Linear Antennas

Linear Antennas

Linear antennas are normally supplied with your transmitters and receivers, linear is now considered by most as old school technology for FPV with the realization of circular polarization, but it still has its place. We will take you through a few different types of antenna here, the names, properties and uses for each. Terminology used too.

Antenna Terminology :

Omni :

The standard whip or rubber duck antenna are all omni antennas. The rubber duck type are the main antennas you get with transmitters / receivers

Yagi :

A receiver antenna that you see everyday ( your TV antenna )

Patch :

A panel antenna with large gain properties

Patterns :

With antenna specifications you sometimes get a graph like this showing the pattern of reception :

So what are you looking at here ? Well take the first graph, the vertical. What this is showing you is the pattern of reception as if you were stood on the floor looking at your ground station. So from the graph above you can see the antenna in the middle receives both front and back with a decent amount of height to its pattern.

The graph on the right, the horizontal is a picture of the reception you would see if you were above the antenna in the sky looking directly down at it. So you can see from the graph that it receives in a circular pattern.

When you combine the two you can see the antenna receives in a doughnut shaped pattern. So there are gaps above and below the center.

Gain :

Not a measure of range, an often made misconception. Easily mistaken for range value because as the gain value of the antenna increases so does its working range.

Some get really confused with how gain works.  Ill try describe it in a manner that should be easy to picture. Imagine your antenna is a light.

An Omni antenna one of low gain like 3 dbi is just like an inspection lamp used by mechanics. If you stand it upright, light is emitted all around 360 degrees. Imaging the light is the receivers view. If your flying round it in the light your antenna can see it. If you go too far away then its not in the light, then your receiver cant see it.

Right, now for a Yagi or Patch antenna imagine a torch in your hand. Both are directional much like a torch. You have to point them. When you twist the front of the torch the beam emitted from it gets either narrow or wider.

OK so take a 8dbi patch antenna, still fairly low gain so imagine your torch with a wide beam. You can fly your plane in a wide area in front of it, the receiver will see it, but when you go too far to the left or the right, up or down your out of the light. So the receiver will loose the plane.

Now take a Yagi like 16 dbi or something, this is like the torch with its beam very narrow. Because all the light is concentrated in a narrow beam it can reach much much further, just like the antenna, but the beam the plane has to fly in to get picked up by the receiver is much more narrow. To make these antennas work, much like the torch you have to point them right at the plane at all times.

SWR :

SWR is a value used when installing and tuning transmitting antennas. When a receiver is connected to an antenna the impedance of the antenna and feed line must match as close as possible for most efficient reception. The SWR value of the antenna varies because of many factors. The antenna’s natural resonance, the frequency being received and the antenna’s height above the ground.
The SWR reading on an antenna cable is used to determine length from proper matching to the antenna and receiver. A SWR of 1:1 is what your after

Polarization :

This depicts how you mount an antenna to match the waveforms from transmitter to receiver. An omin antenna has a linear polarized waveform so when mounted upright the waveforms are transmitted vertically. The receivers antennas polarization must then match to receive a polarized signal

SMA :

This is the name of the type of connector used to join the receiver to the antenna. The antenna itself has a male SMA end the receiver a female SMA

So lets start with the most common of antenna

The Omni antenna :

These provide coverage in an even circle all around the antenna. They get there gain by focusing all the energy from the sides, this means that above or below them is where the losses are. If there is a height difference between the transmitter and receiver it is possible to get losses twice the gain value of the antenna.

Omni antennas are normally vertically polarized.

The 3 dbi Omni pattern is like this :

This antenna has a good circular pattern meaning you can fly around this antenna in all directions and receive good signal. The downside to this antenna is its range. You cant stray to far away.

Also available are higher gain Omni antennas, some buy these thinking that the higher gain means more range, where this is true the gain as we have said is the pattern not the range.

The pattern for a 7dbi Omni for example is this :

What you have now is a flat disk pattern, meaning if you fly high ( as planes tend to) your now out of its pattern and no longer receiving a picture :(

The Yagi Antenna :

The Yagi antenna is commonly seen over the UK in the form of your TV antenna bolted to roofs for miles around. Yagi aerials are directional, they focus the energy both horizontally and vertically. Both values are not equal that`s why both angles are quoted in specs.

Because Yagis are neither horizontally or vertically polarized, polarization depends on which way they are put up.

This type of antenna has a much higher gain. The pattern below is from a 12dbi Yagi antenna :

From this you can see the Yagi can reach much much further out than the omni, but its field of view is very narrow, it has poor reception to the rear of it as well.

With the higher gain comes a penalty, it has a much more narrow field of view so must be pointed towards the plane as it moves around the sky. You can however against popular belief fly to the sides and behind this antenna to a certain degree.

The Patch Antenna :

Patch antennas are a panel form of antenna, there gain can be very large, they are more compact than a Yagi antenna.
Patch antennas are available in a few formats. Horizontally polarized if mounted at 90 they become vertically polarized and Circular polarized. Circular polarized means the plane can be banked at any angle and still receive a good signal. Circular polarized antennas have a 3dbi loss because of the polarization but the advantage outweighs the loss

Here’s the pattern of a 8dbi patch antenna :

As you can see fairly similar to the Yagi`s pattern, again directional but a wider field of view. If pointed in the general direction of the plane a signal will be received. Good for long distance flying and diversity systems

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