GB2241384A

GB2241384A - T-shaped antenna

Info

Publication number: GB2241384A
Application number: GB9002363A
Authority: GB
Inventors: Douglas Fisher
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1990-02-02
Filing date: 1990-02-02
Publication date: 1991-08-28
Also published as: GB9002363D0

Abstract

A T-shaped antenna comprises an upper element (22) extending between supports (24, 26), and a vertical element (30) extending from a position near the mid point of the upper element (22) to ground (32). The antenna can be a folded antenna. A feeder is connected to the vertical element (30) at a food point (38) elevated from the ground. <IMAGE>

Description

T-Shaped Antenna The present invention relates to a T-shaped antenna of the type comprising an upper element extending generally horizontally between first and second supports, and a substantially vertical element extending from a position near the mid point of the upper element to the ground.

A conventional T-shaped antenna is shown, for example, in figure 1 of the accompanying drawings. A T-shaped wire (10) is suspended between two supports (12). The electrical distance between the feed point (14) and the ends of the wire (10) is typically around a quarter of a wavelength of the feed signal. Although the overall height of the T-shaped antenna is less than that of an equivalent simple vertical antenna (ie. of height a quarter of a wavelength of the feed signal), the height of the vertical element is nevertheless quite considerable. At MF and LF frequencies it is normally necessary to use steel lattice support structures, and consequently the antenna can be visually intrusive. The supports are themselves quite expensive, and the amount of fencing needed for safety and security reasons adds further to the cost.

An object of the present invention is therefore to provide an antenna which can be more cost effective than the above conventional design, and which can be less visually intrusive than that design.

By increasing the impedance at the feed point, this impedance can nearly equal the impedance of the feed means. This allows a less extensive matching network to be used to match the antenna to the feed means than is commonly used. In some cases, by selecting the appropriate elevation of the feed point, the impedance of the antenna can be made equal to that of the feed means, thereby aleviating the need for a matching network.,,.

According to the present invention in a first aspect, there is provided a T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to ground, and feed means connected to the vertical element at a feed point elevated from the ground. The length of the upper element may be greater than the length of the vertical element. Using an elevated feed point increases the ntpedance of the antenna at the feed point compared with the impedance at ground level.

According to the present invention in a second aspect, there is provided a T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to ground, and feed means connected to a feed point on the vertical element, wherein the antenna is folded. In a folded antenna, the limbs of the antenna are folded back along their lengths, and are joined to one another at their ends. Folding the antenna increases the impedance at the feed point, compared to an unfolded antenna of similar dimensions. Preferably the feed point is elevated fram the ground.

It will be appreciated that the antenna may be used for both transmission and reception of radio signals. Accordingly, the feed means may comprise means for supplying a feed signal from the output of a transmitter to the antenna, or means for supplying received signals as feed signals to a receiver.

An antenna in accordance with the invention can be of low profile, to avoid being visually intrusive.

Preferably, the antenna further comprises a third support for supporting the upper element near its mid point. This allows the full height of the antenna to be used for the vertical element by preventing sagging of the upper element at its mid point, and provides a support for the elevated feed point.

Preferably, the electrical length of the antenna should be close to a quarter of a wavelength of the feed signal, for efficient radiation or reception of a feed signal.

The antenna should also be matched to the feeder, by use of a matching network and/or by choosing the position of the elevated feed point and adjusting the electrical length of the antenna for quarter wavelengh resonance.

In one form of the invention, the feed point is elevated fran the ground, the feed means including a coaxial cable extending from ground to the feed point, the outer of the cable forming a portion of the vertical element. Preferably, the position of the feed point is chosen such that the impedance of the antenna at the feed point is substantially equal to the impedance of the feed supply.

Matching the impedance of the antenna to that of the feed supply achieves more efficient operation of the antenna.

mbodirents of the invention will now be described by way of example with reference to the remaining figures of the accoanying drawings, in which: Figure 2 is a schematic diagram of a first embodiment of the invention in a basic form; Figure 3 shows a second embodinf nt of the invention, including a coaxial feed arrangement; Figure 4 shows a third embodiment of the invention, including matching components in the feed means; and Figure 5 shows a fourth embodiment of the invention, including a folded element with an elevated feed point.

A first embodiment 20 is shown in figure 2. An antenna 20 comprises an upper element 22 extending between first and second outer supports 24, 26, respectively. The antenna 20 is also supported by a third support 28 near its mid point. A substantially vertical element 30 extends from a position near the mid point of the upper element, to ground 32. Feed means shown schematically as a source 34 is connected to the vertical element 30 at a feed point elevated from the ground 32. The vertical element is thus split into an upper radiating portion 30a and a lower radiating portion 30b. The length of the upper element 22 is significantly greater than the length of the vertical element 30, giving the antenna a low profile.

Typically, the length of the upper element is at least twice the length of the vertical elements. The upper and lower elements 22, 30, respectively, may be formed integrally as a Shaped wire suspended from the supports.

A second embodiment similar to the first embodiment is shown in figure 3. In this embodiment the feed means includes a coaxial cable 36 extending from the ground 32 to a feed point 38. The outer 36a of the cable 36 can form the lower radiating portion 30b of the vertical element 30. The outer 36a terminates at the feed point 38, and is earthed at the ground.

The electrical length of the antenna is equal to a quarter of a wavelength of a feed signal. This achieves quarter wave resonance of the antenna, in use. At quarter wave resonance, there will be a current antinode at the base of the vertical element, where it connects to ground. The impedance of the antenna at this point is low typically, and may be of the order of a few ohms. The position of the feed point 38 is elevated from the ground 32, and is sufficiently removed from the the current antinode of the antenna that the the impedance of the antenna at the feed point 38 is raised to that of the feed supply. For coaxial cable, this impedance is typically 500hums.

A third embodiment is shown in figure 4. In this embodiment, the feed means includes matching means 39 in the form of lumped impedances to match the impedance of the antenna to that of the feed supply, The electrical length of the antenna is therefore less critical. The components for the matching means are contained in a weatherproof housing mounted on the third support 28. Preferably the height of the feed point 38 is again chosen such that the impedance of the antenna at the feed point is comparable with that of a feed supply, to minimise losses in the matching network. This embodiment may be useful in situations where, owing to the dimensions of the antenna, the impedance at an elevated feed point on the vertical element will still be less than that of the feed supply.The elevated feed point will, however, raise the impedance of the antenna at the feed point, enabling a less extensive, and hence more efficient, matching network to be used.

In one form of the embodiment used at medium wave frequencies, the height of the supports is 18 metres, and the span of the upper element on each side of the vertical element is approximately 30 metres.

A fourth embodiment is shown in figure 5. In this embodiment the antenna is folded in the form of a continuous loop. Two sections 41, 43 of the loop form the upper element 22 extending between the outer supports 24, 26. The vertical element 30 also comprises two sides. One side 33 of the vertical element 30 is connected to a position near the mid point of one section 43 of the upper element 22. The other side 31 of the vertical element extends between the mid point of the other section 41 of the upper element 22 to ground 32. The feed point 38 is elevated from the ground and is located on one side 31 of the vertical element 30. The folded antenna raises the feed point impendance, by a factor of approximately 4. This allows a shorter vertical element 30 to be used, while still achieving a direct match to the feed supply, as in the second embodiment.

It will be appreciated that the above embodiments, enable a Tshaped antenna to be constructed having a low profile, but still achieving adaquate efficiency of radiation or reception. At MF frequencies the arrangement allows wooden 'telegraph' poles to be used for all three supports. The low-profile and simple construction of the suports allow the antenna to be very cost effective.

It will be further appreciated that the use of an elevated feed point on a Shaped transmitter antenna enables a live feed supply to be connected to the antenna more safely than in a conventional antenna where the feed point is at ground level. In such a conventional arrangement, expensive protection and security fencing is often required to keep people away from the dangerous live feed supply. With an elevated feed point, the security fencing may be replaced by a comparatively inexpensive anti-climbing barrier positioned on the middle support, below the feed point.

Claims

1. A T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to the ground, and feed means connected to the vertical element at a feed point elevated from the ground.

2. A T-shaped antenna according to claim 1, wherein the length of the upper element is greater than the length of the vertical element.

3. A T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to ground1 and feed means connected to a feed point on the vertical element, wherein the antenna is folded.

4. A T-shaped antenna according to claim 3, wherein the feed point is elevated from the ground.

5. A T-shaped antenna according to claim 3 or 4, wherein the upper element is a loop element having two sections extending between the supports, and the vertical element has two sides connected respectively to positions near the mid points of the sections of the upper element and to ground, and wherein the feed point is on one of the sides of the vertical element.

6. A T-shaped antenna according to any of claims 1 to 5, further comprising a third support for supporting the upper element near its mid point.

7. A T-shaped antenna according to any of claims 1 to 6, wherein the antenna has an electrical length substantially equal to a quarter of a wavelength of a feed signal.

8. A T-shaped antenna according to any of claims 1 to 6, wherein the feed means includes matching means for matching impedance of the antenna at the feed point to that of a feed supply.

9. A T-shaped antenna according to any of the preceding claims, wherein the feed point is elevated from the ground, the feed means including a coaxial cable extending from ground to the feed point, the outer of the cable forming a portion of the vertical element.

10. A T-shaped antenna according to any of claims 1 to 9, wherein the position of the feed point is chosen such that the impedance of the antenna at the feed point is substantially equal to the impedance of a feed supply.

11. A T-shaped antenna substantially as hereinbefore described with reference to any of figures 2 to 5 of the drawings.

Amendments to the claims have been filed as follows 1. A T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to the ground, and feed means connected to the vertical element at a feed point elevated from the ground.

3. A T-shaped antenna comprising an upper element extending between first and second supports, a substantially vertical element extending from a position near the mid point of the upper element to ground, and feed means connected to a feed point on the vertical element, wherein the antenna is folded.

9. A T-Shaped antenna according to any of the preceding claims, wherein the feed point is elevated from the ground, and the feed means is an integral part of the vertical element.

10. A T-shaped antenna according to claim 9 wherein the feed means includes a coaxial cable extending from ground to the feed point, the outer of the cable forming a portion of the vertical element.

11. A T-shaped antenna according to any of claims 1 to 10, wherein the position of the feed point is chosen such that the impedance of the antenna at the feed point is substantially equal to the impedance of a feed supply.

12. A T-shaped antenna substantially as hereinbefore described with reference to any of figures 2 to 5 of the drawings.