The J-type antenna is also called the Polish antenna. It is characterized by low radiation elevation angle and a certain gain, and the efficiency is twice that of the 1/4 GP antenna.
The upper part of the J-type antenna is the radiating section, and the lower part is the matching section. The adjusted J-type antenna has a pure resistance of 50 ohms and can be directly connected to a 50 Ω coaxial cable.
Because of its simple production and easy material selection, J-type antenna has been favored by FM transmitter enthusiasts. It can be used with air-conditioning copper tubes of about 1 mm. It can be produced according to the calculated size and can achieve good results without professional tools. If you can't find the copper tube, you can also make it with 2.5 square copper wire.