Mag loop antenna
When it comes to radio antennas, it's pretty much a case of bigger is better. The more metal you can get in
the sky, and the higher you can get it, the better. (This isn't to say that you can't have a great deal of
success with smaller, simpler antennas.)
At my home, I don't have access to an attic, garden, or roof. This makes putting up amateur radio antennas quite tricky.
After reading about mag-loop antennas, I decided to try one.
Some people make their own, but they're hard to tune and require expensive capacitors, so I decided to buy the MFJ-1788. They're not cheap, especially with the GBP-USD exchange rate, but it's a very nice antenna. It tunes from below 40 metres to over 15 metres, which is a pretty decent range. The bandwidth is small - a few kHz before you need to retune, and the efficiency isn't great on the lower frequencies, but I've made SSB contacts over thousands of miles, and a 5 watt WSPR signal of mine was received in Australia, so it does radiate effectively.
I run it horizontally polarised. The narrow bandwidth of the mag-loop also helps if you suffer from QRM.
Information about mag-loop antennas often comes with the (sensible) warning about the thousands of volts present when transmitting, but also warnings about the magnetic fields present. This puzzles me. As far as I know, magnetic fields aren't harmful to humans. MRI scanners have huge magnetic fields, but are totally fine for humans. Is there something different about the magnetic fields produced by a mag-loop antenna that make it more dangerous to humans?
Check out Tim, W3UE, who's gone one better, and made a retractable, mobile mag-loop, and installed it on his car roof!