...I've read lots of conflicting info regarding is matter. Some say it's sufficient to just feed a full wave loop with coax, directly, some say use a 4:1 balun, I've seem it written a 1:1 balun is preferrable. I guess when using this antenna on multiple bands, there are going to be compromises.
There are many topics that you'll find conflicting information about on the internet.
A good article shouldn't just tell you what to use, but explain why, so you can make
your own decisions whether those recommendations actually apply to your own
situation.
There are two basic approaches to feeding a loop for multiband operation: you can
adjust it to resonance and feed it such that it has a low SWR on multiple bands, or
just feed it with a low loss coax and use a tuner in the shack regardless of the SWR.
If you make the loop resonant on the low end of 80m then the resonances will fall
in (or near) 40m, 20m, 17m, 15m, 12m, and 10m. (The 30m resonance falls too
high, around 10.7 MHz. The 17m resonance is actually a bit low, around 17.8 MHz.)
Just because the antenna is resonant doesn't mean that the SWR is low, however.
My computer model of a 280' loop up 30' suggests the follow approximate input impedances:
50 ohm 200 ohm
band impedance SWR SWR
80m: 90 ohms 2 : 1 2 : 1
40m: 240 ohms 5 : 1 1.2 : 1
20m: 250 ohms 5 : 1 1.25 : 1
17m: 390 ohms 8 : 1 2 : 1 actual SWR will be somewhat higher
15m: 420 ohms 8 : 1 2 : 1
12m: 360 ohms 7 : 1 1.8 : 1
10m: 250 ohms 5 : 1 1.25 : 1
You can see how the SWR is lower on most bands when using a 4 : 1 balun than
with a 1 : 1 balun: good enough that you can often operate on some bands
without needing a tuner at all! But you may have to adjust the wire length
slightly to get the resonances to fall in the desired bands.
Even if you use a tuner, the lower SWR will mean lower losses in the coax.
While losses aren't generally too bad, if you are feeding the antenna with
75' of RG-8X coax you'd loose about half your power in 15m feeding the 420 ohm
impedance directly with a 8 : 1 SWR, compared to a only a quarter of your
power lost in the feedline with a 4 : 1 balun. (The actual number depends
on the length and type of coax you are using.) Not a big difference, true,
and you'll probably still make plenty of contacts either way.
So from the perspective of lower SWR, the 4 : 1 balun is preferable. The
stress on the balun is relatively low because the SWR is pretty tame.
Now, part of the complaint about a 4 : 1 balun is because most of the
ones offered (especially older designs) are
voltage baluns, rather than
current baluns. Voltage don't necessarily do as good of a job of
equalizing current in the two wires and preventing common mode currents
on the outside of the coax, but it probably isn't any worse than not using
a balun at all, and you get a better impedance match. There are
4 : 1 current baluns available, but they tend to be more of a speciality
item.
The other approach to feeding a loop is to put it up without worrying too
much about the wire length, and feed it with low-loss feedline, typically
twinlead or open wire line. Actually, 300 ohm twinlead isn't a bad choice
here, either, as the SWR at resonance (at least on the bands above 80m)
is usually better than 1.5 : 1. But if the antenna length isn't actually
resonant, the antenna will still operate, though at higher SWR. That's
why
low loss feedline is important here: the SWR can easily be
over 20 : 1 when using 50 ohm coax if the antenna isn't adjusted for
resonance, and at that point the losses in your coax become more
significant. With the same coax arrangement I used above, now you
may be getting 30% or less of your power reaching the antenna. Using
twinlead is more efficient in this case, probably around 90%.
If you are feeding a non-resonant loop, the SWR on your feedline
may be quite high, and that presents a problem for many baluns.
The 4 : 1 types built into many antenna tuners will step the impedance
down, and if it happens to be low anyway, it can affect the
efficiency of the tuner. (Tuner efficiency is generally a problem mostly
for low impedance loads on the lower bands, and sometimes on 10m.)
Common tuners can match higher impedance loads more efficiently,
which is why the are often recommended for use when the impedance
may vary over a wide range. The actual impedance will depend on both
the impedance at the antenna and the length and type of the feedline.
So if you are putting the balun directly at the feedpoint of the loop and
adjusting the loop length for minimum SWR, the 4 : 1 balun is the better
choice as it may allow you to feed the antenna without a tuner on several
bands, and it minimizes coax losses.
If you are feeding a resonant loop with 300 ohm twinlead, a 4 : 1 balun
to coax (possibly 75 ohms) isn't a bad approach, either.
If you are feeding a semi-random length loop with ladder line to a balun
at the tuner in the shack, a 1 : 1 balun may work better, as it likely will be
under greater stress / higher SWR. If you put the balun outside the house
and run coax to the rig, keep the coax as short and low loss as possible
because you can get very high SWR: it isn't uncommon to see losses as
high as 10dB in a 20' length of what otherwise would be reasonably
good coax in such an installation.
So there isn't a single simple answer: in some circumstances a 4 : 1 is
better, sometimes a 1 : 1. But if you are putting the balun at the antenna
and trimming to length, the 4 : 1 will give lower feedline losses.
Can you operate without a balun? Obviously you can if you have been
doing it this far. Will you see a big improvement adding a balun? That's
hard to say, since whether or not you have common mode currents will
depend on such factors as the length of the feedline and how your rig
is grounded, making it difficult to generalize.
