Sounds great, doesn't it? There's always a nod to the fact that an outside antenna will be much better, of course. But just how much worse could an antenna isolated from the big world outside by a few millimetres of slate or tile actually be? Surely, it's worth a try if total failure is the only other option.
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| What could be simpler? Two bits of wire and a feeder into the loft. |
So, one wild, wet June morning, I set off with dipole centre, wire and some string betwixt my teeth into the loft. Working in a loft is never comfortable, especially when it's covered in mineral wool insulation, which quickly smothers you in a fine dust of glass. You've also a good chance of going through the ceiling, so good life and household insurances are prerequisites. Already, the whole thing looks much less attractive.
I had a spare set of 20m dipole quarter wave legs in the junk box, and these were strung up, standing well clear of the roof timber work to avoid any flashing over and consequent fire risk. Our 19th century rebuilt cottage is just about long enough to accommodate this dipole. Make note that our roof covering is Welsh slate, noted elsewhere for its poor compatibility with loft antennas.
Connecting up, I thought I would probably hear quite a bit more noise, and that signals would be down by maybe 2-3 'S' points.
The reality was rather worse. Firstly, getting the dipole to match with a reasonably competent ATU proved impossible. OK, an ATU isn't necessary for a dipole cut properly, but going up and down to trim is a real nightmare and so it's just much simpler, especially for initial testing. Twin wire feed would probably have managed a better match, but I didn't have any spare, so that was that.
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| Yes, this wire on the lawn is a big compromise but, unlike the loft dipole, will give you reasonable results. Image: W4CNG |
Not that it was worth pursuing twin feed. The noise was not too bad, but the signal strength was pathetic. Indeed, it was to all purposes non-existent. The only thing I could hear was a single Italian station at S5, who would be about 59+20dB on anything else, including a simple end-fed vertical with no radials. Indeed, I tried a horizontal end-fed coupled to a 9:1 unun in the loft, too, and although a bit better on signals and reaching a far better match, was still useless in practice.
So on that single, scientifically dubious assessment, the loft dipole - saviour, it is claimed by the books, of all antenna-restricted hams - came in at a whopping 44dB down on any bit of wire I would care to string up outside. If you're not getting the message yet, this means that the slate-roofed loft dipole is as good as useless. Other roofing materials seem to be less harsh on hams.
I should also mention that the same dipole has been used to great effect in an inverted vee configuration whilst portable, so there's nothing wrong with the wire itself!
My advice? If there's any way at all you can get any sort of non-ideal wire outside, do so. The loft is simply useless, even from this electrically quiet, rural area. Even a 'grasswire' antenna is infinitely better than a dipole in the loft, and is much more interesting to try and understand. In fact, I recently tried to see if I could make use of the lead flashing on my house as an antenna, which, although very poor, was just about making it to first skip distance, so again, much, much better than the loft dipole.
All I can say about many of the books and other sources of reassurance about loft antennas that I've read can only be endlessly and mindlessly repeating what someone, somewhere once decided was worthwhile information. In my experience, loft antennas are absolutely worthless, certainly as far as a slate covered roof is concerned. After all, there's a reason why WW2 spies in France and elsewhere risked their very lives by stringing their antennas outside: they just didn't work inside!
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