Circular polarisation question

[Matt]

lhcp definition

If I am setting up a line of sight RF link, which is the best polarisation to transmit to my RHCP antenna?

Thanks in advance.

 

[djalli]

LHCP would be the transmitter polarization for 100% efficieny to your RHCP antenna.

 

[dowjones]

I disagree. To have bood power transfer both antennas have to have the same circular polarization (RHCP in your case). Combining LH and RH you will only receive the crosspolarization signal level.

 

[salem_sajid]

Well i might no be knowing much but one defacto rule i read somewhere said that both Tx and Rx should have same polarisation for good reception.

 

[Matt]

salem_sajid,

That certainly is the case for linear polarisation, but I tend to agree with djalli (except for 100% efficiency bit, that would be nice).

Thanks for your opinions.

Matt

 

[dowjones]

OK, it is up to you, but I recommend to read about "polarization mismatch loss" topics. If you make a link between LH and RH you will receive nothing (in the ideal case).

regards

 

[djalli]

Maybe I am wrong here but please correct me. It maybe educating to me.

In a circular polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave.

If the rotation is clockwise looking in the direction of propagation, the sense is called right-hand-circular RHC. If the rotation is counterclockwise, the sense is called left-hand-circular.

So if you send in LHC wave imagine what receiving antena at the other end (in line of sight) expects? A RHC wave.

 

[djalli]

Forgot it!

Have an imagination as you stand behind of transmitter antenna and you are looking at plane of revolution. It rotates left

Now do the same thing as you go in receiving antenna.

I am trying to visualize it.

 

[vfone]

Each end of a link must use antennas with the same sense ( RHCP/LHCP )of polarization.

For reference go to:
Richard C.Johnson " Antenna Engineering Hand book . 3rd.Edition " Choice of Polarization , P 27-5.

 

[juppydu]

Hello,

I see there is a lot of confusion about RHCP and LHCP definition for Tx and Rx antenna. In paricular what says djalli is "extremly wrong", a collegue of mine has done this mistake and it was close to be dismissed by my boss (he built the RX antenna in the wrong polarization! )

There is a simple way to identify if an antena is Tx or Rx is RHCP or LHCP, you have two situation (be aware about the Rx definition, it is the less intuitive !)

1 Tx antenna: a Tx antenna is LHCP or RHCP "if and only if" a people behind the antenna see propagate from the antenna a LHCP or RHCP polarization.

2 Rx antena: a Rx antenna is LHCP or RHCP "if and only if" a people looking toward the Rx antenna see wave in LHCP or RHCP goes to the RX antenna.

the trick is that in both cases you have to stay looking the wave in the direction of propagation (TX-Behind, Rx-in front of)!

in conclusion a RX antenna is RHCP if it is abe to recive a RHCP Wave transmittet from a TX antenna in RHCP.
REMEMBER, ALWAYS BOTHRX AND TX ANTENNA MUST BE HAVE THE SAME POLARIZATION SENSE!

I'm sorry for my english lenguage !

 

[djalli]

Hello,
the trick is that in both cases you have to stay looking the wave in the direction of propagation (TX-Behind, Rx-in front of)!

I'm sorry for my english lenguage !

Thank you juppydu, I have always assumed you must stay behind both antennas.

 

[djalli]

Hello,

a collegue of mine has done this mistake and it was close to be dismissed by my boss (he built the RX antenna in the wrong polarization! )

Indeed this cases confuses a lot of people. Thank god I am still an undergraduate student. Should be enough to explain my in-experience in antennas.

This way we learn in elektroda. That is why I love elektroda.

 

[sergio mariotti]

I think is important here distinguish between opinions and certainties.
What is sure? What has been tried, tested, and used for many years in TLC.
It's sure that a RHCP RX will receive the signal transmitted by a RHCP TX.
It's sure that a RHCP wave is that regardeless the sense of the path (TX or RX)
It's sure that exist 2 opposite definitions; the IEEE one is opposite to the Phisical Optic definition.

Opinions: In the Radio field, i've worked only with IEEE definition.
I disagree the 1st post of djalli.
While i agree the 2nd post of djalli, it's the "source" of the name used for circular polarization.

I've seen that many perople, in practice, confuse RHCP and LHCP; i've observed that the reason maybe psicological and the explain fall out the subj of this post.

 

[juppydu]

Ciao Sergio !

I agree with you, it's obvious a definition (as other in electromagnetic field and in the all days life), anyway stated a definition it is very important to follow it.

 

[sergio mariotti]

Here a graphical view of a rotating polarization.
Ref.: MK3 VLBI Manual (It's a pictural view of what is explained on the Kraus-Antennas)