FPV Antenna Fundamentals
Antennas are one of the easily overlooked things by FPV’ers. Your car is only as good as your tires. FPV Antennas collect the video signals generated by the VTX and transmit it wirelessly where another Antenna placed on the receiver collects this video signal and process it and display it onto our goggles. All of this happens in less than 40ms, in comparison an average human takes 100-150ms (1/10th of a second) to blink.
FPV antennas play an integral role in having a good quality video feed, meaning to say static free and reduced noise and interference, video being displayed on our goggles.
Not only is a good quality antenna essential but the configuration of antennas used also matters. Today we are going to do just that, find out what makes for a good FPV Antenna.
How does an Antenna Work?
Before we move onto what makes a good antenna, let’s take a look at how an antenna works. Anything wireless must have a transmitter antenna and a receiver antenna. Our smartphone’s Wi-Fi, Bluetooth etc.., all have a transmitter antenna and a receiver antenna.
In FPV there is no difference between a transmitter antenna and a receiver antenna. But the type used on the transmitter and the receiver matters.
Transmitter antennas receive the video signals, in our case, and convert this electrical signal into electromagnetic radiation known as radio waves. Antenna’s do the job of converting the electrical signals into electromagnetic radiation by vibrating back and forth.
This transmitted signal is interpreted by the receiving antenna which converts the radio waves back into electrical signals. Hence the FPV antenna has a direct impact on how efficiently the video is transmitted.
Factors to consider
Below listed are some of the specs to keep in mind when buying the ideal antenna for FPV
fpv antenna types
There are 2 major types of antenna categories- Omnidirectional and Bidirectional antenna. Though how they work is quite similar but the mode of transmission of the signal widely varies.
Directional antennas transmit the radio waves in a very narrow beam, typically 120° or less. So if the transmitting antenna is not in the 120° receiving area, then the receiving antenna does not receive any signals and you lose the video feed, more like a focussed beam of flashlight.
On the other hand, Omni directional antennas ideally transmit at 360°, meaning to say no matter where or what orientation the transmitting antenna is placed, the receiving antenna always receives part of the video signal which is sometimes enough to steer your quad out of trouble, more like a light bulb radiating in all the direction.
There are a few types of Directional Antennas- Patch Antenna, Helical Antenna to name a few. In a diversity receiver, the common rule of thumb is to equip one Directional antennas such as a Patch antenna and one Omnidirectional antenna, to get the best of both worlds.
A Helical Antenna is mostly used for long range quads where the orientation of the transmitting antenna does not change much with respect to the receiving antenna.
Patch antennas on the other hand are used to receive signals when the quad is in front of you and within that 120° bandwidth.
Types of Polarization
Linear antennas transmit the waves in a single plane ie., either the horizontal or the vertical axis. They have a concentrated emission and hence have a longer range. But there’s catch to this increased range, the antenna from the quad and the receiver must more or less be oriented to each other.
If the orientation is lost, the image/video feed is lost. They are suitable for long range setups as the orientation of the quad with respect to the receiving antennas does not change significantly.
Circularly polarized antennas transmit waves in 2 planes ie.., both the horizontal and the vertical simultaneously. This results in a slight loss of the transmission power and hence a reduced range.
We don’t have to worry about the orientation as the receiver keeps receiving part of the video no matter the orientation of the quad. If you plan to race, the quad tends to keep changing its orientation rather often. Hence:
Omnidirectional antennas = Exceptional image quality at close proximity flying
Bidirectional antennas = Long range flying
LHCP VS RHCP - Circular Polarization
LHCP stands for Left hand circular polarization and RHCP stands for Right hand circular polarization. LHCP and RHCP refers to the direction the wave is rotated. If when viewed from the back, the wave is rotated to the right then the wave is Right hand circularly polarized. Likewise, if the wave is rotated to the left, the wave is known as Left hand circularly polarized.
Apart from how the wave is rotated, the antenna does not have any other difference. But as a rule of thumb, I would recommend getting a RHCP antenna for the sole reason that most pilots run RHCP antennas and spectating would be cool at an event.
Frequency is one of most important aspects to consider early on in the hunt for a suitable Antenna. Each antenna is tuned to work well with a particular range of frequencies. So a 2.4GHz antenna will not work well with a 5.8GHz setup and vice-versa.
Also omnidirectional antennas are only available for 5.8GHz and are not available for lower frequencies such as 2.4GHz or 1.2GHz, usually used for long range FPV. Why you may ask? Long range quads usually fly 10+ Km and since the quad is so far out, the orientation of the transmitting antenna with respect to the receiving antenna is very minimal.
Also Omnidirectional antennas are good for close to medium range and the strength of the received signals diminish greatly with increase in distance.
Free Space Path Loss
Now let us get a little geeky. Beware it involves a bit of math!!!
Free space path loss is the loss in signal strength of a signal as it travels through free space. The loss in power should be unity.
Pr = Receiving power
Pt = Transmitting power
f = operating frequency
c = Speed of light in vacuum (meters per second)
c = 299792458 m/s
This is totally unnecessary and your Antennas will work regardless of what the “Free space Loss” of your Antennas are, but hey wouldn’t hurt to learn new stuff right. Ideally speaking if all the transmitted radio waves is received at the receiving antenna, then the ideal Pr/Pt ratio equates to one.
If the received power is lesser than the transmitted power, then the ratio falls less than 1 which is likely going to be the case.
As always form factor or size matters. Why? Because a large and bulky antenna will weigh down on your face when the goggles are worn for long periods of time. A larger antenna being placed on the quad is not also ideal, as a bulky antenna can raise the overall weight of the quad, not by a significant number.
The only point of using bulky Helical antennas would be only for long range quads where the pilot will more or less be running a ground station. FPV Ground stations are FPV receiving stations that have FPV receivers equipped and placed on tripod to keep it as high as possible to avoid ground interference. The point of using a ground station is, the Ground stations can be equipped with heavier antennas without taking a toll on the user’s comfortability.
There are 2 major types of antenna connectors- SMA (sub-miniature version A) and RP-SMA (reverse polarity SMA). SMA has a male and female connector and likewise the RP-SMA connector has a male and female connector. So it is important to pay attention to the connector present on the VTX.
If the VTX has SMA male connector, then the antenna connector should be the corresponding female. The same goes with the RP-SMA connectors too.
One of the newer types of connector being used more and more by manufacturers is the MMCX (Micro miniature coaxial) type connector.
The increasing popularity is due to the fact that MMCX connectors have a swivelling head that rotates, getting rid of the stresses developed on the VTX connectors.
How does Antenna Gain Work?
Antenna gain is a number that defines the electric efficiency of an antenna. For a transmitting antenna, antenna gain defines how well an antenna converts the input signals into radio waves radiating in a specific direction. For a receiving antenna, gain defines how well an antenna converts the radio waves into electrical signals. Higher is better, generally.
So one might assume, a high gain Omni-directional antenna might work well at long ranges, but no, it doesn’t work that way. As the gain of an antenna increases, the directivity decreases. What this means is that as the gain increases the, the directional coverage decreases. It is not uncommon for Directional antennas to have gains of over 14 and the Omni-Directional antennas mostly top out at a gain of 5.
Long Range FPV Antenna’s
Long range FPV is a whole another discussion on its own. Long range FPV-ing typically is flying the quad more than 5KM out from the pilot. At these ranges, the radio waves transmitted from an Omni-Directional antenna is so faint that the receivers are not sensitive enough to pick up the signals. Hence a pilot who is planning to fly at these long ranges, using Bidirectional antennas are the only viable option.
Another thing to consider would be a FPV Ground station. Since the quads at long ranges transmit the videos horizontally, an average 6 feet tall man might not be receiving all the signals coming from the VTX. But when the Antenna is placed as high as possible, the received signals would be much more and the video might contain lesser noise.
Antennas play a major role in determining the video quality of the transmitted video signal. Hence buying from reputed manufacturers is recommended to cheap out on antennas as they can always be reused for further builds. Paying attention to Polarization of the antenna and keeping an eye on the antenna gain is recommended