[time-nuts] Antenna question about RHCP/LHCP I'm sure a time-nut can answer

Magnus Danielson magnus at rubidium.dyndns.org
Mon Jun 4 23:26:24 UTC 2012


On 05/06/12 00:30, Dr. David Kirkby wrote:
> This is not exactly a time related question, but I'm sure the subject
> must be of interest to time-nuts using GPS.
>
> If one transmits from an antenna such as a helical one, RHCP, can the
> same antenna be used for reception, or does the helix need to be wound
> the other way?
>
> If you google this topic, there seems to be a lot of confusion about
> whether the TX antenna and RX antenna need to both have RHCP or whether
> one needs to be LHCP and the other RHCP.
>
> Given GPS uses circular polarization, I'm hoping someone here will know.
>
> It would appear there are different definitions of "circular
> polarization", with one considering it from the point of view of the
> source, and the other considering it from the point of view of the
> receiver. The IEEE apparently uses the former, and others (especially
> optics) use the opposite.
>
> http://en.wikipedia.org/wiki/Circular_polarization
>
> My aim was to make a gain measurement of two circular polarized
> antennas. I have two identical antennas, but are unsure if the signals
> should be received strongly, or whether theoretically no signal would be
> received. (Of course in practice, one never achieves perfect
> polarization, so there will always be a signal detected, even if
> cross-polarized.

They would have to have opposite rotation.

The waveform rotation will follow the transmitter antenna into the 
receiver antenna. The receiver antenna follows the same rotation that 
the transmitter antenna has, it's just that the face each other, so when 
you turn one of the 180 degrees such that they face the same direction 
you would see that they are in fact rotated in opposite directions.

I'm sure the sat folks can confirm this.

Cheers,
Magnus



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