Wednesday 30 August, 2006, 08:06 - Amateur RadioSomething that I found confusing when first learning about the 'black art' of radio is that there seemed to be some argument about the gain figures quoted for antennas. Take a look at this example: The gain figure quoted for this Comet GP-3 dual-band 144/432 MHz antenna is shown as 4.5 and 7.2 dB respectively for the two bands. But any 11th grade physics student will tell you that dB or 'deciBel' is just a ratio between two numbers. So a figure of 4.5 dB is meaningless unless it is 4.5 dB with respect to some reference or other - and this is where the confusion arises...
To circumvent this confusion, an additional letter or letters is usually added after the dB to indicate what the reference point is. So 'dBuV' is 'dB relative to 1 microVolt (uV)', and 'dBm' is 'dB relative to 1 milliWatt (m)' and 'dBW' is 'dB relative to 1 Watt'. So what is the reference point for our antenna gain, and why is there potential for a mix-up?
Antenna gain can be measured in 2 different ways. In it's purest form, antenna gain calculations are made with reference to an 'isotropic source' - the measure being dBi. This theoretical antenna radiates signals equally in all directions, up and down, left and right, backwards and forwards. However an isotropic source is just that - theoretical - as if one could be constructed, the maths of antenna design tells us that it would have to be infinately small. So there is another reference point, a standard half-wave dipole (as this is the most basic form of antenna) - indicated by dBd. Now dBd is, in my mind, a more sensible measure as a simple dipole has a gain of... 0 dBd. However the same antenna could be said to also have a gain of 2.15 dBi as a dipole is somewhat larger than our theoretical isotropic source and thus radiates better.
All very straightforward you might think, so what's the problem? As long as I know that I need to subtract 2.15 dB to convert from dBi to dBd then everything's hunky dory isn't it? Well, no. Click again on the link to the antenna specification. It shows the gain as 4.5 and 7.2 dB... it doesn't say whether this is dBi or dBd so in fact, the antenna could have a gain of only 2.35 and 5.05 dBd, which doesn't sound quite so good. Indeed if we check the manufacturer's web-site we discover that the quoted gain figures are, indeed, in dBi.
The upshot of all this is that when comparing manufacturers' quoted gain figures to decide whether one antenna claims to be better than another, make sure you're not comparing apples and pears by making sure that you convert all the quoted gain figures to be relative to the same reference point. It doesn't matter whether it's dBi (which manufacturers prefer as it gives a bigger number) or dBd (which 'feels' more sensible) but always be careful if the gain figure is just quoted in dB.
Wednesday 19 July, 2006, 15:56 - Amateur RadioTuning around the amateur bands during recent sporadic-E openings, I regularly stumbled across voice traffic in a slavic language on 29612.0 kHz USB (upper side band). The format of the traffic was clearly not amateur, and given this, and the fact that it is right in the middle of the relatively active 10 metre FM section of the band but was not FM made me wonder what it might be. Enjoying a mystery I managed to make a short recording of some of the traffic [304kB windows media file] at around 14:15 GMT on 17 July 2006.
Not speaking whatever language it is I posted a message on the Utility DXers Forum message board. What a great set of individuals! Within half a day I'd received a response that the most likely culprit of the signal I heard was the Russian Air Defence (the Protivovozdushnoy Oborony or PVO) who track all aircraft through their airspace giving a continuous read-out (quite literally!) of the sector quadrant and the corresponding coordinates of each aircraft.
It turns out that the PVO also run a morse code based variant of this service which has been given the designator 'M21' by Enigma 2000, the group who monitor 'number stations' such as the Lincolshire Poacher. Looking back through their records shows that a station called 'V21' was active on 30220 kHz in the past. I don't know whether V21 is the voice equivalent of M21 or not, but if it is, presumably they have now moved to 29612 kHz for the time being in a random attempt not to be intercepted. Bit of a silly exercise really given where they've plopped themselves. It makes you wonder whether military radio operators have any idea what goes on around them. To change frequency in order to maintain some kind of covertness is vaguely clever, but to have changed into the middle of a fairly active section of an amateur band is just a bit daft.
I also dropped a note to the UK representative of the IARU Region 1 Monitoring Service, who record and monitor all 'intruders' into the amateur bands. Don't know whether this will make their offender list or not, but it's nice to be on the discovery end of something rather than just following the lead of others!
Wednesday 14 June, 2006, 08:22 - Amateur RadioFor 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!
However, 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.
Installation 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!
One 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!
Tuesday 21 February, 2006, 14:26 - Amateur RadioAs 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.
However 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.