Ham Radio wire Antennas

July 21, 2016
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long wire antenna shack groundBecause the radiating area is often brought into or near the operating position, longwires often create RF interference to consumer goods or RF in the operating room. The easy installation part comes from generally needing only two supports, and not having a heavy feedline hanging from mid-span like a dipole. The long expensive feed cable normally associated with a doublet or dipole is not needed, the antenna wire itself serving as a "feedline".

The antenna itself works just as well as any other wire of similar height and length. Any or all problems are in the counterpoise and feed system. The difficult problems associated with random wire or longwire antennas are caused by ground currents and radiation from the single wire feeder.

End-fed antennas, or antennas with the single wire feeder brought into the shack, come with a little misconception. One commonly repeated myth or "theory" is that half-wave antennas, being resonant, do not require a counterpoise, or that some magical length of antenna will prevent RF in the shack. This does not mean the antenna will be worthless and not make contacts, it simply means something else replaces the missing counterpoise area and we also bring RF fields right into the shack. The feedline, as well as everything connected to and surrounding the single-wire feedline and counterpoise, becomes part of the radiating system. This creates three potential problems:

1.) The feedline, mast, and things around the feedline connect through the antenna into the receiver. This brings noise into the receiver.

2.) The feedline, mast, and things around the feedline become part of the radiator. This brings voltage (electric fields) and current (magnetic fields) directly into the shack.

3.) The feedline and grounding affects SWR and tuning.

Since we often do not have a baseline for noise, unnecessary additional noise will often go unnoticed. The remaining two issues are more likely to be noticed, but only if we run enough power to cause RF burns, power supply shutdown, or other forms of RFI.

Transmitter power levels, feedline length and routing, and the susceptibility of equipment to RF problems greatly influence things we most likely notice. This is why some people (usually with QRP power levels) swear by end-fed half-waves, while others (usually with higher power) avoid end-fed antennas. The reason for that is simple, end-fed half waves have common mode feedline current problems affecting their performance, and these common mode currents cause inconsistency in user satisfaction.

In nearly all cases, if we notice it or not, an inadequate counterpoise hurts antenna pattern and efficiency. This is why high power stations often have more efficient, more ideal, antenna systems. Higher power very often excludes use of power wasting systems, because the wasted power often creates significant local problems. If 5% of 10 watts is exciting the desk with RF, it isn't any big deal. If 5% of 1500 watts excites the desk with RF, the result can be hazardous.

I wouldn't have a problem with a 1500 watt transmitter into a longwire antenna with a tuner remote from the shack and house. I would likely have a fire, or damage equipment, if I brought a single-wire feeder into the house! With 5 or 10 watts, I wouldn't care.

How the Longwire or Random Wire Antenna Works

The single wire feeder not only radiates electromagnetic energy, it has very strong electric and magnetic induction or energy storage fields surrounding the wire for some distance out from the wire.

In order to force current up into the feed wire and antenna, the matching or feed system has to "push against" something else. For every milliampere of current flowing into the feed wire of the longwire antenna, an exactly equal current has to flow into a ground system of some type! In any non-terminated antenna, currents and voltages are transformed along the antenna. This transformation is caused by standing waves. This means ground lead currents can increase or decrease along the ground wire and everything connected to the ground wire or ground system. Voltage changes also along the ground or counterpoise system, just as it does in antennas. The voltage caused by antenna return currents, and the return current, will become stronger (more intense) or weaker (less intense) because of standing waves on wiring and equipment cases.

These ground currents, displacement currents, or common mode currents cause everything connected to the matching system to become "hot" with RF. The result is generally all sorts of RF interference to active devices or even physical harm to the operator, such as actual burns or on lower bands like 160-meters. electrical shocks! These unwanted but very necessary currents ideally should flow through the lowest impedance path and widest area path we can manage.

Noise from Longwire Antennas

Radiation and fields surrounding the single wire feed system not only leak out, unwanted noise and signals can also leak in. Radiation from the feeder and everything connected to the matching system, as well as common mode currents, also allows . This deteriorates receiving system noise performance.

Common mode currents and induction field coupling also decreases transmitting efficiency. This effect adds unnecessary loss to the system.

If things are just right (like having a fairly good RF ground in the shack) and power is fairly low, we can often "get away" with having a random wire or longwire brought directly into the shack, but that is more a matter of good fortune than good planning.

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.

Source: www.w8ji.com
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