http://blog.oscarliang.net/linear-circular-polarized-antenna-fpv/

Circular or Linear Polarized Antenna For FPV

Posted on October 19, 2013 by Oscar

The antenna directly affects the FPV range. There have been discussions about what type of antenna and how much gains should be used for RC airplane, quadcopter or multicopter FPV, so we are going to explore the types of antenna, the difference between them and the pros and cons of these options. Check out my previous post about how Antenna gain would affect the range.
Also here is how you can make your own DIY cloverleaf antenna.

Common Types of Antenna

The dipole – This is the antenna to which all others are compared. This is typically the stock whip antenna that comes with most transmitters.

Skew-Planar Wheel – this antenna is circularly polarized and has excellent multipath rejection capability.  This antenna is used for general purpose flying where aerodynamic drag (e.g. wind) is not a critical factor. While generally regarded as a receive antenna, it works well as a transmit antenna too.

The Cloverleaf – this antenna is an improvement to the skew-plannar mainly as a transmitter. It is larger so it’s affected by aerodynamic drag more. It can be coupled with a Skew Planar wheel for long range or obstacle penetration.

Types of Polarization

Polarization means the way the radio signal travels in space, it is related to the performance of your radio system (both transmitter sticks and FPV system). There are 2 types of Polarization: Linear and Circular.

Linear Polarization

Normal antenna uses linear Polarization, such as most of the original transmitter stick antenna. Basically the linear polarized signal oscillate horizontally or vertically in one plane while travelling.

This type is affected by the antenna alignment. When the receiving antenna is placed in the same alignment as the transmitting one, the signal is transmitted at maximum effectiveness. But if you turn one of the antenna 90 degree, you will only get a small overlapped area (an 90 degree polarization angle) and the signal strength becomes bad (30 dB loss). This is referred to cross polarization.
Because our airplane is moving constantly, if it is tilted to the left, you have a larger polarization angle and therefore reduce the signal strength. To solve this problem, we have circular polarization.

Circular Polarization

In this type of polarization, we are transmitter the signal on both horizontal and vertical planes with 90 degree phase shift, therefore instead of having a sine wave signal, we now have a corkscrew-like signal.

However the rotating direction also matters. For example if we have a right-handed polarized transmitter antenna and a left-handed polarized receiver antenna, it would result in a 30 dB signal lost (cross polarization)

Pros and Cons of both Polarization

One of the good reason we use circular polarized antennas is because of multipath interference.
Multipath interference is the most common reason for bad video quality. It happens when the signal is reflected back from something and it gets distorted or might change its polarization. Both the good (direct) and the bad (reflected) signals are received which could cause a bad image. The distortion could be shown as a color change, large bars, scrambling image even drop-outs.
The causes of multipath could be the position of your antenna, or obstacles and objects in your flying environment, so there are ways to prevent it.

Linear polarization is more common because of its simplicity. The antennas tend to be smaller, cheaper and easier to build. It is capable of longer range than circular polarization, but is more susceptible to multipathing. The range advantage is seldom realized due to multipath interference.
Not only the advantage on multipath interference with the circular polarization, it also does not lose its polarization when a plane roll to turn.
Here are some of my personal ideas on when to use which type.

When to use circular polarization

• your receiver is close to large obstacles such as trees, or buildings (within 30meters)
• acrobatic flying (constantly changing the position of the plane)
• low altitude flying
• flying very high altitude (you need a directional antenna)

When to use linear polarization

• flying out in the open field with no obstacles
• very stable fly, does not roll or pitch a lot
• long distance flying
• standing on top of a hill
• have no space for large antenna

Conclusion

Polarized antenna tends to be quite expensive, and it’s not very straigh-forward to make either. So i will try to write a post about that.