For microwave applications, solid rectangular waveguides and coaxial transmission lines are used due to their low losses at high frequencies. However, when scaling up in frequency and down in physical feature size, they experience some practical problems when integrated in a high-frequency system. Other waveguides have been introduced but often require electrically conductive sidewalls and good alignment. Even though some structures do not require solid walls they still need electrical contact between separately manufactured pieces. Traditional machining techniques for metal waveguides operating at millimeter-wave frequencies, specifically above 100 GHz, are very complicated and costly. Also, when realized as components and manufactured in two blocks, it is difficult to achieve the low loss and high Q-values at high frequencies. The reason is usually due to the field leakage through the tiny gaps, originating due to manufacturing imperfections or metal deformations due to thermal expansion, of two split blocks.
Apart from these manufacturing issues at high frequency, the integration of the active microwave electronic circuitry with a metal waveguide at high frequency is not very easy and often challenges the engineers. Today's planar monolithic microwave integrated circuits (MMICs) are incompatible with non-planar metal waveguides and require the use of different transitions, which adds more complexity in the overall system. This is e.g. discussed in P.-S. Kildal, E. Alfonso, A. Valero-Nogueira, and E. Rajo-Iglesias “Local metamaterial-based waveguides in gaps between parallel metal plates”, IEEE Antennas and Wireless Propagation letters (AWPL), Vol. 8: pp. 84-87, 2009.
On the other hand, microstrip and coplanar waveguide lines are the most representative planar transmission lines and these are robust, low-cost solutions which are very suitable for integrating active microwave components on circuit boards. But both these lines suffer from high insertion loss in the millimeter wave frequency spectrum due to the presence of lossy dielectric material. Apart from this, the coupling between the substrate mode and the desired mode is very crucial beyond a critical frequency. So, despite many attractive properties of the existing transmission lines, their applications in the millimeter-wave frequency range are still critical and not immune to problems.
A new waveguide technology, called ridge gap waveguide has been presented in the article by P-S Kildal et al discussed above, and is also disclosed in US 2011/0181373 A1. This technology is based on local wave phenomena appearing along the ridges of corrugations in parallel-plate waveguides. This is further discussed in Valero-Nogueira, E. Alfonso, J. I. Herranz, P.-S. Kildal “Experimental demonstration of local quasi-TEM gap modes in single-hard-wall waveguides”, IEEE Microwave and Wireless Components Letters 19 (2009) 536-538.
The ridge gap waveguide itself was demonstrated between 10 and 20 GHz and realized using conventional fabrication methods. See e.g. Valero-Nogueira, E. Alfonso, J. I. Herranz, P.-S. Kildal “Experimental demonstration of local quasi-TEM gap modes in single-hard-wall waveguides”, IEEE Microwave and Wireless Components Letters 19 (2009) 536-538.
The structure uses metamaterial surfaces in the form of metal pins to create a parallel-plate stop band, thereby confining the wave to metal ridges in between the pins. See e.g. M. Silveirinha, C. Fernandes, J. Costa, “Electromagnetic characterization of textured surfaces formed by metallic pins”, IEEE Transactions on Antennas and Propagation 56 (2008) 405-415. Metamaterials are artificial materials engineered to have properties that may not be found in nature. Metamaterials usually gain their properties from structure rather than composition, using small inhomogeneities to create effective macroscopic behavior. There is no need for electrically conducting sidewalls or accurate alignment between the two parallel metal plates. The stop band can also be designed using other periodic structures than pins. See e.g. E. Rajo-Iglesias, P.-S. Kildal, “Numerical studies of bandwidth of parallel plate cut-off realized by bed of nails, corrugations and mushroom-type EBG for use in gap waveguides”, IET Microwaves, Antennas & Propagation 5 (2011) 282-289.
The initial study of the newly proposed gap waveguide technology shows that this new technology has much lower loss than microstrip lines or coplanar waveguides and is also much more flexible and easy to manufacture than the conventional metal waveguides. This newly proposed microwave solution based on gap waveguide technology thus gives a very good trade-off between the two opposing criteria of low-loss and manufacturing flexibility. Also, this gap waveguide has the property of suppressing the cavity modes and unwanted propagation within a microstrip circuit over a significant bandwidth and is proposed as a packaging solution. See e.g. E. Rajo-Iglesias, A. Uz Zaman, P.-S. Kildal, “Parallel plate cavity mode suppression in microstrip circuit packages using a lid of nails”, IEEE Microwave and Wireless Components Letters 20 (2009) 31-33 and A. Uz Zaman, J. Yang, P.-S. Kildal, “Using lid of pins for packaging of microstrip board for descrambling the ports of eleven antenna for radio telescope applications”, IEEE Antennas and Propagation Society International Symposium, 2010, pp. 1-4.
Despite their advantage over rectangular waveguides when it comes to assembly, these waveguides are very challenging to produce for frequencies above 100 GHz due to the small dimensions of the pins.
There is therefore a need for an improved and/or more cost-efficient manufacturing method for microwave/millimeter wave devices of the above-discussed type.