Radio Frequency (RF) & Wireless Design

The most basic learning element for designing an antenna is the half-wavelength dipole. It was part of the experiment in which Hertz used to discover radio transmission :

https://engineering.purdue.edu/wcchew/ece604f19/Lecture%20Notes/Lect25.pdf

The Hertzian dipole used balls at the ends of wire with a spark gap in the middle. The wires formed the inductance and the spark gap formed the capacitance of this primitive early antenna.

Over the years the dipole have been experimentally characterizes and is simple to create, but difficult to master. The reasons are because of the many small details that are required to eke out the maximum performance and efficiency.

https://en.wikipedia.org/wiki/Dipole_antenna

The steps to create a Family Radio Service (FRS) dipole are listed below :

1.Determine the length of each 1/4 wavelength element :

1/4 * [wavelength] = 1/4 * [(speed of light) / (frequency)]

e.g. 1/4 * wavelength = 1/4 *[(300E6 m/s) / (462.5 MHz)] = 0.1622 meters = 16.22 cm

2.Determine the half wavelength of the wire or tubing to the diameter as a ratio :

e.g. 0.25" tubing = 6.36 mm = 0.635 cm ; so the ratio is 16.22 cm / 0.635 cm = 25.53 so the k-factor is about 0.91

3.Multiply the k-factor for the material and reduce the length :

e.g. 16.22 cm * 0.91 = 14.76 cm

4.Attach the elements to an open wire transmission (twin or ladder) feed line that is *at least* 1/4 wavelength long and it should be cut in 1/4 wavelength increments if a longer feed line is required.

https://en.wikipedia.org/wiki/Twin-lead

5.Connect the feedline directly to a balun that matches your open wire feed line impedance to your transmitting / receiving device impedance.

e.g. feedline is 300 ohms and transceiver is 50 ohms, a 6 to1 (6:1) balun is required.

(50 ohms could also be a coax - known as an unbalanced transmission line).

6.Mount the antenna *at least* 1/2 wavelength over Earth ground.

Note : this is easy for UHF and even VHF antennas but becomes difficult in the HF and lower spectrum.

I found this calculator online so if you want to play around with different frequencies you can easily : https://www.translatorscafe.com/unit-converter/en-US/calculator/dipole-antenna/

Note : A dipole has an impedance of about 75 ohms and therefore is mismatched to a 300 ohm impedance feedline. It will work nonetheless, but for a better match, a folded dipole is a better choice. I will try to explain that in another post.