Coaxial line to planar RF transmission line transition using a microstrip portion of greater width than the RF transmission line

A coaxial transition includes a first conductor aligned along a first axis. The transition also includes a ground shield surrounding the first conductor such that a first gap exists between the first conductor and the ground shield. An electric field radiates between the first conductor and the ground shield through the first gap. The transition further includes a second conductor aligned along a second axis and coupled to the first conductor. The second conductor forms a second gap between the second conductor and a portion of the ground shield. A first portion of the electric field radiates between the second conductor and the ground shield through the second gap. The transition also includes a top ground plane aligned substantially parallel to the second conductor. A third gap exists between the top ground plane and the second conductor. The second gap and the third gap are substantially parallel with the second conductor therebetween.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to radio frequency transmission, and in particular to a radio frequency coaxial transition.

BACKGROUND

The transition between a vertical and horizontal radio frequency (“RF”) propagation path within circuit boards has proven to be inefficient. Such transitions are typically mismatched and include inductive discontinuities in the circuit path and have relatively high insertion loss and poor return loss. Previous attempts have included capacitance compensation on the center conductor of the vertical coaxial structure, processing smaller external vertical coaxial features, or using smaller size external surface mount coaxial parts to reduce the inductive parasitic.

SUMMARY

The teachings of the present disclosure relate to a coaxial transition that includes a first conductor aligned along a first axis. The transition also includes a ground shield surrounding the first conductor such that a first gap exists between the first conductor and the ground shield. An electric field radiates between the first conductor and the ground shield through the first gap. The transition further includes a second conductor aligned along a second axis and coupled to the first conductor. The second conductor forms a second gap between the second conductor and a portion of the ground shield. A first portion of the electric field radiates between the second conductor and the ground shield through the second gap. The transition also includes a top ground plane aligned substantially parallel to the second conductor. A third gap exists between the top ground plane and the second conductor. The second gap and the third gap are substantially parallel with the second conductor therebetween.

Technical advantages of particular embodiments include a coaxial transition that has little or no inductive break therein. Accordingly, a coaxial transmission line may transition (e.g., change directions from horizontal to vertical) more efficiently than a traditional coaxial transition. This may reduce insertion loss and obtain an improved return loss compared to a similarly sized traditional coaxial transition.

Other technical advantages will be readily apparent to one of ordinary skill in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1is a side profile view of a radio frequency coaxial transition in accordance with particular embodiments. Radio frequency (“RF”) coaxial transition100(“coax transition100”) allows for the propagation of RF signals to transition between a vertical path and a horizontal path. In the depicted embodiment the RF signal is propagated by electric fields150through coax transition100from the vertical direction to the horizontal direction. This may generally be referred to as the propagation path. While coax transition100is discussed in terms of a vertical to horizontal transition, the transition may be between any two different directions (e.g., ‘North’ to ‘West’). Furthermore, while the illustrated transition is orthogonal other embodiments may have non-orthogonal transitions.

Coax transition100may be used anywhere where a change in direction of the propagation path of an RF signal or transition between a coaxial interface and a planar transmission line is desired (e.g., a microstrip transmission line). Thus, coax transition100may be used in a variety of tasks covering a wide range of RF frequencies. For example, in certain embodiments, such as those involving an RF or microwave circuit board, coax transition100may be used to transition from a surface mounted vertical coaxial launcher to a horizontal stripline transmission line coupled to the circuit board. This may allow for RF signals to be received from a surface mounted coaxial interface and communicated to another part of the circuit board. By employing coax transition100, a better RF/microwave transition from the coaxial surface mount to the circuit board may be achieved. Furthermore, particular embodiments may allow for a vertical conductor110(and thus the corresponding surface mounted coaxial parts) to have a larger diameter while maintaining and/or improving on the efficiency of RF/microwave/millimeter transitions compared to a standard coax transition. The increased size may result in easier printed circuit board manufacturing.

Coax transition100includes a vertical conductor110, a horizontal conductor120, a ground plane130, outer ground walls comprising ground walls140aand140b(outer ground walls140aand140bmay collectively be referred to herein as outer ground walls140, reference number140is not depicted), and via connectors160. These components form the structure of coax transition100, which reduces the problem of mismatched coaxial transitions. In the depicted embodiment vertical conductor110is a coaxial interface that is transitioning to horizontal conductor120a, which in this example begins as a microstrip (120a) and then continues as a stripline (120b) transmission line. This transition may occur over the diameter of vertical conductor110while maintaining a continuous transmission line.

The components of coax transition100are arranged such that electric field150is able to pass through gaps170(comprising gaps170a,170b,170c, and170d(gaps170a,170b,170c, and170dmay collectively be referred to herein as gaps170, reference number170is not depicted) created between vertical conductor110and outer ground walls140, between horizontal conductor120-(comprising horizontal conductor120aand120b(horizontal conductors120aand120bmay collectively be referred to herein as horizontal conductor120, reference number120is not depicted) and ground plane130, and between horizontal conductor120band ground wall140b. Gaps170may comprise any desired dielectric material. Because coax transition100includes gap170dbetween ground plane130and the top surface of horizontal conductor120, there may be little or no inductive break during the transition. Also, a greater percentage of electric field150may be able to make the transition from a vertical propagation path to a horizontal propagation path, as compared to certain prior coax transitions in which there is no ground plane to create a gap above the horizontal conductor. A more traditional coax transition may allow a relatively large portion of the electric field to escape as it transitions from vertical to horizontal propagation paths.

In the depicted embodiment, gap170ais substantially the same as gap170b. This consistent gap may continue until the top surface of outer ground walls140is reached. At this point coax transition100begins to transition from a vertical direction to a horizontal direction. More specifically, the coaxial portion of coax transition100begins to transition to the predominantly microstrip portion (120a).

Electric field150on both sides of vertical conductor110is able to transition from the vertical propagation path to the horizontal propagation path. Furthermore, horizontal conductor120is able to maintain electric field150on both of its sides. This may be facilitated by ground plane130. Ground plane130may continue for the entire length of horizontal conductor130. This may reduce or eliminate inductive discontinuities in the propagation path of electric field150. A traditional coax transition does not include a ground plane130as depicted inFIG. 1.

InFIG. 2, the overhead cross-sectional view of coax transition100, taken along line2ofFIG. 1, shows horizontal conductor120, vertical conductor110, and via connectors160. Horizontal conductor120(comprising horizontal conductor120aand120b) is electrically connected to vertical conductor110. This may help to facilitate transitioning the vertical propagation path of electrical fields150to a horizontal propagation path. While in the example embodiment horizontal conductor120ais a microstrip conductor and horizontal conductor120bis a stripline conductor, other embodiments may comprise any other type, or combination of types, of conductors that may be desired.

In the illustrated embodiment, horizontal conductor120includes quarter-wave impedance transformer180. Quarter-wave impedance transfer180may aid in transitioning from the substantially cylindrical vertical conductor110of the coaxial interface to the substantially planar horizontal conductor120of the stripline transmission line.

Via connectors160may electrically connect the top ground plane to outer ground wall140. In the depicted embodiment, via connectors160surround both vertical conductor110and horizontal conductor120. Thus, both vertical conductor110and horizontal conductor120remain enclosed as coax transition100transitions from vertical to horizontal. This may be different than a traditional coax transition in which the horizontal conductor is not covered above by a ground plane or corresponding via connectors. The enclosure may help to preserve more of the integrity of the electric field150during the transition. This may result in a more efficient transition than would occur in a traditional coax transition.

FIG. 3is an overhead cross-sectional view of the radio frequency coaxial transition ofFIG. 1along line3, in accordance with particular embodiments. From this view, the bottom surface of ground plane130can be seen. The shape of ground plane130corresponds with the shape of outer ground wall140(depicted inFIG. 4). Ground plane130may be electrically connected to the outer ground wall through via connectors160. Thus, ground plane130may be at approximately the same potential as the outer ground wall. As discussed above, ground plane130may extend the length of the horizontal conductor which it covers. Because ground plane130, in essence, covers the top of horizontal conductor120it may facilitate in providing a continuous transmission line through the transition from a coaxial interface to a microstrip transmission line. By covering the outer ground wall and the horizontal conductor, ground plane130may reduce the amount of RF radiation that escapes.

FIG. 4is the overhead cross-sectional view of coax transition100, taken along line4ofFIG. 1. InFIG. 4outer ground wall140completely surrounds vertical conductor110. Because the depicted embodiment is a coaxial transition, vertical conductor110and the area enclosed by outer ground wall140may have axes that are substantially aligned and/or collinear. Thus, gap170may be of a consistent size providing a substantially constant distance between the outer surface of vertical conductor110and the inner surface of outer ground wall140for the entire circumference of vertical conductor110. This may allow for a relatively even electric field150to be used to propagate the RF or microwave signals along the coaxial line. As alluded to above, vertical conductor110and outer ground wall140may form a coaxial interface for surface mounted components.

Although particular embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope the appended claims.