enough time to see whether its DX performance can equal my inverted-L, but what I can say is that (a) subjectively, signals on 10, 15 and 20 metres seem a good bit stronger all round than they did before and that (b) my neighbour's television now gives S9 of noise on 20m instead of S7!

mfj 902One final note - whilst my ATU will tune the antenna on the 12, 17, 30 and even 40 metre bands, unlike the inverted-L which comprised nothing other than a piece of wire, the WiMo antenna has a 1:1 balun at the centre. This ensures a much better match with the coax feed, and means that the coax down-lead does not radiate. However it does mean that, if the feed-point impedance of the antenna is not near 50 Ohms, there is a potential for much of the power travelling up the feeder to be absorbed by the balun, rather than making it to the antenna. Thus, whilst it might be possible to tune the antenna on these bands, there is a significant danger of burning out the balun - sodon't do it!
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Free Ham for Lifesignal strength
Tuesday 21 February, 2006, 14:26 - Amateur Radio
cqAs a Valetine's Day gift to the UK Radio Amateur community, Ofcom has decided to modify its radio amateur licensing policy. From 1 October 2006, instead of having to renew licences annually (at a price of GBP15 per annum), a lifetime licence will be issued. This licence will remain valid until either it is revoked by Ofcom, surrendered by the licensee or the details (such as the station address) become invalid (at which point a new licence will be issued). Licences, however, will need to be revalidated every 5 years. And the cost for this marvellous new licence - amazingly enough - nothing! It will be free. Well, it will be free if licensees use the proposed on-line licensing facility. If you prefer a paper-based licence, a small administrative fee will be charged.

rsgbHowever in the small print of the new amateur radio licensing policy, it states that "Ofcom may allow large organisations to act as agents on behalf of amateur radio licence holders". The agent would be free to issue licences using Ofcom's licensee database and could "charge its clients for providing this service". So in theory then, the RSGB could act as a licensing agent and charge its members for their licence. In fact, any large organisation could act as an agent! Ofcom's idea is that such a service might be of use to those without internet access. Presumably you would have to agree to be licensed through the agent and not just be automatically signed up by some default, 'don't tick this box if you don't want to not sign up' type clause? That much, sadly, isn't clear.
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Over-the-Hill Radarsignal strength
Wednesday 19 October, 2005, 08:19 - Amateur Radio
Listening to the 17 and 15 metre amateur bands recently, I was puzzled by a buzzing that occasionally appeared. Centred around 21300 and 18130 kHz, the odd 50 Hz buzz spread over about 30 kHz (which almost wipes out the whole SSB section of the 17 metre band, which is only 57 kHz wide in total!)

A quick web-search and I discovered the IARU Region 1 Monitoring System (the IARU is the International Amateur Radio Union in case you were wondering) who monitor intruders in the amateur bands. To my surprise (I thought these things had died in the late 1980's) the signal is none other than an over-the-horizon (OTH) radar.

http   www mf2fm com blog  akrotiriBut to make matters worse, this particular one emanates from Cyprus and is brought to us courtesy of the British Military base at Akrotiri (see the picture on the right) - my own side! - where there have recently been riots due to the installation of new antennas...

And it seems that Cyprus is not the only one. The Russian Woodpecker is still drilling holes in the amateur bands, and has been joined by Iran who have updated their OTH radar which can apparently be heard on 14000 and 21000 kHz from time to time. There are also some civil radars that occasionally splash against the amateur bands which use HF frequencies to monitor the state of the sea (cold, wet, salty... what more do you need to know?)

The real surprise in all this is that with modern day spy satellites and other surveillance mechanisms, it's amazing that such primitive technology is still useful. Apparently the radars are only used in times of heightened security risk but what with the atrocities of 9/11 in the US (and the UK's much smaller 7/11 bombings in London) I guess risk is high again. Shame. And shame on the British Military for causing all that nasty interference!

Postscript: I was (cough, ahem) checking the frequencies used by the RAF for 'Architect', their HF communication network and lo and behold up cropped the Cyprus radar centred on about 9040 kHz, so they are 'crapping in their own back yard' too!
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