This post will be covering some of the terminology related to the antennas we carry (also applies to receivers, transmitters, and transceivers, and anything related to RF). Some of the words used are quite confusing to customers who are just starting to research this kind of technology.
Decibels are a unit of measurement to express a ratio of one value to another in a logarithmic scale. In the realm of audio, decibels (dB) are the units used to measure how loud something is. RF applications also use different forms of decibels such as dBA, dBm (sometimes just written as dB), dBi, and dBV. Since dB is “unitless”, certain suffixes are added when discussing specific values. Here is a list of where these are used:
- dBA: This stands for decibel Amps, it is used in context when measuring the amplitude of current in RF applications.
- dBm or dB: These two are common when just describing power (watts) in an RF application. The "m" usually calls out the prefix "milli". Usually, RF power measurements don't measure very high (depends on application), dBm tends to be more common.
- dBi: This measurement is specific to directional gain for antennas.
- dBV This stands for decibel Volts, it is used in context when measuring the amplitude of voltage in RF applications.
The frequency range is the effective frequency where an antenna operates. There is usually a minimum frequency and maximum frequency given. The component will be able to receive or transmit within this range at different “efficiencies” depending on center frequency/frequencies. An antenna may also have several frequency ranges listed if they are broadband capable.
The center frequency is where an antenna will produce or pass on the largest signal strength (better gain). Some antennas are capable of having multiple center frequencies, these antennas may be capable of broadband communication. You do not need to match center frequencies when developing an application, sometimes it isn’t feasible to get an exact frequency. It is better to get close to the center since it will perform better.
After reading this section down to broadband, I’d recommend checking out this related post since this is a broad overview: Frequency Filters Explained. Bandwidth is the total width of a frequency range. The minimum frequency subtracted from the maximum frequency an antenna is rated for is equal to its bandwidth. For example, if an antenna has a minimum frequency of 1Mhz and a maximum frequency of 50Mhz, the total bandwidth would be 49Mhz. You cannot guess at a frequency range if you are only given a bandwidth. You would need the minimum rating or the maximum rating to derive the other if you also have the bandwidth.
Band-Pass and Band-Reject
Two other words are related: band-pass and band-reject. These words typically apply to special filters that either “pass” or “reject” frequency ranges. A bode-plot is used when looking at RF filters, the X-Axis is the ever-increasing frequency (usually in logarithmic scale) while the Y-Axis is usually the amplitude of the signal in dBV (decibel Volts). Here is what a bode plot of a band-pass signal I drew as an example (both diagrams use dBV on the Y-Axis, I forgot to add that):
So why are diagrams like these used? It would look rather messy if it was a continuous sine wave that grew in frequency and amplitude at various points.
The band-pass diagram would look like the graph above if it was a sinewave with increasing frequency, it becomes a mess of what looks like vertical lines as frequency increases.
This term is often used to describe internet connectivity, however, it is also a general term. An antenna that has a wide range of frequencies along with several center frequencies is known to be a broadband antenna.
Gain can describe several attributes without context, but it usually describes an increase in some sort of signal property. If you are talking about antennas, gain is not increased power (antennas are not capable of boosting power) but rather a sort of “directional gain”. Signals produced from antennas have directionality due to design. More gain is not always beneficial, less gain is required if you don’t want the signal in specific directions. Directional gain is dependent on the application, that is why some antennas have a negative gain (losses). If you are talking about filters or boosting signals, gain can apply to other units of measurement. You can have an increase in power, current, and voltage as well. Boosting these require some amount of external power.
Return loss is the ratio of frequencies that are accepted and rejected by an antenna.
VSWR stands for Voltage Standing Wave Ratio. Standing waves represent power not accepted by a receiver and reflected back on a transmission line. VSWR is the ratio between the maximum and minimum voltage on a loss-less line. Standing waves highly depend on impedances of the transmission line, receiver, and transmitter.
Impedance is a combination of reactance and resistance. Reactance is also measured in ohms but is completely dependent on the frequency of the signal. Here is a post about impedance for some context: A Quick Explanation of Impedance. I don’t cover impedance of transmission lines or other components, however.