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Two-Way Radio Antenna Basics

Thinking about building or installing your own two-way radio antenna? That can be a very interesting project, but there are some very important points to understand before you get started.

For the beginner, the usual antennas that come to mind are the half-wave dipole and quarter-wave ground plane. The half-wave dipole is probably the easiest to fabricate but has lower gain than the quarter-wave ground plane. Gain is the ability of an antenna to focus transmitted signals in a desired direction.

The fabrication of these two types of antennas is beyond the scope of this article. The focus here is to explore these two antennas from the standpoint of an entire two-way radio system… consisting of a transceiver, transmission line, and antenna.

Initial setup of a transmitting antenna system is a two-step process. First, the antenna must be tuned to resonance. Second, the antenna must be power matched to the transmission line that’s driving it. Completing both these steps is critical in obtaining maximum performance.

Let’s start by saying that most transmitting type antennas need to be resonant. This is probably the most important factor (there are others) in assuring optimal antenna performance. If either of the two antennas mentioned are not tuned to resonance, one can expect only lackluster performance. Most of us have only seen the “resonant effect” at audio frequencies. A common demonstration of that effect is when a powerful audio tone is directed at a glass object… the tone is varied in frequency… once the resonant frequency of that object is reached… the glass shatters! When electronic resonance occurs in an antenna at much higher RF frequencies, it certainly doesn’t shatter the antenna… but it does produce maximum RF currents that allow the antenna to transmit/receive signals energetically.

So, what is electronic resonance?... and is that all that’s necessary to get good radiation from an antenna? Electronic resonance is defined as a condition… where a load impedance seen by an AC source is purely resistive and contains no inductive or capacitive reactive component. All AC (RF is AC) impedances are complex in nature and contain both a “real” and “imaginary” parts. The “real” part is frequency independent, while the “imaginary” part is frequency dependent. This frequency dependent “imaginary” part directly relates to behaviors defined for inductive and capacitive reactance. So electronic resonance is when an AC source sees only a “real” part of an impedance… and the “imaginary” reactance part is zero. Getting to this zero-reactance point involves a cancelling strategy. Any existing load reactance (inductive or capacitive) is negated by introducing an equal but opposite reactance.

So how do you achieve antenna resonance? Resonance tuning is accomplished by adjusting the length of certain antenna elements. In the case of the ½ wave dipole… this adjustment is performed by monitoring the antenna feed-point impedance at a chosen frequency… and then adding/subtracting length equally from both elements until the “imaginary” reactance equals zero. In the case of the ¼-wave ground plane… the vertical radiator length is adjusted until that condition is reached. These resonance adjustments can only be done with an appropriate antenna analyzer or VNA (vector network analyzer) that show both the “real” and “imaginary” parts of the complex impedance.

So now you’ve reached resonance… everything is perfect, and the antenna should radiate great… right? Well, not so fast. Without going into a long discussion… suffice to say that transmitting antennas that are significantly shorter than ¼ wavelength are inefficient… even if they are resonant. But the two antennas we mentioned earlier don’t have that issue because both meet the minimum ¼ wavelength requirement defined for efficient operation. There’s also another issue that must be addressed before the resonant antenna goes on the air. I’m referring to the impedance match between the antenna and transmission line. Just because an antenna has been tuned to resonance, doesn’t mean its properly power matched to the transmission line. Power matching involves the “real” part of the complex impedance. If the “real” part of the antenna impedance doesn’t match the characteristic impedance of the transmission line feeding it… the transmitter won’t be able to transfer all available output power to the antenna. This power mismatch shows itself as reflections on the transmission line. This phenomenon is readily detected with an SWR (standing wave ratio) meter.

Here’s where it gets a little tricky. Many radio enthusiasts think that by adjusting the length of an antenna… you should be able to get a perfect SWR of 1:1… and everything is optimized. That sure would be nice if it worked that way. But here’s a few examples of why that doesn’t work. Let’s take the two antennas we mentioned earlier. Start with the ½ wave dipole. This antenna theoretically has a “real” impedance of about 73-Ohms after tuned to resonance. So if you just hook your resonant dipole to your 50-Ohm coax cable… the match you will see on your SWR meter will be 73/50=1.46:1. Now, that’s certainly not a match that is of great concern… but it’s important to understand that you will NEVER see a perfect 1:1 SWR reading with this properly tuned antenna… unless an additional matching circuit is added at the antenna feed-point (preferred)… or an antenna tuner is used. Now let’s try that same experiment with the ¼ wave ground plane antenna. This antenna theoretically has a “real” impedance of about 37-Ohms after tuned to resonance. So if you just hook your resonant ¼ wave ground plane to your 50-Ohm coax cable… the match you will see on your SWR meter will be 50/37=1.35:1. Again, that’s certainly not a match that is of great concern… but it’s important to understand that you will NEVER see a perfect 1:1 SWR reading with this properly tuned antenna… unless an additional matching circuit is added at the antenna feed-point (preferred)… or an antenna tuner is used.