Understanding Antenna Gain, Beamwidth and Directivity
I want to start off with the fact that terms can often times be confusing or used incorrectly at times. My goal tonight is not to make anyone antenna engineers but to clear some of the fog or misconceptions about what we are reading or hearing about antennas. Remember some ham/cb antenna manufacturers are interested in selling antennas only, so watch the numbers that they list.
A lot of this information can be found in the ARRL Antenna Handbook, Radio Engineers Hand book as well as from various reputable antenna manufacturers like AH Systems.
I’m encouraging a lot of input, comments and questions from everyone this evening.
Antenna gain is a performance indicator gauged in comparison to a reference source. In compliance engineering, antenna gain is measured in decibels over isotropic (dBi), referring to an isotropic antenna — an “ideal” antenna that transmits/receives energy uniformly in all directions, exhibiting a gain of 0 dBi.
In a transmitting antenna, gain describes the antenna’s ability to convert input power to radio waves sent in a specified direction. In a receiving antenna, gain describes the antenna’s ability to convert radio waves (incoming from a specified direction) into electrical power.
How Does Antenna Gain Work?
First, it warrants mention that in some industries — such as broadcast engineering — manufacturers may utilize dBd (gain relative to a dipole antenna) as a metric, rather than dBi. Note that dBd is inherently greater, defined as 2.15 dBi gain.
This disparity is due to the antennas’ differing radiation patterns; picture an isotropic antenna’s gain pattern as a sphere, and a dipole antenna’s pattern as donut-shaped, resultant of the latter’s more focused beam.
Similarly, when dealing with transmitting antennas, numeric gain may be used in lieu of dBi to calculate the field intensity an antenna is likely to produce. For purposes of this talk, we will remain focused on receiving antennas whose gain is quantified in dBi.
Next, let’s examine how gain functions: picture a perfectly symmetrical balloon representing an isotropic antenna. If you were to squeeze the balloon’s sides, the ends would bulge. That analogy is the essence of gain; the pattern created by the theoretical balloon represents the gain of the antenna under test in this scenario — altering the gain “squeezes the balloon” and changes the antenna’s radiation pattern.
Further, an antenna’s gain can vary across its frequency range for a number of reasons.
For example, a broadband antenna is either tuned to one part of frequency range or another, or a multitude of antennas may be combined in an array, creating a ripple effect.
Some antennas may exhibit gain from zero up to 10 and back down again across their frequency ranges depending on how they are fabricated (they are intended to cover a broader frequency range). An example is a log periodic antenna.
#1C Log Antenna
Even “constant gain” antennas operate within a narrow gain range.
The frequency range trade-off comes in power; by creating an antenna to operate over a wider frequency range, you give up some of the antenna’s performance.
This reality directly counters the common misconception that antenna gain is analogous to amplifier gain: additional antenna gain does not create power, either in added field or voltage.
Antenna gain calculator
Antenna Factor (or correction factor) is defined as the ratio of the incident Electromagnetic Field to the output voltage from the antenna and the output connector. This can include:
- Loss due to mismatch of impedance between the antenna output connector and transmission line.
- Loss due to attenuation of the transmission line.
- Loss due to VSWR at the antenna and/or the receiver.
- Gain due to a preamplifiers located at the antenna.
- Loss due to the mismatch of impedance at the input of the receiver.
Gain (dBi) The ratio of the signal received or transmitted by a given antenna as compared to an isotropic or dipole antenna.
Antenna gain can only be achieved by making an antenna directional, that is, with better performance in one direction than in others.
* Valid only for 50 ohm systems
Creating the AF Data
By way of further explanation, a not particularly accurate yet simple way to generate AF data for an antenna would be to illuminate it with a known field strength at a specific frequency, and use a receiver to measure the volts produced at the antenna connector.
The reason it is not particularly accurate in this example is because we will use the RF train used to create a known field strength (say 10V/m) over the antennas frequency range (covered in equally spaced spot frequencies). This is shown in #2. The field probe is then replaced by the antenna in question, the spot frequencies stepped through and the voltage at the antenna connector measured and recorded.
Using This calculator can help you to determine gain (dBi or numeric) and antenna factor based on your antenna’s frequency range and one other parameter.
Antenna gain also has a direct correlation to both antenna directivity and beamwidth. Higher gain antennas achieve extra power by focusing on a reduced area; thus, the greater the gain, the smaller the area covered (measured in degrees of beamwidth). Antenna gain and beamwidth always are inversely proportional.
Returning to the balloon analogy from above, the harder you squeeze, the further out other areas of the balloon will go as its energy is directed to a smaller area. This act of focusing directivity reduces the beamwidth; consequently, the coverage of the antenna under test is reduced. This scenario represents increasing gain.
This analogy can be related to all variances of gain. Gain may vary across the antenna’s whole frequency range, which means your coverage is not consistent across that frequency range.
Thus, when designing test specifications or performing a test, ensure that you are properly applying appropriate beamwidth coverage to the antenna.
Say, for example, your directional antenna boresight (the axis of maximum gain/maximum radiated power) is 0. Then, if you vary left or right a certain degree, and your reading hits about 3 dB fewer, that’s the point you’re basically looking for.
Typically, in compliance testing, a 30-degree beamwidth provides decent coverage at one meter away (#2).
Beamwidth is the angle from which the majority of the antenna’s power, as illustrated on the radiation pattern’s main lobe, radiates. It may be measured in the horizontal or vertical planes and is the distance between two points where the power is less than half of the maximum. This applies to dipoles also.
You calculate an antenna’s maximum (beamwidth) coverage from a specified distance, as well as its half-power beamwidth. Additionally, this can help you determine the actual field intensity or power density (in V/M) at a given distance with a known antenna gain.
The ARRL Antenna Handbook and the Radio Engineers Antenna Reference manual are excellent resources on this.
Again, it bears stressing that higher antenna gain is not always advantageous. The advantages of lowering/raising gain depend on the application (though the inherent trade-offs remain the same).
For example, if you’re testing a vehicle, you’re going to have to sweep multiple times and you want a wider beamwidth, because it means fewer test setups.
Conducting a similar test on a cell phone, meanwhile, doesn’t call for a wide swath of beamwidth.
Thus, ideally, higher gain is more advantageous in a smaller product.
That said, antennas all must abide by the laws of physics; you can only draw so much gain out of certain antennas.
Typical Properties of Common Antenna Types
Gain: 2.15Dbi over an Isotropic
- #5G Small Loop Antennas or Mag-Loop Antennas
- Gain: 2dbi (max)
- Half-power beamwidth: 80 deg x 360 deg
- Monopole Antennas
- Gain: 6 dBi at best (combination of wave lengths)
- Half-power beamwidth: 45 deg x 360 deg
- Biconical Antennas
- Gain: up to 4 dBi
- Half-power beamwidth: 20-100 deg x 360 deg
- λ/2 Dipole (Half-Wave Dipole Antenna
- Gain: 2.15 dBi (max)
- Half-power beamwidth: 80 deg x 360 degrees
- Log Periodic Antennas
Varies by manufacturer and theirs designs and frequency ranges.
- Gain: 6 to 10 dBi (typical)
- Half-power beamwidth: 60 deg x 80 deg
- Flared Horn Antennas
- Gain: 5 to 24 dBi (typical)
- Half-power beamwidth: 40 deg x 40 deg (dependent on gain)
- Double Ridge Guide Horn Antennas
- Gain: 0 to 18 dBi (varies over operating band)
- Half-power beamwidth: 5 to 100 deg both polarities