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RADIO-MONITORING STATION

ANTENNA HANDBOOK (PART 2).

Edited by Radio Canada International , Montreal, Canada. 1988.


The antennas in the following pages can be built economically at home. The necessary materials (antenna wire, lead-in wire, insulators, lightning and static arrestors, rope, electrical tape, aluminum tubing, etc.) are readily available from most hardware or radio service stores.

If reception is still unsatisfactory after you have tried one or more of our proposed antennas, a more elaborate system may be the only solution. Your local radio service store can supply information on antennas appropiate for your particular area.

If you recently purchased a transistor portable receiver, you can probably get your first introduction to antennas without making a shopping trip at all. Most manufacturers now provide an antenna kit which includes 10 metres of insulated wire complete with terminal, insulator and connector.

While hardly robust enough for permanent installation, such kits are fine for experimentation at home or temporary installation while on a business trip or vacation. A horizontal antenna can be arranged on a balcony; a vertical antenna can be suspended from a window and held motionless by a weight at the bottom.

Let's start our detailed discussion of antenna construction by assuming the following conditions: You have a portable receiver with a built-in whip antenna and, for any of the reasons noted you decided to erect an external antenna.

THE VERTICAL WHIP ANTENNA

Omni-directional. Impedance: 75 ohms. Vertical. Broad band. End-fed.

This antenna is ideal for city and apartment dwellers with limited space. It's effective and simple to erect. It can be made either from an automobile antenna or from a length of small diameter tubing, such as aluminum. Commercial versions often include a combination tuner and amplifier. The lead-in can be either shielded co-axial or regular insulated wire.

Since it is omnidirectional, best results are obtained by locating it on the roof, a chimney or a high pole. Extending the antenna from a window sill or mounting it alongside a building limits the directions from which a signal can be received. However, provided the desired signal is unobstructed, this is an easy solution. Being a vertical antenna, it is very susceptible to electrical noise. If the listener lives in an area where electrical noise is not too high, using insulated wire as a lead-in increases the effective length of this particular antenna - and its ability to capture signals. Figure 4 shows a typical installation of a whip.

[FIGURE 4]

THE INVERTED L OR MARCONI ANTENNA

Omni-directional. Impedance: 50 - 75 ohms. Horizontal. Broad band. End-fed.

The horizontal section should be about 17 metres (50 feet) long. See Figure 5. Attach the insulated lead-in directly to the built-in whip antenna with alligator clips. The whip antenna should be in the collapsed position to reduce the amount of noise pickup. It this arrangement results in signal overload, wind the lead-in arourd the base of the whip antenna (don't remove the insulation!) a few times and secure it with electrical tape. This is called inductive coupling.

Portable receivers equipped with an external antenna terminal or jack often have a signal reducing device, called an attenuator, in the input of the receiver. The attenuator ensures that the strength of the signals from an external antenna will be comparable to those obtained from the receiver's built-in whip antenna. This in turn reduces the possibility that the external antenna will produce a signal overload in the receiver.

[FIGURE 5]

THE VERTICAL ANTENNA

Omni-directional. Impedance: 52 onms Vertical. Broad band. End-fed

It consists of a wire, 6,1 to 18.3 metres (20 to 60 feet) long, which can be suspended from the overhang of the roof, or supported by a pole, or hung from the branch of a convenient tree. Anchor the lower end of the antenna securely to the ground. Because the lead-in is very short, the amount of noise pickup is kept to a minimum. However, the closer the antenna is to the house or other obstructions, the more its omni-directional properties are reduced. If you are unable to receive the desired signal, install the antenna as far away from obstructions as possible and use a shielded lead-in. Figure 6 shows a typical installation.

[FIGURE 6]

THE WINDOM ANTENNA

Omni-directional Impedance: 75 ohms. Horizontal. Multi-band. Off-centre-fed.

The Windom is a half-wavelength antenna cut for the lowest frequency band in which it is to be used. The lead-in is connected about one-third from the end of the antenna to provide a good impedance match. It you have sufficient space, make the Windom about 38.4 metres (126 feet) long and ccnnect the lead-in 12.8 metres (42 feet) from the end nearest your receiver. (See Figure 7). This should provide good reception in an international shortwave bands from 11 - 75 metres. For maximum reception, the Windom may be cut to a particular shortwave band, according to the following formula:

L= 142
     F

where L is the required length of the antenna in metres, and F is the known frequency in MHz. For example. divide 142 by 6.075 MHz (the centre frequency for the 49 metre band). The result is 23.4 metres, which is the horizontal length of the Windom. The lead-in for this particular antenna should be soldered 7.8 metres (23.4 divided by 3) from the end nearest your receiver.

[FIGURE 7]

THE HALF-WAVE DIPOLE

Directivity: right angles to the signal. Impedance: 75 ohms. Narrow band. Centre-fed.

This antenna gives best results in a panticular metre band. Start by determining the centre frequency within the desired metre band and then calculate its wavelength.

The total length of the antenna is half the wavelength of the centre frequency. The half-wave dipole is split in two at the centre by an insulator. The result is two sections each a quarter of a wavelength long: in other words, it is a quarter-wavelength from the centre insulator out to each of the insulators at either end of the antenna.

The 75 ohm lead-in can be either co-axial or twin lead. It a co-axial lead-in is used, then the centre conductor should be connected to one half of the antenna at the centre insulator and the outer shield should be connected to the other half of the antenna at the other end of the centre insulator. If twin lead is used, then the conductors should be connected one to each side of the antenna at the centre insulator. As with the Windom, the lead-in should be allowed to drop freely at right angles from the antenna for as great a distance as possible before any bends are made. See Figure 8.

[FIGURE 8]

Since the half-wave dipole is cut for a specific frequency band, you must erect a dipole for each band you wish to use. It you erect a series of half-wave dipoles, connect all the lead-ins to a multi-position switch, available at stores which set electronic parts and supplies or at ham radio shops. This will make it easy to select the antenna to match the band to which the receiver is tuned. Table 2 contains measurements for constructing a half-wave dipole for the various shortwave bands. Note that these are the overall lengths, so the centre insutator should be located at the middle point for each band. The lead-in should run from the antenna at right angles for as great a distance as possible before bending toward the receiver location.


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