I wonder what kind of tuner you have, all of them are not created equal, some do well enough, some don't.
I'm assuming you will be using a horizontal loop? If so, you can do the matching using a 1/4 wave of 75 ohm coax, and then feed it with whatever length of 50 ohm coax to get to the radio. That makes it a single band loop, not a multi-band one. Your tuner might handle it that way on other bands but don't count on it.
The easiest way of feeding one through a tuner that can handle parallel feed line is just run the 'ladder-line' from the tuner to the antenna. Not everyone can do that for whatever reasons, so the shortest coax from the tuner to a balun (outside) and then ladder-line from there will certainly work. The problem with doing it that way is that the coax used will have some very high voltages in/on it, so has to be able to withstand those voltage levels. Since break down voltages aren't given anymore, use the shortest length of coax possible with the highest power rating possible. That coax doesn't have to be of any particular length, just as short as possible. Hang a 1:1 balun outside (on the eaves?) and run ladder-line from there.
A typical tuner should be able to make that work on bands higher than 40 meters. If not, then adjust the length of ladder line, shorter/longer, till the tuner can handle it. That's a fairly common way of doing it, the ladder-line length adjustment thingy.
There are some 'catches' when doing it this way. When using a loop as a multiband antenna (any other kind of antenna too) the resulting input impedance will be all over the place, high/low/etc. That means that having a tuner with a fairly broad matching range and kigher power rating is a must. If the tuner doesn't have a balun to start with then use a 1:1 rather than a 4:1. A balun is 'directional' in that if may change impedance in a 4:1 ratio, but you have to know which way it has to do that, up or down. See how that can be a problem? Let the tuner do the transformation, don't count on the balun in that respect.
A 'trick' with ladder-line to keep it from flexing enough to destroy it's self is just to 'twist' it. Make it into a spiral, one twist to every couple of feet, sort of. It'll still move around in the wind, but not nearly as much as a 'straight' run of it. Another 'trick' is to use the highest impedance rated ladder-line as possible, 450 ohm, 600 ohm etc. The gimmick with that is that the higher impedance rated stuff is usually heavier wire, and because of it's high impedance there's usually less voltage/current going through it, it won't destroy it's self. That isn't a biggy at all unless you run gobs of power. The common 450 ohm stuff will last you a long time and handle anything you can put into it, almost.
The hardest part about using parallel feed lines is that it's 'different', not all that difficult to use, but you gotta learn how to use it, it ain't coaxial cable.
Have fun.
- 'Doc
