The potential of a nanoantenna-coupled thermocouple to convert untapped infrared (IR) energy from low-temperature heat sources into direct current (DC) energy has recently gained attention. In a typical nanoantenna with a thermocouple, heat spread at the antenna center leads to a limited temperature difference and lowers the open-circuit voltage (Voc). Therefore, a membrane or air gap under the antenna has recently been used, but when a massive array is constructed for high DC voltage output, stability and fabrication issues can arise. To solve this, this work utilizes the constructively coupled fields at the antenna center in both vertical and horizontal directions via a grounded and open-ended substrate and maximizes Voc of the thermoelectric nanoantenna. In the antenna structure, a titanium (Ti)-based bowtie nanoantenna and nickel (Ni)-Ti bimetal nano-thermocouple are used for high Seebeck effect. The thermoelectric bowtie nanoantenna is designed and fabricated on a metal-backed and open-ended silicon dioxide (SiO2) substrate with optimum dimensions, demonstrating the maximum Voc of 2.03 µV at λ0 = 10.6 µm which is five times the maximum Voc of 0.4 µV from the state-of-the-art substrate-mounted thermoelectric nanoantennas. It is expected that the fabricated SiO2-mounted thermoelectric nanoantenna with the maximum Voc can be used for a high DC output IR-harvesting massive nanoantenna array.