Loops!
How much area do you have to put one up in? That's the biggy. From there it's merely a matter of having enough wire to put up. If you have the option, two or more times the full wave length of whatever the lowest frequency you'll be using is VERY nice to have! That typically means some 'gain' on bands higher than the design frequency/band. It also means one heluva big loop, lots of area required. But in most cases, it's worth it.
The shape of a loop, round, square, rectangular, triangular, some 'other' weird shape isn't all that important, sort of. The more area -inside- that loop the more efficient it will be. That difference in efficiency isn't going to be huge, but there is a difference. A triangular shaped loop is probably the least efficient, but the easiest to get up. That round loop is the most efficient, but the hardest to get up. A rectangular (or close to sort of rectangular) shape is probably the most common. Until that rectangular thingy gets real 'skinny', that loop ought'a work okay. When it get's real 'skinny' you ought to call it a 'folded dipole', sort of, and then things start changing rapidly.
Can you have stuff inside that loop, like a house, trees, and like that? Yes. That stuff does affect things to some extent, but probably not all that 'badly', sort of. (Tear that house down and live in a cardboard box? You gotta put priorities where they belong!)
A typical horizontal full wave loop has an input impedance somewhere around 100 - 150 ohms (give or take a little). Feeding a loop with coax can certainly be done if you use an impedance matching device of some kind (gamma? 75 ohm electrical 1/4 wave section?). That pretty well limits you to one band though. If you can, feed the thing with ladder line through a tuner and use it on any higher bands. You are going to loose some efficiency (power transfer) with any multi-band antenna like that so it's not really a big deal, sort of.
Polarity with horizontal loops depends on where you feed the thing and it's general shape. Square loops are easy to figure for polarity. A corner means vertical polarity, the center of one side means horizontal polarity. When the loop's shape isn't exactly 'regular' that polarity thingy gets more complicated. This is one of those 'generalized' thingys, meaning it just depends on a lot of factors, and is going to be different for each installation because of what's around it. So change the feed point till you find the 'sweet spot' you're looking for.
Ladder line and tuners.
Lots of misinformation about those things. Is there losses when using a tuner? Sure, but not as much as you might think. Is ladder line difficult to 'work' with? I wouldn't say it's more difficult than coax but it's certainly different. Just a different set of "do's" and "don't do's". What's the advantages? Biggest one for me is that it doesn't really matter what that SWR is, it isn't going to hurt/damage the ladder line like it can coax. If that tuner can handle whatever the impedance mismatch is, then who cares! The typical $19.95 tuner isn't gonna work in most cases, so get ready for it. 'Nuther one'a them 'bigger is better' thingys.
And lastly, tuners and ladder line don't do miracles, they have limits. Sometimes those 'limits' can be in unexpected places, not where you'd expect them to be. Most of those limits deal with the harmonic relationships between bands. Or maybe the different portions of bands?
Loops are not the best answer for -every- antenna choice/requirement. They do have some characteristics that are nice, and I've found that they will usually work fine for most of my antenna requirements. I like them! (You couldn't tell that, could you?)
- 'Doc