## 1 Introduction  
A whip antenna is widely used in a wireless communication system due to its simple structure and omnidirectional radiation pattern [1]. In particular for a terrestrial communication system, using an omnidirectional radiating antenna is necessary because a receiver (or transmitter) can be located in any direction from a transmitter (or receiver) along the ground at a distance. Figure 1 shows a whip antenna mounted on a vehicle for military terrestrial communications at the high-frequency (HF) or very high frequency (VHF) band [2,3]. The whip antenna is long compared to the vehicle size, since its length is proportional to the wavelength of operation frequency. For instance, the ideal length is 375 cm at 20 MHz (HF band) and 83 cm at 90 MHz (VHF band). Such a high profile not only makes a vehicle more likely to be exposed to opponents but also degrades its mobility. In this context, a whip antenna is often pulled and bent as in Figure 1b on the move. However, this can result in a poor antenna performance due to the change in the antenna impedance and radiation pattern.  
Contrary to a whip antenna, a patch antenna [4] has a planar low-profile form factor, thus is easy to install on a vehicle roof. Nevertheless, it is not preferable for terrestrial communications as its radiation pattern is directional toward the zenith. Figure 2 shows a 3D radiation pattern of a patch antenna mounted on a vehicle. The patch antenna consists of a lower ground plane and an upper radiator, and they are separated by the air. It was modelled and its radiation pattern was calculated using full-wave simulation software (Ansys\({}^{\text{\text{\textregistered}}}\) HFSS). As can be seen in Figure 2, most of the electromagnetic (EM) energy is radiating vertical to the antenna plane.  
To overcome the above-mentioned limitations of whip and patch antennas, researchers have developed several patch-like low-profile antennas exhibiting a whip-like omnidirectional radiation pattern. One solution is the so-called monopolar wire-patch antenna [5, 6, 7, 8] which incorporates shorting pins (i.e., vias) connecting the upper radiator and the lower ground. By adjusting the patch size, the strong currents flowing along the feed and shorting pins become in-phase, thus the EM energy is radiating in all directions, presenting a whip-like omnidirectional radiation pattern. They were implemented at 5.8 GHz [5], 4 GHz [6], 2.45 GHz [7], 4.1 GHz [8], which are relatively high frequencies compared to the VHF band. Another solution is to utilize a substrate-integrated waveguide structure (SIW) [9, 10, 11]. SIW is an EM waveguiding structure which emulates a rectangular waveguide in a planar form, e.g., a printed circuit board (PCB). A SIW has arrays of vias forming a fictional magnetic wall. With this, the antenna's patch size can be reduced to half. Ref. [10] reported an SIW-based antenna operating at 5.9 GHz and [11] designed an SIW wearable antenna at 2.5 GHz.  
In this paper, we designed a planar patch-like antenna that can be mounted on a vehicle's roof based on an SIW structure. Different from previous SIW-based antennas operating at GHz frequency ranges, the proposed antenna was designed for the VHF band, more specifically 88 MHz, which is approximately 100 times lower than the GHz range. This required a careful optimization of the antenna geometry other than a simple scaling. In addition, the proposed antenna adopted the monopolar wire-patch antenna design by adding shorting pins in the SIW structure to generate omnidirectional radiation patterns. Followed by the simulation studies, an antenna prototype was fabricated and tested to demonstrate its feasibility. The size of the antenna was 125 cm \(\times\) 125 cm \(\times\) 9 cm which is 0.37\(\lambda\)\(\times\) 0.37\(\lambda\)\(\times\) 0.026\(\lambda\) in terms of the wavelength at 88 MHz. The measured antenna's reflection coefficient was less than \(-\)10 dB at the target frequency and the measured received power showed a similarity to the simulated gain.