Sunday, 7 June 2015

Attic (receiving) antenna

We used to be warned the we should avoid placing a transmitting antenna in a loft or attic as there is a danger of interaction with domestic wiring and the transmission of interference.

I recall that, but I do not remember where it came from. It may have been in the notes to the Amateur Radio (Sound) License we all were issued with. I cannot find my books at he moment, but it was drilled into us that this was a bad idea.

Having said all that, I have been watching the success which Gianfranco, IU1DZZ,  has been having with an attic fan antenna. I had some idea of making one of these covering 40 metres (which is a band I have some interest in because I know nothing practical about it) and maybe some more bands.

It looked unlikely that my WSPR signal, say maximum 5 watts, would cause any problem with the house wiring. I might even be able to use JT65 at 25 watts or so. I suspected that my 500mW and less, my usual WSPR power would be too low for noisy 40 metres.

If that was my total knowledge of the issues, I was pretty ignorant. Well, I am pretty ignorant anyway.

To begin with I needed "proof of concept", which means a simple 40m dipole. I can leave the multi-band fan antenna idea until it is working smoothly.

Gianfranco said "time to measure the attic" and he was right. I did not immediately go up to the loft, but I measured the outside of the house, reckoning that the attic would probably be the same length as the house as otherwise the rain would come in. That was a mistake.

The dimensions showed that a 40 metre dipole would fit, and so I got the bits together. I wanted to try 40m most of all. I think my lack of knowledge about 40m is because I thought it was good on dark nights in winter, and I am a lazy sod who does not like to stay up at night. But then, WSPR never gets sleepy.

My "spares" box (a.k.a. junk box) had all sorts of useful bits and pieces. One was a dipole centre which was once in the middle of the W3DZZ antenna I used for my first HF QSOs. Then there were some "dog bone" insulators and some copper wire to cut to length.

Surely that is all you need for a 40m dipole? It seems to be simple compared to many other antenna designs. If I was to buy a commercial "super duper" antenna it would cost £150. This cost more or less nothing: I bought some cup hooks to hold it up. Even the tie-wraps I was going to use were re-used ones from other projects (yep, I reopen and reuse my tie-wraps!).

I like balanced antennas, and I dislike "ground plane" types (except at VHF where you can make a sensible ground place). Verticals loaded against earth might work in some places, but not here on our sandy soil. Just personally, I do not like "off centre" fed dipoles either, and I especially dislike the "G5RV" design, but more of that another day.

I did have to clean up the dipole centre and file off quite a lot of corrosion, but that is not a big chore. This antenna was not intended for use outdoors, but it was made with that possibility in mind. It should be possible to use it outdoors with a bit more weather proofing.

Could anything be simpler than that?

So up I went into the attic to find that the 40m dipole would not fit in it. Is this how Dr Who's TARDIS gets to be bigger on the inside than the outside, because my attic is bigger on the outside than the inside?

It eventually dawned on me that I extended the house and converted an attached garage into two more rooms. The garage has a different roof, and a different attic, separated by a wall. Now I could have drilled through the wall, but I left it for now. A "proof of concept" experiment can cheat a bit, and drilling holes in walls could wait for later. So the 40m dipole was bent down for a couple of metres at each end. It would fit in along the entire length through the hole in the wall, should it work well.

Up went the dipole and a quick check with the antenna analyser showed it was resonating at about 6.5MHz with an SWR of nearly 2:1. No doubt bending it back did not help, but at least it was too long and I can fix that easily later. On 15m the same antenna was resonating too high, and I decided to abandon any ideas of using it on 15m for now.

So I connected it to the FT-817 and set the power to 2 watts to see what it could do. 5 watts would be better, but the 817 needs a fan to do that  (how I miss the Flex 1500). First tx cycle and the house burglar alarm went off. Or more accurately it put out a huge continuous scream and the readout said "WARNING TAMPER".

I was not really expecting the burglar alarm to go off, especially as it was not set. However, I had also forgotten that, as the replacement garage is separate from the house, it is connected to the alarm system by a UHF radio link. and the transceiver for the link is mounted ... in the attic. Actually about half a meter from the antenna.

The radio link is encrypted and if the receiver detects a signal which it does not recognise, it thinks someone is trying to tamper with it. It then sets off the alarm, whether the alarm was set to start with or not. I thought that the alarm was "off", but it checks for tampering 24 hours per day.

Resetting the alarm and some tests later, revealed that 1 Watt was the maximum power which did not set it off. I did not think that 1 Watt would do much on 40m but it was worth the test.

I was being heard in Europe but I then made the mistake of putting a fan behind the FT-817. It runs more efficiently when cool. The power then crept up to 1.2W, and set the burglar alarm off again. This brought questions from Mrs FVM about whether there was any danger of the alarm going off overnight, so I decided to set it to 500mW until bedtime and then just receive overnight.

Half an hour produced this result

I was not expecting to be heard in Tasmania on half a watt ...

2015-06-06 19:44  GM4FVM  7.040070  -28  0  IO85wu  +27  0.501  VK7DIK/P  QE28sf  17216  10698 

I was fairly happy with Northern Norway, so anything else was a real bonus.

OK, -28dB is a bit weak, but a VK7 is a VK7.

It seems that from a radio point of view, the concept is proved. It works. However, the presence of the burglar alarm transceiver means that I need to use the services of the inverse square law to stop interference setting off the alarm. I reckon it is not actually picking up the signal as such, so filtering will not work. I think that the receiver is being overloaded by the RF and that is causing it to register an unexpected signal and think it is being tampered with.

I could try moving the end of the antenna, but it is already too long. The best way to put distance between the antenna and the alarm radio is to move the antenna out of the attic.

But before I move it outdoors, I decided to let it run overnight and see what I could hear (no tx-ing!!!). I received more that 1750 decoded WSPR spots, including quite a few from the USA between 00:00 and 06:30.

So what have I learned from this? :-
1) certainly, a simple attic antenna will work on 40m (Gianfranco was right of course)
2) in my particular case, the burglar alarm will remain  a problem unless I convert the garage to a wired system (difficult)
3) I knew nothing about 40m
4) I still know nothing about 40m but it looks like a great band.

That is it for the moment. I may try using it again, maybe with 100mW or something like that. Whatever the power level, it would need to be low enough to prevent any accidental activation of the alarm. I do not want to wake the neighbours during the night.

I will probably try it outside when next I do some work on the 6m antenna. It should fit below the rotator. Then it will be in the clear, but to be honest, it works fine in the attic.



1 comment:

  1. Don't forget Jim that it is a two way process and you may well be troubled with receiving noise from the house wiring with a loft antenna. Working ZL on 40m will give you more of a sense of achievement than working it on 20m - it's a great band.
    73 Richard