What is the difference between LHCP and RHCP forDJI Goggles 2

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hi i need a bit explain what differents RHCP vs LHCP what mean one left and another right ? need buy both if i want instal on my goggles 2 ? Thank you

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Hi there!

This is a long reply, so I’ll summarise at the end if you want to skip it!

LHCP and RHCP are different antenna “Polarisations” - I’ll explain what this means below! There are other types of polarisation, but the only important other one you need to know about for FPV is “Linear Polarisation”.

When you’re sending a signal from an antenna to another antenna, that signal can take on many forms. If you were able to see the signal from a linear antenna (antennas which just look like a straight pole or wire), you’d see the signal as a 2D sine wave moving through the air between the antennas.This is great, as long as you can guarantee that both antennas are going to be in an upright (or otherwise matching) position at all times.

You can think about the antennas as one of those wooden shape block kits for young children - the square peg fits into the square hole, that sort of thing. The transmitting antenna creates the shape of the block, and the receiving antenna is the hole.

From the perspective of the antenna, the sine wave we were talking about earlier actually looks like a 1-Dimensional line segment. That’s the shape of our block - a line, and the orientation of the line matters. The receiving antenna is like a line shaped hole, but if you’re trying to push the line through at an angle, it won’t fit - this is where we see signal degredation between the two antennas. For linear antennas, they both have to be in the same orientation for the signal to “fit” the receiving antenna. This is a problem for FPV drones, where the drone might be in any orientation, and the receiving antenna on the goggles is essentially fixed in place.

That’s where Circular Polarisation comes in. Instead of a 2D sine wave being transmitted, a Circularly Polarised antenna will transmit the signal as a sort of 3D Spiral towards the receiving antenna. In this case, from the antenna’s perspective, the signal looks like a 2D Circle (think about looking down through the end of the spiral). The benefit is that, on the receiving end, a circular signal (our “block” from earlier) will fit into a circular hole in any orientation. What that means is that for Circular Polarised antennas, orientation doesn’t matter, as the circle will be received no matter how you rotate it. It’s for this reason that Circular Polarised antennas are used in FPV drones, as you can flip and flop around and the signal can still be read by the receiving antenna.

LHCP and RHCP are two different types of circular polarisation - but they’re easy to grasp. Going back to the spiral, you could have a spiral in a clockwise (“Right”) direction, or an anti-clockwise (“left”) direction. That’s the only difference between LHCP and RHCP - LHCP transmits a spiral in the left direction, and RHCP transmits a spiral in the right direction. RHCP stands for “Right Hand Circular Polarised”, I’m sure you can figure out LHCP from that! :slight_smile:

It’s important that the receiving antenna and the transmitting antenna are both the same polarisation. This adds a dimension to the block-and-hole analogy from earlier, but an RHCP antenna can best read RHCP signals, and an LHCP antenna can best read LHCP signals. The two are not compatible - an LHCP signal will be “cancelled out” by an RHCP antenna, and vice-versa.

When an LHCP signal bounces off of a surface, it reverses and becomes an RHCP signal. The same goes for a bouncing RHCP signal - it becomes LHCP. This can lead to some interesting problems - if you’re trying to receive an RHCP signal, how do you know that it’s actually RHCP when it reaches you if it’s been through a complicated environment with lots of obstacles?

That can be where you see a benefit from having both an RHCP and an LHCP antenna. It allows you to receive a signal regardless of whether it’s “bounced” or not on it’s way over to you. However, if you’re only transmitting in one polarisation, it’s generally best (for our purposes) to just have all the antennas on the receiving side be the same type, and instead chase the benefit of having a wider area of coverage for the signal.

In summary:

  • RHCP and LHCP are types of “Circular Polarisation”

  • A “Linear” signal is like a 2D Sine wave, and a “Circular Polarised” signal is more like a 3D Spiral in shape.

  • Where a Linear signal depends on antenna orientation being the same on the transmitter and receiver side, Circular Polarised signals do not.

  • This is beneficial for FPV drones, as the drone could be in any position when it transmits the signal. Therefore, we use Circularly Polarised antennas.

  • LHCP antennas require an LHCP signal, and RHCP antennas require an RHCP signal. LHCP antennas cannot read RHCP signals, and vice versa.

  • When a Circular polarised signal “bounces” off of a surface, it flips from RHCP to LHCP or vice-versa. This can mean that having both RHCP and LHCP antennas on the receiving side is beneficial, as you can pick up multiple forms of the same signal regardless of the path it took to reach you.

  • However, when you’re only transmitting in one polarisation (e.g. RHCP), it’s likely that having multiple of the same polarisation (RHCP) on the receiving side to add more coverage to an area will provide better results for an FPV drone than using a mix of RHCP and LHCP to detect signals from multiple paths.

I hope this helps!

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Best regards,

Cameron

Unmanned Tech

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