 |
 |
 |
|
|
|
|
|
|
||||||||||||||||
|
|
|||||||||||||||||
|
|
|||||||||||||||||
|
|||||
|
|
It is important to consider that few compromise antennas are capable of delivering the performance one can expect from the full-size variety. But the patient and skillful operator can often do as well as some fellows who are equipped with high power and full-size antennas. The "cliff-dweller" may not be able to "bore a hole" in the band as often, and as easily, but DX can be worked successfully when band conditions are suitable.
Situations arise in nearly every amateur's life which call for discreet antenna installations. That is, rather than arouse the wrath of some neighboring nonamateur, it might save a lot of explanation and discussion merely to put up a temporary radiating system which does not resemble the classic wire antenna. A 120-foot length of No. 28 enamel wire, strung between the window of a motel unit and some supporting object at the far end, will usually pass without being observed, and will provide good performance if matched to the transmitter.
A primary consideration with any antenna system, makeshift or permanent, is safety to those who might come in contact with the system. Always keep the antenna well out of reach. The QRP operator who uses only one or two watts of power is not likely to create a hazardous situation with his antenna, but should observe the rules of safety just the same.
Another invisible antenna can be realized by erecting a flagpole and using it as a vertical antenna. Alternatively, the flagpole's halyard can be the kind of plastic clothesline which contains a wire core, the halyard, thereby, serving as the antenna. The latter is especially useful when wooden or fiberglass flagpoles are used. Hollow, nonmetallic flagpoles lend themselves nicely to containing an internal wire or length of copper tubing which can be used as an antenna.
Another disguise is the possibility of using a TV antenna and its feed line as a vertical antenna. The antenna and feeder should be insulated from the supporting mast, and standoff insulators should be used to keep the 300-ohm Twin-Lead from touching the house or other objects. The entire antenna system can be tuned to HF-band resonance by means of a Transmatch and treated as a random-length wire. In the interest of safety, the supporting mast or tower should be grounded. A high-quality lightning arrestor should be used between the feed line and the grounded mast. VHF operators can modify many TV antennas to work as beam antennas on 6 or 2 meters by cutting the elements to the correct length and adding a matching device, such as a quarter-wave universal stub.
Some amateurs have reported good results when using the downspout and gutter system of a wooden house as a radiator in HF-band work. Still others have used plastic clothesline (with a steel core) between the clothes poles in the back yard, placing an insulator at each point where the line is supported by a pole.
Enterprising amateurs should be able to contrive many schemes for installing invisible antennas. The possibilities are unlimited. But always Safety First!
Perhaps the simplest indoor antenna one can use is the attic-installed random-length wire. For single-band use it is convenient to install a Transmatch in the attic and bring coaxial cable down from the antenna system to the operating position. It is seldom practical to route the radiating portion of a wire antenna through the walls to some lower level in the house. Electrical wiring and water pipes will have an adverse effect on the efficiency of the antenna.
Physically shortened dipoles are practical and should be of interest to the indoor-antenna user. When there is insufficient area to mount a full-size dipole, one can install a loading inductor in each leg of the doublet and tune the system to resonance by adjusting the number of coil turns. Fig. 1 is a drawing of such an antenna. The longer the overall length, dimension A, and the farther the loading coils are positioned from the center of the antenna, dimension B, the greater the efficiency of the antenna. However, the greater is distance B (for a fixed overall antenna size), the larger the inductors must be to maintain resonance. Approximate inductance values for single-band resonance may be determined with the aid of Fig. 2, but the final values will depend upon the proximity of surrounding objects in individual installations and must be determined experimentally. The use of high-Q low-loss coils is suggested. A grid-dip meter, Macromatcher, or SWR indicator is recommended for use during adjustment of the system. (Also see A Bite Size Beam)
 |
Fig. 1 - Dipole antenna lengthened electrically with off-center loading coils. For a fixed dimension A, greater efficiency will be realized with greater distance B, but as B is increased, L must be larger in value to maintain resonance. If the two coils are placed at the ends of the antenna, in theory they must be infinite in size to maintain resonance. Capacitive loading of the ends, either through proximity of the antenna to other objects or through the addition of capacitance hats, will reduce the required value of the coils. |
 |
Fig. 2 - Chart for determining approximate inductance values for off-center-loaded dipoles. At the intersection of the appropriate curve from the body of the chart for dimension A and the proper value for the coil position from the horizontal scale at the bottom of the chart, read the required inductive reactance for resonance from the scale at the left. See Fig. 1. Dimension A is expressed as percent length of the shortened antenna with respect to the length of a half-wave dipole of the same conductor material. Dimension B is expressed as the percentage of coil distance from the feed point to the end of the antenna. For example, a shortened antenna which is 50% or half the size of a half-wave dipole (one-quarter wavelength overall) with loading coils positioned midway between the feed point and each end (50% out) would require coils having an inductive reactance of approximately 950 ohms at the operating frequency for antenna resonance. |
If the attic area is large enough to accommodate an almost full-size dipole, simply erect as much of the antenna as possible in a straight line, then bend the ends of the dipole up, down, or sideways from the main portion of the system. It is recommended that the installer attempt to maintain symmetry in the system by bending the ends of the antenna in equal amounts. Some ideas for indoor installations can be gained from Fig. 3.
Fig. 3 - Various configurations for small indoor antennas. A discussion of installation and tuning methods is contained in the text. |
Some amateurs living in wooden-frame dwellings have enjoyed reasonable success when loading the metal screen of a large window. Here again, a Transmatch is almost mandatory in tuning the window screen to resonance. The ground system can be the cold-water pipes, the third wire (ground) of the electrical system, or both.
It is possible to reduce the physical size of an antenna by 50% or more and still obtain good results. Use of an outdoor off-center-loaded dipole will permit a city-size lot to accommodate a doublet antenna on the lower frequency HF bands, 40, 80, or even 160 meters. Short vertical antennas can also be made quite effective by using lumped inductance to obtain resonance, and by using a capacitance hat to increase the feed-point impedance of the system. As is the case with full-size vertical quarter-wave antennas, the ground-radial network should be as effective as possible. Ground-mounted vertical radiators should be used in combination with several buried radials. Above-ground vertical antennas should be worked against at least four quarter-wavelength radials.
