I just got passed my general class exam, so I'm trying to plan my first HF antenna. ...I'm looking into a horizontal loop, sized for 80m. ... A lot of what I'm seeing is causing some confusion though.
There are two basic approaches to feeding a horizontal loop. If the length of the wire is pruned to
resonance at an appropriate point (usually around 3.55 MHz, roughly 280 feet long) then it can have
a reasonably low SWR using coax feed on most HF bands. The impedance ranges from around 100 ohms
on 80m to 400 ohms on 10m, so using a 4 : 1 balun should give you 2 : 1 SWR or better at resonance.
Even at the edges of the bands the SWR rarely will be worse than 5 : 1, so coax losses are low and the
tuner requirements are not too difficult - even most internal autotuners should handle it in most cases.
Such a combination should cover at least 80, 40, 20, 17, 15 and 10m, and you might get 12m depending
on the exact wire length, but the SWR will be higher on 30m and 60m. However, to get the higher
band resonances to line up properly you have to tune it for the low end of 80m: the SWR may be over
5 : 1 at the top end of the 75m phone band, which might be a problem with a limited-range tuner.
If your primary interest is 80m you can feed it with a quarter wavelength of 75 ohm coax, which
should give a pretty good match there, at the expense of a somewhat higher SWR on the coax on
the higher bands.
The second approach is to use a low loss feedline (twinlead or open wire line, etc.) all the way from
the antenna to the tuner. In this case the tuner must have a wider operating range and should be
balanced, but the exact wire length isn't critical - the tuner adjusts for whatever wire and feedline
length you happen to use. This approach will cover all HF bands with low losses (depending on the
tuner.) If your tuner doesn't have a balanced output, you can add an external balun and a VERY
SHORT length of coax. Either a 4 : 1 or 1 : 1 balun will work here: I'd generally choose a 1 : 1
unless the wire and feedline lengths have been optimized for a 4 : 1 balun.
The latter approach is a good choice when it is difficult to adjust the antenna length, while the
former may allow the use of the antenna with a limited-range tuner, or sometimes without a tuner.
Problems may appear, however, when you try to mix these two methods by running balanced
feedline to a remote balun and a length of coax from there to the tuner. The antenna itself will
still work, of course, but you can have higher losses in the coax and in the tuner even though
the tuner will match it. Ideally such a system would be designed with a combination of wire
and feedline lengths (typically a resonant loop with a half wave of feedline on 80m) that gives
a reasonable impedance on the bands of interest.
Also, I've read that the ladder line radiates RF and essentially is part of the antenna--is that correct? I've seen lots of conflicting info on this one, so I'm pretty confused about it.
No. The ladder line is part of the feedline, not part of the antenna. If the antenna is balanced,
radiation from the line is very low. There is a strong electromagnetic field around the balanced
line that may cause coupling to other wires very close to it. While the inner conductor of coax
is shielded, connecting it to a balanced antenna can cause common mode currents to flow on the
outside which will radiate more than the equivalent open wire line. In practice, this shouldn't be
a major issue with either type of line.
If I were going to build it today I'd run the shortest length of 75 Ohm coax I could get away with to a balun, and then 450 Ohm ladder line to the feed point of the loop. Am I way off on this plan?
It depends on how short of a length you can get away with, and the matching range of your tuner.
I'd suggest a coax length of less than 1 foot. If it has to be more than, say, 5 to 6 feet, it may be
worthwhile to optimize the loop and feedline lengths to provide a reasonable impedance to the balun
to reduce losses in the coax.