## 1 Introduction  
For the military very high frequency (VHF) ground communication system, a conventional monopole or dipole antenna is mainly used. It guarantees reasonable gain for reliable wireless communications despite its long length (e.g., 0.5-5 m at VHF band) since the physical size of the antenna is proportional to the wavelength. This limits mobility and installation freedom on a platform. To overcome this, several small VHF band antennas have been proposed [1, 2], but they are mostly not suitable for conformal installation or have inevitable limitation in an operation bandwidth.  
Meanwhile, there have been many studies on the miniaturization of antennas using the composite right/left-handed (CRLH) transmission line (TL) technique [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. CRLH TL is one of the metamaterial-inspired electromagnetic structures which is a composition of right- and left-handed TLs. By combining these two structures, the CRLH TL has the left-handed (LH) characteristics at a lower frequency and the right-handed (RH) characteristics at a higher frequency where the center frequency can be adjusted by the inductance and capacitance of the unit cell of CRLH TL. The LH characteristic provides a negative phase velocity which has an opposite wave propagating direction from the RH one, which is different from natural characteristic. The small antennas using the CRLH TL have been investigated mainly in the ultra-high frequency (UHF) band or higher bands. Recently, we proposed a VHF band small antenna using the CRLH TL in [9].  
Although the CRLH TL can miniaturize the antenna physical size, the operation bandwidth is generally narrow due to its resonating nature. To improve the bandwidth, a non-Foster circuit (NFC) can be used as an impedance matching circuit [15; 16; 17; 18; 19]. However, the non-Foster circuits have not been used for the matching of the small VHF antennas using the CRLH TL. The non-Foster circuit generates a negative capacitance; therefore, it is useful to cancel the high positive capacitance of the antenna over a broad bandwidth.  
In this paper, we applied a non-Foster circuit to the CRLH TL-based VHF antenna to resolve issues by the miniaturization such as bandwidth narrowing and receive power mitigation. The CRLH TL-based VHF antenna combined with the non-Foster matching circuit was designed using a full-wave electromagnetic simulator and radio frequency (RF) circuit simulator. We miniaturized the antenna with the zeroth-order resonant characteristic of the CRLH TL, and we obtained a wide bandwidth of the antenna using non-Foster impedance matching. The optimized antenna and circuit were fabricated, and then their individual characteristics as well as the combined antenna system characteristics were evaluated. The proposed 3-cell CRLH TL antenna consisted of interdigital capacitors and shunt spiral inductors to maximize the miniaturization effect. The size of the antenna was extremely small, \(0.025\times 0.014\times 0.0008\)\(\lambda\) (\(51.4\times 30\times 1.6\) mm), at the minimum operation frequency of the proposed antenna (\(145.5\) MHz). Meanwhile, the size of the non-Foster circuit was only \(0.0093\times 0.006\times 0.0007\)\(\lambda\) (\(20\times 14.4\times 1.6\) mm). The measured 10-dB return loss bandwidth of the proposed antenna system was \(145.5\)-\(500\) MHz (\(354.5\) MHz), which is 18 times wider than the CRLH TL antenna without the non-Foster circuit applied. The received power was also measured outdoors and an average of 17.3-dB enhancement was displayed in the frequency range of 120-500 MHz.