Radio Frequency (RF) & Wireless Design

Created by Steven Minichiello on 27 April, 2018

There are many times when the user doesn't want to have the full length of an antenna either for appearances sake or because the antenna may be too cumbersome to deal with, such as handheld transceiver antennas.


The fact is that antenna really wants to be a large as possible ! The reason for this is simple - surface area. An antenna is most effective when the surface area is at a maximum, however there are (3) electrical factors that must be taken into consideration: resistance, inductive reactance and capacitive reactance.


The first thing to consider is that an antenna is an electrical resonator. That is that it resonates at a specific (center) frequency and may have either a narrow (sharp) bandwidth or a wide bandwidth.


The design of the antenna takes into account the interest of the frequency of which it is centered and the bandwidth. The analogy that we can personalize would be a tuning fork that may resonate on a single frequency or an instrument that resonates on multiple frequencies that we can hear.


Since all Radio Frequencies (RF) are based upon Electro-Magnetic waves, the capturing of the electric field and the magnetic field is what makes the response (gain) of the antenna characteristic of its quality.


Also since most magnetic energy is coupled thru inductors, most antennas try to focus on the inductive reactance coupling as the primary field of interest, and then tune the capacitive reactance necessary to provide the correct center frequency.


Magnetic field energy is best captured when the inductance is large, which requires a maximum length for the largest area. Furthermore is that the electrical length is best when the physical length are equal. The electrical length is the calculated length of the inductance for various wavelengths such as 1/4 wavelength being the most common.


When an antenna is shortened by coiling the antenna, the antenna physical length is smaller, while the electrical length remains the same. This is known as a loaded antenna.


The coil will add and increase the capacitive reactance between each coil and will raise the frequency response of the antenna, so careful design must be considered. Typically short rubber ducky antennas use this approach.


The problem is that the overall area is reduced and since there is more capacitive reactance loading, these antennas typically have virtually no gain ! Moreover considering that the user places the transceiver near their head, a large portion of the antenna is detuned by the users head as parasitic capacitance.


Now for situations such as cars, where many people like the 'cool' appearance of a stubby or short antenna, usually the buyer is shooting-themselves-in-the-foot in terms of performance. Since the height is minimized, the range is lowered. Moreover there is much more ground coupling which also pulls in most of the magnetic energy. The added capacitive loading means most of the EM energy is stored and not radiated.


So if you want a good antenna consider something like this :


Rather than something like this :