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Repeating and expanding on what was said above, radiation and fields surrounding the single wire feed system not only leak out, unwanted noise and signals can also leak in. This is an unsolvable problem with a single wire feed. The very best we can do is relocate these problems to an area where they cause no noticeable problems. We can do this by relocating the feedpoint.
Relocating the feedpoint can move strong magnetic and electric fields away from the operating position, house wring, consumer devices, and our sensitive equipment. This reduces noise into the antenna feed system, and RFI caused by the antenna feed system. Ground rods have limited effectiveness, except perhaps on very low frequencies.
This is not saying we should not have ground rods, but rather depending on them for RF grounds is not a good idea. The RF ground should if possible contain radials or counterpoise wires and not just ground rods. Ground rods do very little good for RF! As a matter of fact adding ground rods can decrease RF efficiency when an insulated counterpoise is used. With fairly low power and good luck, one way to mitigate noticeable problems is a ground plane at room level.
This ground system can be as simple as strips of foil laid under a carpet. These conductive wide strips of foil would then be connected back to a wide station equipment ground buss. Stained-glass hobby suppliers sell adhesive backed copper foil that works very well, and is easily soldered. Another choice is aluminum foil, but this requires pressure connections that tend to be less reliable over time.
As an alternative to copper foil strips, a metal screen or grid of wires at floor level can be used. This system can be right under the carpet, or can even be directly below the floor. This type of counterpoise system makes the entire room, including the operator, "rise" in voltage and match the equipment chassis voltages. It also disperses or spreads the current and voltage around, reducing intensity of localized electric and magnetic fields.
This is very effective for a second floor or higher story RF ground. Like standing on a metal plate carrying high currents, there is little potential difference between different areas of the high-conductivity counterpoise sheet. The antenna lead from the tuner "longwire" terminal to the antenna should parallel and be reasonably close to the outside ground lead, if possible. This forms a two-wire transmission line, which helps to reduce external fields.
The lead to the longwire antenna should be as short and direct as reasonably possible, because the feed wire is a leaky transmission line. The grid of foil, metal screen, or wire grid does not need to be too dense. Below 30 MHz, spacing conductors one to two feet apart is nearly as effective as a solid sheet. But it is fart better to disperse the current in a widely spread-out path, making all areas of the operating area have similar RF potential without concentrated currents.
The best method of taming a long wire is to install a good low-loss current or choke balun just outside the operating room. This effectively puts distance between the leaky feeder and radiating ground leads outside and away from sensitive equipment.
A system like this is ideal for a minimal investment unobtrusive counterpoise. This system, even with a minimal RF ground, keeps common mode currents out of the operating area. This system reduces noise and RFI. It generally eliminates the need for a shack "floor groundplane". The balun must be a reliable current balun with high common mode impedance.
A voltage balun, or a single core balun, will make things worse. The counterpoise can be a ground system like radials instead of a single counterpoise. It just cannot connect back to the station entrance ground, or the balun's ground.
If it is a single wire or a few wires, they should be insulated from earth and kept a little distance above earth. Ideally the single wire counterpoise should be directly under the longwire antenna, and a few feet above earth. Remember the counterpoise will have considerable current and voltage, and might be an RF burn or shock hazard.
Because a counterpoise is less than perfect, and can even have a fairly high impedance on some bands, the counterpoise system will try to "ground" back through the station gear. Unun's and voltage baluns have a low impedance path for common mode currents, and will not isolate counterpoise currents from the shack equipment. A current balun isolates the ground path from the counterpoise to the shack.
The balun must be a high quality current balun, not an unun or voltage balun. It should be a ratio with high common mode impedance and high voltage breakdown. The coaxial feed line, since it operates at high SWR, should be high quality and as short as possible. Ideally the counterpoise wire should be elevated above earth. This minimizes earth losses, and the counterpoise should not be connected to a ground rod or especially to the station ground.
The required lightning and safety grounds must all be on the coaxial side of the balun. Unless the ground system is nearly perfect with near-zero RF impedance, it is best to keep the antenna's ground or counterpoise isolated from the feed line shield and station equipment. At the expense of simplicity, a better ground will improve efficiency. A better ground would be multiple radials, or multiple counterpoise wires. The ideal system, in which efficiency would nearly equal that of a balanced center-fed system, would be a ground system similar to radials for a vertical.
The ground system can include existing wire fences or metal plumbing, or might be a totally new system installed just for the antenna. A good enough ground system, or large area counterpoise system, reduces RF voltage on the ground terminal. If the voltage on the counterpoise or radial ground system is large enough to present a low impedance, station equipment and the antenna ground systems can be tied together.
Isolation, such as a current balun, might not be required. The counterpoise, in effect, becomes "less hot" with RF voltage. Such a system would minimize RFI and electrical noise problems. With a large system of multiple wires, current density in lossy earth surrounding the radials is reduced.
One common misconception is a near-perfect ground needs quarter-wave or half-wave radials. This is not true. Notify me of new comments via email. Notify me of new posts via email. This site uses Akismet to reduce spam. Learn how your comment data is processed.
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Strictly speaking a long wire antenna is just that - a very long piece of wire - many wavelengths long. However the term is widely used to describe a random. A random wire antenna is a radio antenna consisting of a long wire suspended above the ground, whose length does not bear a. The ARRL Antenna Book describes the general characteristics of Long Wire Antennas. Whether the long wire antenna is a single wire running in one.