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For all my wire antenna projects I always use extremely flexible and comparatively cheap black-coloured 4mm²-sized tinned HO7 V-K 4 wire. As a comparison, 4mm² corresponds to about AWG 11. "HO7 V-K 4" is the European harmonisation code for this wire.
That said, with such a broad term as 'wire antenna', the 'best length of wire' depends on what you want to use it for. Some wire antennas with a vertical polarisation like a quarter wave vertical have to have lower take-off angles than a horizontally polarized like a dipole made from wire strung up between the trees in a back yard.
Of course the ironic thing about a random length wire antenna is that the most effective lengths aren't actually random. They work best when the antenna is at least a quarter wavelength at the lowest operating frequency, 65' for 80m, for instance. To maximize the effectiveness, and have something useful down the line, you could build a small tuner.
If the antenna is sloped upwards towards the end of wire, chances are it works as an effective backbeam. Ie. It receives signals better from opposite direction. This can be enhanced by dropping wire vertically over the branch to the ground . That flat height is ideal for antenna that is purposed to receive as well from front and back.
I built a 40m end fed halfwave antenna and unun, and the wire is about 66 ft long. I have some tall trees nearby I can hang it in, semi-permanently. Which of the following would be better? A) hanging the far end nice and high, about 50ft, but then slope down the feed end to about 8ft
To lower the incoming noise on a dipole antenna, connect a ground wire to the coax fitting at the antenna base. Run it to a ground rod and enjoy hearing stations that were covered up before. This does not drop incoming signals it just gives noise that comes in the shack on the shielding portion of the coax a direct path to ground.
My antenna is end-fed wire slung out of a tree and run through a small hole in the wall. There's just a few feet of feed line between the rig and the antenna. The RF ground is a ground rod just on the other side of the wall and connected to a jumper cable wire also poked through the wall.
For me this comes to a fine point in the "random wire" and also "end-fed half-wave" antenna guidance. These antennas — at least supposedly — involve feeding only one element at its extrema, using a transformer "matchbox" to compensate for the impedance at that otherwise unlikely feedpoint. But the question then becomes: what to do with the ...
If attached to a directional antenna, it causes problems transmitting and receiving because the actual radiation pattern is a combination of the intended antenna, and the unintentional antenna formed by the feedline, which probably isn't directional at all. If you've modeled your antenna, the model assumes a feedline that does not radiate.
Since most of the RF current is only on the surface, it often makes sense to build antennas from tubing for antennas requiring rigid elements (a 440 MHz Yagi), or copper-clad steel for wire antennas requiring tensile strength (a 40 meter dipole). This saves weight and cost for a negligible increase in loss.