GB2384367A

GB2384367A - Multi-band small loop antenna

Info

Publication number: GB2384367A
Application number: GB0201313A
Authority: GB
Inventors: Benjamin Edginton
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-01-22
Filing date: 2002-01-22
Publication date: 2003-07-23
Also published as: GB0201313D0

Abstract

A multiple frequency band antenna arrangement comprises a small loop antenna element 1 connected in series to a tank circuit 6. The tank circuit 6 consists of an inductor 15 connected in parallel to a capacitor 14. The tank circuit 6 provides another resonant frequency band of operation, of a quarter wavelength loop antenna, above that provided by the loop antenna itself. This means that the loop antenna may be operated in the extra frequency band without altering a coupling transformer 3 impedance ratio. As a result, it is simple to use a variety of different shaped loop elements. The compact antenna may be used in mobile, military or covert communication systems. The antenna may be formed with elements wound in a non-inductive manner and supported by an inflatable tubular structure. Plug and socket connections may be used such that the tank circuit 6 may be readily changed. The said arrangement may use variable inductor and capacitor components to adjust the tuning. Two whip antennas may be joined to form a loop antenna element. The position of the antenna arrangement, or one or more of its adjustable circuit components, may be altered by a remotely activated motor.

Description

Multi-band Small Loop Antenna.

This invention relates to the Small Loop Antennas used for transmitting and receiving that are usually constructed by forming an antenna element made of wire or metal tubing into a loop which is then brought to resonance at the required operating frequency by using a variable capacitor.

The Edginton loop antenna patent No GB 2285712 uses a ferrite coupling transformer to feed RF power to a wire loop, and to correct for the change in feed-point impedance when changing bands there are adjustable tappings on the primary of the coupling-transformer.

This means that as well as tuning a capacitor to bring the loop to resonance on the desired frequency, the connections to the couplingtransformer tappings must be altered by a switch or relay 7, to present the correct impedance to the feeder 12.

The Edginton loop is also restricted to a loop element made with wire and uses cords and spreaders to support it and form it into shape. This makes it cumbersome when using it as a mobile antenna.

It is the object of the present invention to provide an antenna that can change bands without the need to alter tappings on the primary winding 4 of the coupling transformer 3, and to be adaptable for use with a selection of different shape and size of loop element.

Because the coupling transformer 3 does not require tappings on the primary winding 4 it removes the need to operate switches or relays 7 when changing to another other band.

This can only be possible if the transformer secondary 2 is looking into what appears to be the same value of loop antenna feed-point impedance on all bands.

The optimum loop element size for the Small Transmitting Loop is onequarter wavelength so it will follow that to be able to use a quarter wave

Loop designed to work for example on the 40 metre band at 7Mhz and for it to still work correctly on the 80 metre band at 3.5 Mhz the loop element size would have to be doubled when changing to the 80 metre band.

But because this is not practical when the loop element is made of tubing or rod the problem has been overcome by connecting a coil or inductor and capacitor in parallel to make what is generally called a Tank Circuit and to place this Tank Circuit 6 in series with the Loop Element 1.

This will make a 7 Mhz quarter wave loop still appear to be an electrical quarter wave when it is presented with a signal of 3. 5 Mhz on the 80 metre Band.

If the tank circuit components are selected to bypass the RF power at 7 Mhz the loop antenna will behave like a dedicated 7mhz loop and present a suitable feed point impedance to the transformer.

But when the transceiver is tuned to 3.5 mhz the tank circuit will cease to bypass the RF and introduce into the loop circuit the required amount of inductance and capacitance to create a lumped electrical circuit that is one-quarter wavelength at 3.5 mhz.

Therefore the combination of the loop element 1 and the Tank Circuit 6 will present a feed point impedance of the same value to the transformer secondary when the transceiver is tuned either to the 7 MHz Band or the 3.5 MHz Band.

In effect the Tank Circuit makes the antenna, frequency sensitive in a way that will correct the feed-point impedance by altering the loop antenna electrical length when the operator changes band.

A second Tank Circuit can be placed in line to expand the loop to three bands, for example a loop antenna with two Tank circuits to provide bands of 40-metre 80-metre and 160-metre can provide a useful multiband antenna for the Radio Amateur.

Other combinations of capacitance and inductance can be chosen in order to make additional tank circuits that are suitable for constructing a Multiband Small Loop Antenna that can be used on Military or Commercial Bands.

The Coupling Transformer 3, Tuning Capacitor 5, and the Tank Circuit 6 can all be located in a single container to make a Tuning and Matching Unit or T. M. U. 10.

Shown in Fig 3 is a mobile Multi-band Small Loop Antenna that can be constructed from two Whip antenna elements attached to the container of the TMU 10. A length of nylon cord 16 is used to pull down the tips of the two whip elements. This has a stabilizing effect and also creates the balloon shape. The resulting antenna can easily be attached to the roof of a vehicle or boat, and be controlled remotely from inside.

The other shapes made from tubing or rod, each have a different radiation pattern making them useful in a number of different ways fig 8 is a compressed loop and fig 6 and fig 7 are formed so that they will take up less space and provide a compact Multi-band Loop Antenna.

The compactness of the resulting multi-band loop antennas are ideally suited to radio communications where space is limited, and because these antennas do not require an earth, ground plains, or an additional antenna tuning unit, they have a place in battlefield communications or as a mobile or maritime mobile antenna for use in permanent or temporay locations on military or covert expeditions.

A specified embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which :- Figure 1 shows the original Edginton Loop Antenna patent No GB2285712 using a switching relay 7 to change the impedance ratio of the coupling-transformer 3 when changing bands.

Figure 2 shows the present invention with the switching relay 7 removed and the addition of the Tank Circuit 6 this creates a multi-band antenna that does not require transformer tappings and removes the need for relay switching.

Figure 3 illustrates dual whip antenna elements mounted onto the Tuning and matching unit container 10. They are pulled down by a nylon cord 16 to form a balloon shape loop antenna.

Figure 4 illustrates a loop antenna element constructed using two whip antennas joined at the top to form an inverted V loop antenna element mounted onto the roof of a boat.

Figure 5 illustrates a loop mounted on the roof of a vehicle.

Figure 6 illustrates a three-loop antenna element designed to reduce the overall physical size of an antenna.

Figure 7 is a rectangular variation of the three-loop antenna element shown in 6 Figure 8 shows a compressed loop mounted vertically on to the tuning and matching unit 10.

Figure 9 is a conventional shaped loop mounted on to a tuning and matching unit 10.

Referring to the drawing Fig 1 it can be seen that the tappings on the Coupling-transformer 3 have to be switched with the relay 7 in order to change bands.

In Fig 2 it can be seen that to remove the need for the switching relay 7 a Tank circuit 6 has been placed in circuit with the loop element 1.

The addition of the Tank Circuit 6 has the effect of increasing the electrical size of the loop element when bands are changed in order to make the Antenna Feed-point impedance remain the same.

The antenna will then continue to work like a quarter-wave loop without having to alter the size of the element or alter transformer tappings when changing bands.

The following Example is by way of example only.

This Example of a Multi-band Small Loop Antenna is designed to work on the 40-metre band and the element is ten metres long formed into one of the loops described, and shown in fig 3 to fig 10.

The coupling-transformer 3 in this example is constructed from two ferrite tubes and the primary winding 4 consists of three turns, this is to match the 50 ohms impedance of the coaxial feeder 12 that connects the antenna to the transceiver.

The secondary winding 2 is wound with two turns of wire to match the 22.2 ohms feed-point impedance of the loop to the 50 ohms of the coaxial feeder 12.

The above turn's ratio only applies when the antenna is constructed with the coupling-transformer 3 connected into the loop element at a point adjacent to the Capacitor 5.

If it is required to feed the loop element at any other point around the loop, away from the capacitor 5 the coupling-transformer turn's ratio will have to be modified.

As shown in figure 2 one end of the loop element Ib is connected at 8 to the secondary winding 2 of the ferrite matching-transformer 3 the other connection from the secondary winding of the matching-transformer is connected to the rotating vanes of the variable tuning capacitor 5. The fixed vanes of the capacitor 5 are connected to the Tank Circuit 6 and the other connection to the Tank Circuit is connected to the loop element at 9.

The transformer ratio of 1.5 turns to 1 will transform the 22.2 ohms of the feed-point impedance to 50 ohms to match the transceiver antenna socket impedance.

This Multi-band Small Loop Antenna is not limited to a wire element la and can be constructed using any type of loop element that provides the correct electrical length for the bands that the antenna is designed to be used on.

An example of this is shown at fig 3 it uses two Whip elements that are joined together at their ends and pulled down by cord to form a balloon shaped loop and mounting them onto the container of the matching and tuning unit then on to a vehicle or boat cabin roof When extremely long-wave bands are to be used the elements will be very long, to reduce the size wire can be wound on to fiberglass tubing either helically or in a non-inductive manner by periodically changing the direction of the winding. The resulting elements can then be used to construct an inverted V multi-band small loop antenna fig 10 by connecting the elements to the tuning and matching unit 10.

Claims

Claims Claim 1. A Multi-band Small Loop Antenna comprising interchangeable loop elements connected in series with a Transformer 3 a Variable Capacitor 5 and a Tank Circuit 6 that consists of an inductor or coil 15 connected in parallel with a capacitor 14.

The Tank Circuit 6 is frequency perceptive and used to maintain a suitable feed-point impedance to the Small Loop Antenna when bands are changed, it does this by creating a lumped circuit that remains electrically one quarter of a wavelength at the operating frequency.

The transformer 3 feeds RF power to the loop 1 and signals back from the loop and also matches the feed point impedance of the loop to the feeder 12 that is connected to the transceiver.
2. A multi-band wire loop as in 1 with a socket 11 fig2 connected in line with the loop element into which a selection of different tank circuits can be plugged to allow quick antenna modification in battlefield communication situations.
3. A multi-band loop antenna as in 1 and 2 that can be installed on vehicles or boats for mobile operation.
4. A multi-band loop antenna as in 1 and 2 that uses an inflatable tubing to support the wire element. It can be deflated to pack away and quickly reinflated with a co2 cartridge.
5. A multi-band antenna as in 1.2. 3. constructed with two whip antennas Sand 9 Fig 3 connected together at the top to form the loop element as shown in drawing fig 3.
6 A multi-band antenna as in 1 to be carried in a backpack for portable communications on HF bands during expeditions or military use.
7. A Multi-band antenna as inl using a loop with the element made from metal tubing, mounted on a roof rack and can be raised and lowered remotely by a geared motor.
8. A multi-band antenna as in 1 constructed for covert radio transmitting possibly from behind enemy lines.

<Desc/Clms Page number 8>
9. A multi-band antenna as in 1 for use on Long-wave radio communications by using non-inductively wound elements to reduce the physical size of the small loop antenna.
10. A multi-band antenna as in 1 with a Tank Circuit 6 is constructed from a variable inductor and a variable capacitor to allow the loop feedpoint impedance to be adjusted over a wider range of frequencies.
11. A multi-band antenna as in 1 where the control motors, Tank Circuit 6 Tuning Capacitor 5 and the Transformer 3 are all assembled in a single container to form a Tuning and Matching Unit 10. All that is then required is to connect to the TMU a suitable Loop Element.
12. A multi-band antenna system as in 1 with sockets attached to the Tuning and Matching Unit case so that two whips 8 and 9 can be inserted and joined at the top in order to form the Loop Element 1.

13 A multi-band loop antenna as in 1 to 12 that can be remotely controlled by motors from a computer or control unit.

14 A multi-band antenna substantially as described herein with reference to Figures 1. to fig 10. of the accompanying drawings.