@Harry Bloomfield is correct. If we were playing Indoor Aerial Top Trumps, then the manufacturer's claims is always a low-scoring card.
Look at the top half of the image below. It shows a house where the owner can virtually see the transmitter from a window in their living room. Unless the transmitter was very low-powered, or the aerial really quite poor or incorrectly aligned, then chances are that you could buy any of the indoor aerials, stick it on the windowsill, and get some reasonable reception.
The lower half of the image shows a house where the living room is on the opposite side of the building, so there's no direct line-of-sight to the transmitter. This will change the signal level getting to the aerial. We call this
field strength. Since it's not possible to increase the field strength, then the only practical way of improving the chance of getting some reception is either to increase the efficiency of the aerial, or to amplify the signal. However, the outcome of the two methods is very different.
Increasing the efficiency of the aerial either means adding more metal (a bigger aerial) or tuning the aerial to the specific frequency range that the local transmitter is using. These tuned aerials are referred to as Group aerials*, but they have a design that makes them unsuitable for use in a living room or similar. They're okay for lofts though.
A living room aerial has to be compact, so regular Group aerials are out, they're just too big, but the flat mini Log Periodic type is actually pretty good as an indoor aerial. Compared to the regular amplified indoor aerials it has more metal and so generates a better quality signal in most cases.
In all cases, signal Quality is king. That's where the super-duper-high-gain-ultra-amplified retail indoor aerials suck. They're designed to look acceptable in a living room or bedroom or similar, and that compromises the amount of metal that it's possible to design in to the space. Less metal equals a poorer signal Quality. To try to compensate, they apply massive amounts of amplification, but all they succeed in doing is making a lousy signal much louder.
Where someone can get one of the highly amplified indoor aerials to work, it's more likely to be because the local field strength was good enough to run a simple mini Log Periodic, so it's
in spite of the amplification rather than because of it.
Which ever aerial type you decide to go for, have a look at your neighbours roof aerials. You're looking not only at the direction they're pointing, but also whether the aerial is horizontally or vertically aligned as it points towards the transmitter. This is called
polarisation. It's a way of making sure that the smaller relay transmitters used to fill in the dead spots don't interfere with the main transmitter signals where the two overlap.
* Technically speaking, a Log Periodic is still a Group aerial, but it covers the entire frequency spectrum and we call it a Group T aerial.
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