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I've read that the spiral antenna is circularly polarized, either right or left circularly polarized, depending upon the construction.

I want one antenna be the emitter, and the second one be the receptor. What should I choose: both antennas right (or left) polarized, or differently polarized?

EDIT: more precisely. I want to built 2 spiral antennas, each one looking to the other at a distance of 1m (that is, one in front of the other). The first antenna will emit at 1 GHz, and the second one will receive the signal. Should I build the two antennas exactly the same, or should I revert the chirality of one of the antennas?

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The antennas should be wound in the same direction, which is to say they have opposite polarization, since polarization of an antenna is defined according to what you see when looking down the boresight of the antenna. The image presented by @V.V.T is correct, but the statement that they have the same polarization is incorrect because you must turn your head 180 degrees to look down the boresight of each antenna. In other words, you would purchase oppositely polarized antennas.

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It depends on the application. If you use the pair for a line-of-sight communication, the polarisation is identical in both transmitter and receiver sides. If you use the pair in a ground penetrating radar, the polarisations would be opposite because the reflection reverses the polarisation handedness (left or right), and, with the reversed propagation direction, the handedness for TX and RX antennas would be identical.

Antennas of the two practical communication device, if each device uses one antenna both to transmit and to receive, should be oriented as per @carloc's comment: the antennas must look not to each other, but in one direction along the line of sight. If the devices have separate TX and RX antennas, and for some reason the devices must look to each other when in a communication session, then the handedness of the emitter and the receptor need to be opposite, of course. The permanent issue of the handedness convention and the like considerations.

All this said, the configuration for your scenario ("2 spiral antennas, each one looking to the other") should be like in this picture:

spiral antennas

The antennas are pictured in blue, the line-of-sight axis in magenta.

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    \$\begingroup\$ thx for your answer. What do you intend exactly when you say "metal detection device" ? \$\endgroup\$
    – MikeTeX
    Commented May 9, 2021 at 6:10
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    \$\begingroup\$ @MikeTeX Sounds like radar? Not sure. \$\endgroup\$
    – DKNguyen
    Commented May 9, 2021 at 6:33
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    \$\begingroup\$ Mmmm I believe that in case of circular polarization TX and RX antennas need to be opposite polarization instead. They just face each other, what looks CW one way looks CCW when flipping point of view \$\endgroup\$
    – carloc
    Commented May 9, 2021 at 6:55
  • \$\begingroup\$ Of course, ground penetrating radars, not metal detector coils. Thanks to @DKNguen for correction. AP mines detection was on my mind when I wrote the answer. \$\endgroup\$
    – V.V.T
    Commented May 9, 2021 at 8:57
  • \$\begingroup\$ @V.V.T. I am now even more confused. I realize that it's my fault because I have under specified the conditions of the problem. I'll make an edit to make the situation clear. \$\endgroup\$
    – MikeTeX
    Commented May 9, 2021 at 9:21

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