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Indoor shortwave viability?

tomizett

Active Member
I don't generally come in here, but I'm worried that I might be HAM-curious... I need talking out of it; I already start more project then I finish and I don't need another avenue of distraction.

But supposing for a moment that I was serially interested in getting into amateur radio. I live in a first floor flat, part of a larger building, which has an attic/loft space - I have no outside space - in a fairly elevated location in the south-west of England, with buildings of similar size nearby. I reckon I might have 10m corner-to-corner in the loft.
My question is, then, is there any chance of erecting a worthwhile antenna inside the loft?
I'm interested in building radios rather than contesting etc, so it's shortwave that I'm thinking of - VHF is far beyond my capabilities as a designer. I'm also not talking about running high powers - 10W would probably be all.

Given the space constraints I might be restricted to the 10m band, but it looks like not a lot happens there. 20m looks like the place to be, but how much luck might I have if I can't get a full 1/2 wave dipole up?

Just interested in hearing some peoples views.

I should add that, no, I'm not a license holder; that would be step 1 if it looked like there was any point in trying.
 

RadioRon

Well-Known Member
You have the lack of space working against you. There is a bigger problem than that though. This is that your electronic devices and your neighbour's electronic devices will be very close to your antenna, so the HF noise that they generate will interfere with your operation. It is difficult to say how bad this latter problem will be but in my experience it can be a real tough one.

For your antenna, I would recommend a magnetic loop suitable for transmitting and receiving. There are many for sale, and you can build your own. The idea is that the main element is a loop of pipe as big as you can make it with a diameter of 1m being practical. You have to feed this loop with another smaller pipe loop mounted within the circle of this large loop (kind of like a transformer with one turn on primary and one turn on secondary). You feed the smaller loop with your cable. The larger loop is typically resonated with a variable capacitor. This type of antenna is very high Q, meaning a narrow bandwidth, and probably has to be tuned a bit even within the 40 meter band. On 20m, it might not have to be tuned within the band. Have a look through the ham literature to see examples of these antennas available for retail purchase.

The magnetic loop has some advantages that may help you. Since it tends to be insensitive to electric fields, it is often good at rejecting nearby man-made interference, which is often dominated by electric field rather than magnetic field, at least until you get really close to the interference source.

Another problem that a mag loop antenna may help with is in reducing the coupling to your neighbour's hi-fi equipment and other electronic devices, although this is dependent on so many things. Such coupling will cause you grief when they come pounding on your door and demanding that you quit transmitting. Your neighbours will not care a hoot about any logical arguments that claim it is their equipment that is at fault. So, this can be difficult to deal with in an apartment building.
 

tomizett

Active Member
Hi Ron, thanks for the input.
As I've alluded, I'm pretty ignorant about radio (which is the main reason why I'm toying with getting into it) and your rather obvious point about the amount of interference in the vacinity of the building hadn't crossed my mind.
I appreciate the pointer towards magnetic loops - I'd never heard of them before and will certainly look into them.
I suspect the best plan of action might be to get some kind of antenna up and equip myself with a suitable receiver and see whether I can get anything at all, before dedicating too much to the project.
 

RadioRon

Well-Known Member
Yes, it is always best to just try something simple and basic to start with, to see how it goes. Perhaps a thin (invisible) wire hung from a balcony railing?
 

tomizett

Active Member
One thing I'd like to ask about. Ron says:
...man-made interference, which is often dominated by electric field rather than magnetic field, at least until you get really close to the interference source.
My understanding is that a propagating radio wave has electric and magnetic fields in a fixed ratio, which "re-generate" each other as they travel - that's my rather crude way of visualising it.
If the electric field dominates, is this because we are in the near-field of the transmitter? ie, the electric field we're seeing is not part of a propagating wave?
And, if that's the case, would we expect the interference to drop off more rapidly than the inverse-square that we'd expect in the far-field, as we moved the receiver from the near- to far-field?
 

RadioRon

Well-Known Member
I'm a bit rusty on the theory, but you are right about the presence of both electric and magnetic fields in a propagating EM wave. My comment refers to my understanding that the majority of our interference from household devices can be near field when we are as close to them as you would be. With near fields, there is no consistent relationship between electric and magnetic field amplitudes and I believe I read that, while the typical realistic situation is very complex, with a mix of stuff from many sources, that there tends to be more electric field noise than magnetic field. I hope to hear from our other members with more authoritative info on this.
 

tomizett

Active Member
That makes sense to me, and certainly sounds quite feasible. If we're talking about, say, 80m, then I suppose the "near" field is going to be quite large!
 

JimB

Super Moderator
Most Helpful Member
I reckon I might have 10m corner-to-corner in the loft.
My question is, then, is there any chance of erecting a worthwhile antenna inside the loft?
Yes is the quick answer.
Some kind of a balanced antenna such as a dipole or a loop would probably be best.

Also consider that a dipole does not have to be in a straight line, it can bend about a bit to fit more wire in the available space.

But as Ron comments, your first big problem could be all the noise from the multitude of electronic devices very few of which are compliant with EMC legislation.

Having said all that, the best thing to do would be to beg/borrow a receiver, put up an antenna and have a listen, all the theorising from the four corners of the earth cannot predict your exact situation.

JimB
 

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