GB2192311A

GB2192311A - Waveguide switch/limiter

Info

Publication number: GB2192311A
Application number: GB08616018A
Authority: GB
Inventors: John Kitchen
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1986-07-01
Filing date: 1986-07-01
Publication date: 1988-01-06
Anticipated expiration: 2006-07-01
Also published as: GB8616018D0; GB2192311B

Abstract

A waveguide mounted microwave switch/limiter including a solid state switching element 3, has blocks 8 of material (e.g. PTFE) having a higher dielectric constant than the contents of the waveguide (e.g. air), which are mounted at the same longitudinal position in the waveguide as the solid state switching element. These blocks case the switch/limiter to have a larger bandwidth and lower signal attenuation. <IMAGE>

Description

SPECIFICATION Mierowave swftch/lim iter This inert:inn isco ncern ed with imprevements to waveg u idemaunted microwave switch/limiters including solid state switching elements.

Microwave switchesand microwave limiters can be identical in physical structure, differing only in their selected mode of operation. Since the present invention is applicable to both microwave switches and limiters they will be referred to as switch/limiters.

Solid state switch/limiters mounted within wave guides are well known. A common aim in designing such devices isto increase the bandwidth overwhich the switch/limiter operates and to reduce power losses due to the switch/limiterwhen it is setto allow power to pass.

This invention was made while attempting to produce an improved solid state microwave switch/ limiter.

This invention provides a microwave switch or limiter comprising: a waveguide containing a first dielectric material; a solid state switching element mounted within the waveguide; and second dielectric material having a dielectricconstantwhich is higher than that of said first dielectric material and which is mounted within the waveguide such thatthe solid state switching element and the second dielectric material are atthe same longitudinal position in the waveguide.

The invention is, of course, equally applicable to systems where a number of solid state switching elements are used in series or parallel in a waveguide, in this case dielectric material would be provided at the same longitudinal position in the waveguide as each of the switching elements.

A preferred embodiment of the invention will now be described with reference to the accompanying figures, where: Figure 1 shows a known solid state microwave switch/limiter mounted in a waveguide in crosssection, Figure 2 shows an equivalent circuit to the switch/ limiter shown in Figure 1 when setto pass microwave power, Figure 3 shows an equivalent circuit to the switch/ limitershown in Figure 1 when setto block microwave power, Figure 4 shows a solid state microwave switch limiter constructed according to this invention, in cross-section, Figure 5 shows a longitudinal cut-away view of a solid state microwave switch/limiter constructed according to this invention.

The same reference numerals are used for identical parts throughout the specification .

Refe rri.nto Figure 1. a waveguide ifermed; by an, airfilLedicav7.ty 1. surrounded by wa.d:uzti,vewalLs24 The waveguide cross-section in the vicinity of the switch/limiter is the same as the emss-section at any other point The waveguide cross-section is a rectang le having two sides of length A and two sides of length B, in this case A is the larger ofthetwo dimensions and so determines the cut-off frequency of the waveguide in normal operation. An active element ofthe switch/ limiter is formed by a PIN diode 3, which is supported by a conductive boss 4.The conductive boss 4 can be screwed in oroutofthewall 2to altertheoptimum point of tune of the switch/limiter. Conductive post 5 can be screwed up and down through a choke 6.

Choke 6 is electrically separated from the conductive walls 2 of the waveguideby an insulating layer7. The choke 6 and insulating layer 7 form a high impedance between conductive post 5 and conductive wall 2for d.c. and a low impedance for RF alternating current.

In operation, the d.c. isolation of conductive post 5 and choke 6from the conductive walls 2 allows diode 3 to be d.c. biased.

The switch/limiter described is designed to operate at the usual (TE10) microwave transmission mode in rectangularwaveguides.

Referring to Figure 2, when set to pass microwave power by the application of an appropriate d.c. bias to the diode 3, the switch/limiter is equivalent to a capacitor9and an inductor 10 in parallel, forming a resonant circuit connecting the top and bottom of the waveguide. There will always be some absorption of power by a switch in this mode because a leakage currentwill flow between the top and bottom of the waveguide when an electric field is present across the switch.

Referring to Figure 3, when set to block microwave power bythe application of an appropriate d.c. bias to the diode 3, the switch/limiter is equivalentto a capacitor 9 and an inductor 10 in series between the top and bottom of the waveguide, this allows current to flow easily between the top and bottom walls of the waveguide at microwave frequencies, and so will reflect or absorb much of the incident microwave power, although due to resistances inherent in a real circuit, some power will pass.

Referring to Figures 4 and 5 the switdh/limiter is formed in the waveguide as before and two blocks 8 of dielectric material are added. The two blocks8 of dielectric material are placed at the same point along the waveguide as the switch/limiter with one on each side ofthe switch/limiter. The dielectric constant of the blocks 8 is relatively high compared to that of air, typically being made of P.T.F.E.with a dielectric constant of 2.1 or Nylotron with a dielectric constant of 4.

All dielectrics are inherently power absorbing and so it is advantageous to keep the size of the dielectric blocks along the waveguide to a minimum. It is preferred to keep the size of the blocks along the waveguide less than or equal to Ag/2, where A is the wavelength of the optimum tuning frequency of the switch/limiter and g is the dielectric constant of the dielectric blocks.

In operation, these dielectric blocks increase the frequency bandwidth of the switch/limiter, and reduce attenuation of signals when the switch/limiter is setto allowthem to pass, attenuation meaning the reduction in signal strength and including both reflection and absorption. The addition of the blocks 8 will alter the optimum frequency ofthe switch/limiter, but the switch/limitercan easily be re-tuned to allowforthis.

The maximum attenuation when the switch/limiter is setto block microwave power will be reduced, but in practice it has been found thatthis disadvantage is outweighed by the increase in bandwidth and reduction in attenuation when set to pass power.

Although the blocks shown are rectangular, the amountof microwave power reflected, andthusthe attentuation, could be reduced further bytapering the ends ofthe blocks awayfrom the switch/limiter in orderto prevent sudden change in impedance.

The addition of dielectric material at the same longitudinal position as the switch/limiter has these effects because the e el ectric field,a nd thus the power travelling along the waveguide, passes through the dielectric in preference to the air around the diode. So the electric fields applied to the diode are reduced with the effect that the electric currents flowing between top and bottom of the waveguide in the equivaient circuits is reduced.

Although the preferred embodiment uses blocks of dielectric in an airfilled waveguide, the invention can be used with-a waveguide filled with anytypeof dielectric material, where dielectric material is defined to include a vacuum, provided that a dielectric with a higher dielectric constant is used in a position analogous to the dielectric blocks described. The dielectric blocks described can also be replaced with dielectric blocks that onlypartlyfill the height ofthe waveguide or with dielectric in othershapesprovided the conditions mentioned above are fulfilled.

Although two blocks disposed on either side ofte switch/limiter were used for reasons of symmetry a switch/limiterwith only one block could be used, but it might then be necessaryto move the diode and associated parts offthe centre line ofthewaveguide, involving significant re-design of the system.

The switch/limiter described and the addition of dielectric blocks described are designed for use in a waveguide carrying microwave radiation in theTElO mode, becausethisisthemostcommon mode.

Should it be needed to use some other mode, the switch/limiterwould be altered, as would the positioning of any dielectricadded, However, once useofthe invention in theTElO mode is understood, application to other modes is simple.

Claims

1. A microwaveswitch-or limitercomprising: a waveguide containing a first dielectric material; a solid state switching element mounted within the waveguide; and second dielectric material having a dielectric constant which is higherthan that of said first dielectric material andwhich-is mounted within the waveguide such that the solid state switching element and the second dielectric material are atthe same longitudinal position in thewaveguide.

2. A microwave switch or limiteras claimed in claim 1 where the waveguide cross-section is the same at the switch or limiter as it is elsewhere along the waveguide.

3. A microwave switch or limiter as claimed in claims 1 or 2 where said second dielectric material is arranged symmetrically about a plane of symmetry of the waveguide.

4. A microwave switch or limiter as claimed in claim 3 wherein said solid state switching element is arranged so that itforms part of a path between two opposite walls ofthewaveguide,this path having an impedance at microwave frequencies controlled by the solid state switching element, and said second dielectric material being arranged intwo rectangular blocks each arranged to contact both of said two opposite walls and one otherwall ofthewaveguide

5. A microwave switch or limiter as claimed in claim 4where said two rectangular blocks of dielectric material are parallel to the walls of the waveguide in the vicinity of the solid state switching element but taper in both directions along the waveguide.

6. A microwave switch or limiter as claimed in any preceding claim comprising a plurality ofsolid state switching elements each having at least one associ atedsecond dielectricatthe same longitudinal position in the waveguide.

7. A microwave switch or limiter as claimed in any preceding claim where said first dielectric is air and said second dielectric is P.T.F.E.

8. A microwave switch or limitersubstantially as illustrated in figures 4 and 5 of the accompanying drawings and substantially as described with refer ence thereto.