The antenna is the essential component of every FPV system. When we say “essential” we mean it. A wrong antenna can degrade the range of the system to very low values, making flying impossible.
- polarization (vertical, horizontal, circular)
- directivity (omni, directional)
- SWR (standing wave ratio)
- “gain” = directivity
- transmit power
One of the major parameters used in analyzing the performance of radio frequency (RF) communications links is the amount of transmitter power directed toward an RF receiver.
This power is derived from a combination of:
- Transmitter power
- The ability of the antenna(s) to direct that power toward an RF receiver(s)
The directivity of the antenna is determined by the antenna design. Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving. To determine the directivity of an antenna, we need a reference antenna with which to compare our antenna’s performance.
Over the years there have been several different reference antennas used; however, today an isotropic radiator is preferred as the standard antenna for comparison. As noted, the isotropic antenna transmits equal amounts of power in all directions (like a light bulb).
To increase the directivity of a bulb’s light (the antenna’s energy), similar to a flash light or automobile head lamp in this example, a reflector (antenna) is added behind the bulb. At a distance, in the light beam, the light bulb now appears to be much brighter. The amount that the bulb appears brighter compared to the bulb without a reflector is the directivity of the reflector (antenna).
When the directivity is converted to decibels we call it the antenna gain relative to an isotropic source (dBi). Typically the higher the gain, the more efficient the antenna’s performance, and the farther the range of the antenna will operate. For every 6 dBi in gain, you double the range of the antenna.
The two antenna characteristics, among others, that impact link performance to a large degree are directivity and polarization. Some people tend to confuse the two but it’s important to sort it out. Another factor that’s often quoted (mostly to sell after-market antennas) is “gain”. Antenna gain can be a useful number if understood in the right context. Gain is a product of antenna directivity and to be meaningful, requires a reference antenna for comparison, which is most commonly an isotropic radiator. This is a hypothetical antenna that radiates (or receives) equally well in all directions i.e., it is an antenna where the radiation pattern is a sphere with the antenna located at the center. Units for gain using this reference are dbi, meaning decibels improvement (or loss -dbi) compared to the isotropic reference.
A simple dipole antenna has a gain of 2.15 dbi and has a directivity pattern, in free space, shaped like a torus or donut. Since the radiation is much less in the “hole” of the donut, it holds that it has to be greater in some other directions. Thus, the dipole has gain in its favored directions compared to isotropic. This can be an advantage in some circumstances but in RC systems, that “ hole in the middle” can be a real problem. The usual transmitter antenna is a dipole or monopole and If pointed straight up, the donut pattern is horizontal and minimum radiation is from the tip. This is why we are told to not point the antenna at the plane. This is counter-intuitive. Do we always do this while concentrating on flying? I doubt it. It’s too bad that we even have to be concerned about it.
Linear antennas with higher gain (>2dbi) need to be used with caution. The directivity pattern often has multiple narrow lobes with nulls between. This results in more signal fading as the plane/transmitter orientation changes and monitoring the transmitter position in your hands becomes necessary. We don’t need more things to think about while flying. If more antenna gain is needed to extend range, there are antenna types with increased gain without breaking up the pattern into narrow lobes but you’re not going to find them at your favorite RC supplier.
Antenna polarization is the orientation of the electric field component of the transmitted radiation and for simple linear antennas, is the same as the orientation of the antenna conductor (wire). If the wire is vertical the polarization is also vertical, if horizontal the polarization is horizontal and everywhere in between it follows the wire orientation. The main point is that the polarization is linear and is independent of directivity. The same linear polarization is present in all directions of the directivity pattern. Transmitting and receiving antennas behave the same in this regard.
One characteristic of linear receiving antennas is that they respond best to an incoming polarization that matches their own. At any other linear polarization the signal is reduced and at 90 degrees it can reach –25db or more. This is a huge penalty in signal strength and glitches are probable, particularly near maximum range.
As the plane orientation changes – banks, climbs, dives and inverts, the signal level delivered to the receiver by the antenna(s) is all over the place. This signal fading is due to two primary factors – the mentioned polarization difference between the transmitter and receiver antennas and changes in gain of both antennas because of the directivity pattern response. Taken together, signal level changes of more than 50db are possible. Fortunately, they’re usually momentary but still represent a degradation of link integrity and can occur at a bad time, like when close to the ground.
Many of today’s systems use a form of receiver antenna diversity, which can help with the fading issues, both polarization and directivity. That’s why they usually have multiple receive antennas. If these antennas are oriented and spaced properly, they can definitely help.
There is another type of polarization that solves some of these problems. Unfortunately, it is not used very often in RC due to greater complexity and cost. However, if you’re willing to DIY it can be a big help and not cost very much. Circular polarization results when the electric field vector continually rotates (at a very high rate) and because of this, will respond near equally to any linear polarization. There is a slight downside in that a circular antenna’s response to linear polarization is down –3db compared to its own polarization response – a small price when compared to –20db plus fades.
Circular polarization comes in two flavors (sense), left hand (LHCP) and right hand (RHCP), which refers to the direction of rotation of the wavefront. Like two opposite linear polarizations, the two circular types don’t talk to each other well. RHCP response will be –20db or so to LHCP and vice versa. Actually, this turns out to be a significant advantage to using circular for RC.
One issue of using 2.4GHz compared to the lower VHF bands is that reflection from surrounding objects is more prevalent at the higher frequency because of the shorter wavelength, which can result in signal interference at the receiver. Circular polarization has the property that upon reflection, the polarization sense reverses. RHCP becomes LHCP and vice versa. This is not true for linear polarization – the polarization remains the same. So, if we are transmitting and receiving circular of the same sense, any reflections, having been reversed, would be reduced considerably. A BIG advantage!
Basically, antennas are reciprocal devices – they behave the same whether transmitting or receiving. However, there are some parameters that apply more significantly to one mode or the other. The feed point of an antenna, where a transmitter is connected, has a certain impedance (RF resistance), which ideally should be the same as the transmitter output impedance for maximum power transfer. If there is a transmission line (coaxial cable) connecting the transmitter to the antenna, the line also has a characteristic impedance (most commonly 50 ohms) and again, for best performance, all three impedances (transmitter, feed line and antenna) should match. If the antenna impedance is mismatched to the transmitter or feed line, a standing wave results, which can represent a power loss and the transmitter isn’t very happy with a mismatched load.
You may see a SWR or VSWR (Voltage Standing Wave Ratio) specification stated for a given antenna. A perfectly matched antenna (referenced to 50 ohms) would have a SWR of 1.0, but generally a number less than 2.0 is acceptable. The lower the better. Depending on receiver design, antenna SWR may not be as critical for receiving, however some of the newer telemetry radios both transmit and receive using the same antenna, so it’s worth considering.
Another parameter of importance is the antenna bandwidth (don’t confuse with beamwidth). An antenna designed for a specific frequency has certain characteristics at that frequency only. As the frequency is changed, plus or minus, from this center frequency, those characteristics will change to some degree. The degree of change vs. frequency determines the range of frequency where the antenna performs in an acceptable manner. This is the bandwidth, which is highly dependent on the antenna type and design.
For radio systems that operate on a single frequency, bandwidth isn’t much of an issue. However, a number (if not most) of 2.4 GHz systems use spread spectrum or frequency hopping technology, which require operation over a range of frequencies. For these systems, bandwidth is significant.
Two of the more significant parameters connected to bandwidth are impedance and directivity (pattern or gain). An example is that an antenna may have an SWR of say 1.2 at its center frequency and 2.0 at some plus and minus frequency, which would represent the bandwidth if 2.0 is our specification limit. We would also like the directivity to be reasonably consistent over the bandwidth for predictable results.