Several designs involving EM waves has been studied, most of the studies used horn antenna which is large and non-compact in size. Thus, it's not appropriate to be utilized in a commercial respiratory monitoring. To decrease size patch antenna was used in studies but problem for the patch antennas includes that it is tough to attain wide bandwidth which can be a problem if it is desired to work at multiple frequencies. An antenna particularly designed and made for EM wave non contact respiratory sensing is not found. Also with the shorter wavelength, the high frequency waves, they suffer from various attenuations like atmospheric attenuation and absorption by gases in environment. These are also affected by rain and humidity, which reduces its signal strength and range [10]. For transmission of high frequency waves, metal or micro-strip devices are not efficient as their manufacturing requires very tight tolerance [11–13]. The transmission lines if used for higher frequencies results in copper loss, skin effect, and radiation loss. For high frequencies a waveguide is preferred [14].
For guidance of high frequency signals a transmission line is used. Most commonly used transmission lines for higher frequency are rectangular waveguide and microstrip. Rectangular waveguide (RWG) structure is known for low losses and high power handling ablitiy, but size is bulky also cost is high and it is difficult to integrate with other structures. Microstrip line has advantage of easy integration and low cost production but power handling ability and quality factor is less [15].
Substrate Integrated Waveguide (SIW) is transition from RWG. SIW is structure having two planes which are separated by pillars of two rows known as vias. Structure of SIW is shown in Fig. 10.
4.1 Cutoff Frequency of SIW
The cut off frequency is given as [16]
fc\(=\frac{c}{2\pi } \sqrt{{\left(\frac{m\pi }{a}\right)}^{2}+{\left(\frac{n\pi }{b}\right)}^{2}}\) (1)
Where m and n are the modes, c is speed of light 3x108 m/sec, from the shown Fig. 11, a is width and b is height of the waveguide. Design equation for SIW can be expressed as
as\(={a}_{d }+\frac{{d}^{2}}{0.95p}\) (2)
where d is via diameter, p is distance of vias, ad is width
4.2 Leaky Wave Antenna
The slot length is given by
$$\frac{{\lambda }_{0}}{\sqrt{2({\in }_{r}+1)}}$$
3
λo is wavelength at mid frequency.
The radiation beam is
ϑ = sin− 1\(\left(\frac{\beta }{ko}\right)\) (4)
The mode of propagation should be β < ko for radiation in structure, Where β is the phase constant [17]