GB2287615A - Telescopic aerials; testing transmitters - Google Patents

Abstract

A screw-in telescopic aerial 10 is removable from a base portion including a trough transmission line 14. The base portion includes contacts A, B, C for making selective connection either to the aerial 10 or to a substituted test adaptor 20 for connection to test equipment monitoring the transmission characteristics of the apparatus. <IMAGE>

Description

TELESCOPIC AERIALS This invention relates to telescopic aerials.

In some types of mobile radio transmitting apparatus, such as pagers provided with a "talk back" facility, a telescopic aerial is generally provided for transmitting radio signals, a separate receive aerial being integral with the unit. Testing of such pagers can be undertaken with a field strength meter which provides a measurement of the radiated power of the transmitted signal. However, testing is facilitated if the pager includes a test port, such that a conductive measurement of the transmitted electrical power can be obtained instead.

According to the present invention there is provided a telescopic aerial arrangement comprising a base portion and a telescopic aerial removable from the base portion, the base portion including means for making selective electrical connections both to the telescopic aerial when fitted and to a test adaptor, when substituted for the telescopic aerial, the test adaptor being connectable to equipment for testing the transmission characteristics of the apparatus connected to the aerial arrangement.

When a telescopic aerial is stowed within a solid conductive housing cylinder, radiation of the transmitted signal is effectively suppressed by virtue of the screening effect of the housing. Thus if the equipment user attempts to answer a received call and omits to raise the aerial, it is likely that the radiated signal strength will be insufficient for the purpose.

According to another aspect of the present invention there is provided a telescopic aerial arrangement in which a base portion within which the aerial is to be retracted has at least one electrically open portion to allow leakage radiation to pass therethrough with the aerial retracted.

In a preferred embodiment of the invention, to be described in greater detail below, the aerial arrangement for a mobile unit transmitter has optimum radiation efficiency when fully deployed, but also maintains good performance (for example, lOdB degradation at 460 MHz) when fully retracted.

The mobile transmit aerial, according to the preferred embodiment of the invention, provides in conjunction with the transmitter power, the correct Expected Isotropic Radiated Power (EIRP) to meet the pager GS SPEC. FTZ TR2036. After due allowance for fading and system operating margin a range of some 5 miles is possible, although most of the pager users will be within hundreds of metres of the base station.

For a range of 3000 metres the aerial gain could be reduced by (3E3/5*2E3)2=l/10 or -lOdB. The design aim was to achieve this reduced gain when the aerial is stowed, and maximum gain when deployed.

A telescopic monopole aerial has been designed that has the necessary radiation resistance to achieve the required EIRP, and can telescope into a 50mm stowed length inside the pager transceiver.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figures 1A, 1B and 1C show, in schematic cross-section and respectively, a cylindrical coaxial transmission line, a square-section coaxial transmission line, and a trough coaxial transmission line similar to the square-section line of Figure 1B but with one open side; Figure 2 shows a schematic trough line aerial assembly with a telescopic aerial in place; Figure 3 shows the schematic assembly of Figure 2. but with a test adaptor in place of the telescopic aerial; and Figure 4 is a partial perspective view of the test adaptor within the assembly.

Referring to Figures 1A to 1C, there are shown various configurations of coaxial transmission line. A concentric line has a certain characteristic impedance and when correctly fed and loaded with resistive impedances no standing waves are present and the transverse electric and magnetic fields are completely contained inside the line.

Figure 1A shows a cylindrical coaxial line. The outer conductor can take on a square shape in section, as shown in Figure 1B, and still maintain the same operating characteristics. If one side of the outer conductor is removed to form a trough line, as shown in Figure 1C, transmission line operation is still maintained but a little leakage radiation occurs, although on a well matched line this is low. If now the line is progressively mismatched then large standing waves are present and the line will radiate. Experiments have shown that if the stowage length of the telescopic aerial is kept to 50mm or about 1/9 wavelength which forms the inner of the coaxial trough line assembly, then the radiation from the trough line is about lOdB down.

The polarization of the monopole aerial is in a parallel plane to the principal axis of the transceiver, which the user, when talking, will usually hold at 45 degrees to his/her vertical. The polarization of the trough line when radiating is 90 degrees to the monopole polarization, so when held at a 45 degree angle to the vertical is also at 45 degrees.

One example of the aerial and matching assembly is shown in Figure 2. A telescopic aerial 10 includes an insulated sleeve 12 at the end of the outermost section, which fits within a trough line 14.

The telescopic aerial 10 is retained by a screw-in connection for ready removal. Transmitter (TX) power is fed, via an inductor L1, to the aerial 10 by means of a tap on the trough line 14, connection being made by a sliding contact A as the aerial 10 is screwed into position.

A second "butt" contact B at the end of the trough line 14 "makes" when the aerial 10 is fully screwed home connecting the end of the aerial 10 to ground via an inductor L2. The insulated sleeve 12 prevents a ground connection being made via a contact C.

The aerial 10 when fully deployed has an impedance made up of a series resistive component (radiation resistance + loss resistance) and a series capacitive component. The inductor L1 in the input feed line from the transmitter partially matches out the series capacitance, and the shunt inductor L2, via the contact B, ensures a 50 ohm matched condition.

It is most convenient when testing transmitters to be able to do this via a monitor point. Design considerations for the adaptor may exclude a separate monitor point in view of the required size of the transceiver, and also because of the very severe spurious specifications, built-in diodes for monitor points may be excluded.

Since the telescopic aerial 10 is removable by virtue of the screw-in connection, the design utilises the stowed aerial base dimensions to form the trough line coaxial inner (as for the aerial) for the probe adaptor, which terminates in a coaxial 50 ohm BNC socket.

The preferred complete probe assembly in position in the aerial base assembly is shown in Figure 3. A test adaptor 20 replaces the aerial 10 within the base assembly and trough line 14. The test adaptor 20 includes a probe head 22 which fits within the trough line 14, an outer conductive sleeve 24, a semi-rigid copper coaxial cable 26 and a BNC coaxial output connector 28 for providing a connection point to test equipment (not shown). When screwed into position the test adaptor 20 makes contact with the transmitter power input sliding contact A but not the "butt" contact B in the housing. A series capacitor C1 built into the probe head 22 matches out the feed series inductor L1 such that the BNC socket presents the transmitter 50 ohm output impedance as a source impedance to the test equipment. The outer body of the coaxial probe is connected to the transceiver groundplane by the sliding contact C connecting the conductive sleeve 24. Development testing has shown that a 50 ohm source impedance within 1.35:1 is maintained at the BNC socket over the transmit frequency band.

Figure 4 shows a perspective view of the test adaptor 20 positioned in the aerial base acting as the trough line 14. As shown, the base is made up of electrically conductive parts and is electrically open at the top and bottom so that leakage radiation can be emitted in both directions. When assembled, the pads A, B, C and D are soldered to ground on a chassis/printed circuit board (not shown) fitting over the top of the assembly, so that the base is physically closed at the top, but remains electrically open.

Thus the preferred embodiment of the invention provides a telescopic aerial arrangement in which a base portion within which the aerial is to be retracted has at least one electrically open portion to allow leakage radiation. The aerial can be removed leaving the base portion acting as a test port for connection to a test adaptor for enabling power measurement, suitable connection contacts being provided in the base portion for matched connection both to the aerial and to the test adaptor.

Claims (10)

1. A telescopic aerial arrangement comprising a base portion and a telescopic aerial removable from the base portion, the base portion including means for making selective electrical connections both to the telescopic aerial when fitted and to a test adaptor, when substituted for the telescopic aerial, the test adaptor being connectable to equipment for testing the transmission characteristics of the apparatus connected to the aerial arrangement.

2. A telescopic aerial arrangement according to claim 1, wherein the base portion includes a trough transmission line.

3. A telescopic aerial arrangement according to claim 2, wherein the trough transmission line comprises conductive parts on opposite sides, and a chassis or printed circuit board attached to the conductive parts and acting as the third side of the trough transmission line.

4. A telescopic aerial arrangement according to any one of the preceding claims, wherein the means for making selective electrical connections includes a plurality of contacts operable to bear on corresponding parts of the telescopic aerial and/or the test adaptor.

5. A telescopic aerial arrangement according to claim 4, wherein the telescopic aerial includes an insulated sleeve so as to prevent electrical connection with one of the contacts intended for the test adaptor.

6. A telescopic aerial arrangement according to claim 4 or claim 5, wherein one of the contacts in the base portion is arranged to provide a connection to a part of the telescopic aerial when fitted but not to the test adaptor.

7. A telescopic aerial arrangement substantially as herein described with reference to Figure 2 of the accompanying drawings.

8. A test adaptor for connection between test equipment and a telescopic aerial arrangement in accordance with any one of the preceding claims.

9. A test adaptor according to claim 8, including a series capacitor matching out inductance in a transmitter feed arrangement so as to present substantially the same output impedance to the transmitter as that of the telescopic aerial.

10. A test adaptor substantially as herein described with reference to Figure 3 and/or Figure 4 of the accompanying drawings.