Wellbrook Communications

The ALA 100 has two modes of operation:

at Medium and Low Frequencies the antenna is a high efficiency broadband Loop; for High frequencies the antenna has a response similar to a Longwire. The ALA 100 is primarily designed to reduce local interference. It is suited to users, who don't have a lot of space to erect large wire antennas.

The ALA 100 is broadband and does not have to be tuned in step with the receiver. The loop can be mounted remotely from the receiver away from local interference. Whereas, traditional antennas require a lot of space and can pick-up local noise.

ALA 100 FEATURES

• Very low intermodulation products ensures good performance in a strong signal environment.
• Large aperture provides lower noise reception and reduces fading of High Frequency signals.
• Figure of eight directivity and deep nulls to reduce interference for MediumWave and LongWave.
• Antenna is balanced/isolated from ground and feeder induced currents.
• Very high rejection of mains borne noise.
• Supplied with Antenna Interface and 12 volt PSU.
• Broadband, no tuning necessary or matching unit
• No planning problems, mounted outside at ground level or can be used indoors

ADVANTAGES

Active antennas are necessary for several reasons:

the difficulty of matching a conventional wire antenna to a 50 ohm feeder and still retaining a broadband response is not easy. At certain frequencies, conventional wire antennas can deliver excessive signal strength - leading to receiver overload.

Planning regulations restrict antenna erections.

The active antenna solves the problem of impedance matching to the feeder and yet the performance is comparable with larger antennas. However, most active antennas are of the whip or dipole type and respond mainly to the electric-field.

The ALA 100 is balanced antenna and responds primarily to the magnetic-field at medium and low frequencies, this ensures high rejection of nearby electric-fields. The intensity of the electric-field is usually higher than the magnetic-field when an antenna is close to interference sources such as TVs florescent lamps, mains wiring etc. Therefore, by rejecting the electric-field there will be a reduction in local interference compared to other types of active and passive antennas.

The large aperture of the ALA 100 improves the signal pickup to optimise the signal to noise ratio and also reduces fading at HF, this is a very important advantage compared to small active antennas.

INTERMODULATION

The second order intermodulation performance of a broadband active antenna is very important because active antennas do not have any selective circuits to reduce intermodulation. The ALA 100 has been specifically designed to reduce intermodulation products to a minimum. The second order and the third order intercept points are typically +78dBm (IP2) and +43dBm (IP3) respectively. Thus the level of the intermodulation products are generally below the atmospheric and man made noise.

ANTENNA DESIGN

The ALA 100 consists of a 8-18m circumference wire Loop and a balanced gain optimised broadband amplifier using low noise RF power transistors. The amplifier is encapsulated in epoxy resin and housed in a ABS box, this ensures reliable operation in all weather conditions.

The ALA 100 provides very low noise performance and a large signal handling ability. Rejection of mains borne noise is accomplished by a balanced amplifier and a feeder isolation transformer in the Antenna Interface.

INSTALLATION

The ALA 100 Loop Antenna comprises of a loop/amplifier Head Unit together with an Antenna Interface and a 12 volt regulated power supply. RG58C 50 ohm coaxial feeder cable is recommended for the antenna. The maximum feeder length is 100m.

The Antenna Interface feeds the 12 volt dc power to the antenna. A 1m coax. lead connects the Antenna Interface to the receiver.

The wire Loop can be supported by a tree or light weight wooden poles 2-4m high spaced 2-5m apart. Alternatively the Loop can be loft mounted or simply attached to the inside or outside wall of the home.

However, for lower noise performance, the ALA 100 should be positioned away from sources of interference such as fluorescent lights, TVs, computers and electrical wiring. In most cases satisfactory results can be obtained by mounting the antenna near ground level and at least 6m from buildings.

The Loop size can be altered to match the receiver performance and thus reduce overload problems when used with medium priced price receivers. The ALA 100 can also be mounted on to an antenna rotator using a simple 2m x 2m wooden (Bamboo) frame to exploit the Loops directional characteristics.

TECHNICAL INFORMATION

TYPICAL INSTALLATION

CONCEPTUAL ILLUSTRATION OF THE HIGH ELECTRIC FIELD COMPARED TO THE MAGNETIC FIELD CLOSE TO A LOCAL INTERFERENCE RADIATOR

N.B. at medium and low frequencies the Loop will reduce local radiated E-Field interference by up to 30dB compared to an active whip or dipole antenna placed at the distance from the interference source.

ACTIVE ANTENNA CONSIDERATIONS

Currently there are several active antennas available providing similar performance.

These active antennas generally fall into two types:

1) Whip and dipoles, responding mainly to the electric field.

2) Loops, responding mainly to the magnetic field

Whip and dipole antennas are more numerous because they easier to design for broadband performance.

The loop antennas are either manually tuned or wideband. The manually tuned antennas can provide excellent results because their selectivity reduces intermodulation products. However, having to manually tune the antenna in step with the receiver is a considerable disadvantage. Thus the broadband loop antenna offer considerable operational advantages. Also it is nearly impossible to obtain a tuned loop antenna that will provide coverage of LW through to SW. In most cases several tuned loops would be required at a considerable cost.

Where active antennas are used in the presence of local interference. The balanced dipole and balanced loop designs provide the best interference rejection. Whip antennas that use the feeder as an RF return path are prone to interference induce on the feeder screen and mains borne noise.

Generally the loop antenna is less sensitive to local interference and static. Also a balanced loop antenna will have a "figure of eight" directivity pattern with deep nulls to further enhance interference reduction.

N.B. Some loop antennas are not balanced, especially at the higher frequencies and therefore do not provide deep nulls.

SUMMARY

If a medium output antenna is be used (<30mV) then chose an antenna with an IP2 >73 dBm and an IP3 of >30 dBm. However ensure that the antenna does not overload the receiver by using an attenuator if necessary.

For best rejection of local interference, a balanced loop antenna is probably the only choice.

CIRCUIT DIAGRAM FOR SIMPLE ANTENNA INTERFACE
FOR USE WITH ALA 1530 HEAD UNIT

Components Housed in small plastic box

Wellbrook Communications
The Farthings
Beulah
Llanwrtyd Wells
Powys
Wales LD5 4YD
UK
Phone 01591 620316