How is Gain and other 'effectiveness factors' of single-wire long wire antenna, with antenna tuner (ICOM AH-4 type), compare with single frequency or multiple frequencies dipole HF antenna?
What is practical range of length of long-wire, for fixed home station?
Gain and radiation pattern are both a function of the wire length in wavelengths.
A half wave end-fed wire has a broadside pattern, just as if it was fed in the center as a
dipole. A full wavelength has major lobes at about 45 degrees to the wire (all around
the wire) and a null broadside to it. As the wavelength gets shorter (frequency is higher)
maximum radiation will shift more towards the end of the wire, with various nulls and
weaker lobes in other directions. That assumes the wire is horizontal.
A sloping wire can give some gain if you align the angle of radiation from the wire
for that frequency with the angle of the wire. This primarily gives vertically polarized
radiation.
Typically when I set up such an end-fed wire, I aim the end in my desired direction for
longer paths on the higher frequency bands, and use it for NVIS (nearly omnidirectional)
local contacts on 80m. This works as long as the wire is 1/2 wavelength on the frequency
you want to use for NVIS: with a 40m long wire, 80m NVIS works well, but on 40m there
is an overhead null in the pattern, so it isn't as good for close contacts when the band is
open. When I had an 80m long wire, it worked great for local contacts on 60m, but was
marginal on 80m. Then one night after a net another station called me and asked if I was
using an amplifier: the high angle radiation off the ends was particularly effective at 500 -
800 km, even if the antenna didn't work as well for stations out to 100 km.
Can you make a wire too long? Certainly, if in doing so it doesn't meet your objectives
for radiation pattern. The 80m long wire didn't work as well for me as a wire 40m long,
both due to the 80m coverage, and because the pattern on 15m and 10m got too sharp,
so it wasn't as good for general operation. But if you want to optimize for a specific direction
and have supports that can run it nearly in that direction, you can choose an optimum
length to focus it where you want. (And you can get more creative by carefully putting a bend
in the middle of the wire to make the equivalent of a vee beam, but that gets more complex.
I knew one station who had good results with a 400 foot (130m) long wire, particularly on
40m and 80m. (At least, she had a good signal at my location - I don't know where the
other lobes ended up.) I put up a 200m wire one year for Field Day, and it was spectacularly
poor, perhaps because it angled down a hill. (The best thing about that antenna was asking
a particularly annoying person to go untie the other end of it, without telling him how
far down the hill it ran...)
I have used many such antennas over the years, particularly when I had only a single far
support for it. If you can point it in a useful direction, it may work OK. (One ran down the
peak of the roof of my apartment building, tucked out of sight under the wood shingles.)
But for a lot of other purposes I tend to use dipoles as my default antenna, or, for multiband
operation, a horizontal loop. It really depends on my desired coverage area, what supports
are available, and what other options I have.