I have a copper wire antenna about 100 feet long that I have relocated and now need about 75 feet of lead in wire. What type & size of wire should I use?
Your question does not have enough information to give a specific answer, so here are some general guidelines.
The type and size of wire needed is unrelated to the antenna. The factors that are important here needed to answer your question are:
- the maximum power you will transmit with
- the amount of loss you can tolerate (greater distance has more loss)
- the frequencies in use
Higher power needs heavier wire to handle the power.
Low loss requires some type of transmission line with an impedance matched to the radio and antenna. Typical choices are parallel line (such as twin lead or ladder line), coax, and wave guide.
Wave guide is typically used for very short distances at high frequencies (1GHz and up); since your antenna is 100ft long, it's probably safe to assume you are working at much lower frequencies.
Parallel transmission line is typically extremely low loss even if the antenna is unmatched with high SWR, but is sensitive to surrounding metal objects. This works well outside, but if the transmission line has to enter a building, it may couple to wiring and other metal and the building and have high loss.
Coax works well in buildings and near other metal objects, but has high loss if the antenna is not well matched enough to have low SWR, and may have moderate loss (typically measured in dB per 100 feet) even with good SWR. Different grades (i.e., more expensive) of coax have better shielding or better dielectric to reduce loss. Loss in coax is also frequency dependent, with higher bands (above 1GHz) needing different coax than lower bands. In extreme cases where either long distance is involved (more than 100 ft) or extremely low loss is desired, coax typically called hardline can be used.
Both parallel line and coax have power ratings that can also be used as a guide when choosing transmission line.
For amateur radio use, the size of the transmission line is typically not a concern, as your choices in transmission line come first with the above concerns, and there are limited choices to increase wire size. However, in commercial broadcast radio where the power may exceed 10Kw and loss becomes critical, hardline can have large diameters and come in the form of pipes with a diameter measured in feet.