144 MHz Antenna Length Calculator

In this post, we provide a calculator to find the length of a 144 MHz antenna. In this case, we consider a dipole. Using the calculator on this page, the total length of the dipole is 990 mm or 39 inches.

Calculator


	

			

		

						

				

				

                

			

			
						

		

		











Background





A 144 MHz antenna is designed for use in the 2-meter band, which ranges from 144 MHz to 148 MHz in the amateur radio frequency spectrum. 





This band is primarily used for local communications, including base station to base station, mobile to mobile, and for handheld portable operations. Antennas designed for this frequency band can vary in type, including Yagi-Uda antennas, verticals, dipoles, monopoles and loops, depending on the specific application and performance requirements.





A half-wave dipole antenna is comprised of two quarter-wavelength conductors (l) placed end to end. Its physical length (L) is half the wavelength of the transmitted or received RF signal. 










Practical design





In the video below, TA5LVC shows how to build a dipole for the 144 MHz frequency band













You will notice that there is a gap between the two elements of the dipole (shown in the picture below). The gap actually adds about 15 mm of length to the dipole to make the total length 975 mm. The length of each element is 480 mm. 










There are multiple reasons for this gap:





    

  1. Feeding Point: The gap serves as the point where the antenna is fed by the transmission line (such as coaxial cable or a balanced feeder). This is where the electromagnetic energy from the transmitter is converted into radio waves radiated by the antenna, and vice versa for received signals.
    2. 




  2. Balanced Antenna Design: A dipole antenna is a balanced antenna, meaning both halves are symmetrical and operate in an equal but opposite manner. The gap ensures that each side of the dipole is properly balanced in terms of electrical length and impedance, which is crucial for optimal radiation and reception of radio waves.
    3. 




  3. Impedance Matching: The impedance at the feed point of a dipole antenna (typically around 73 ohms for a half-wave dipole in free space) is determined by the geometry of the antenna and the point where it is fed. The gap allows for the connection of transmission lines or impedance matching devices to efficiently transfer power between the antenna and the radio equipment without excessive reflection or loss.
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  4. Voltage and Current Distribution: In a dipole antenna, the maximum current and minimum voltage occur at the center (at the gap), while the ends have maximum voltage and minimum current. The gap ensures there is a point for the voltage to be fed into the antenna, aligning with the way the electromagnetic waves are generated and radiated.
    5. 




  5. Isolation and Decoupling: The gap helps to electrically decouple the two halves of the dipole, ensuring that they function as two separate but connected radiating elements. This decoupling is essential for the dipole to produce a radiation pattern that is as theoretically predicted, with most of the energy radiated perpendicular to the axis of the antenna.
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