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

Dave Martindale dave.martindale at gmail.com
Tue Jun 5 00:12:13 UTC 2012

```I don't think that's correct.  A right-hand spiral (however you define
right-hand) remains right-handed if you rotate the whole object in space so
the centre axis of the spiral points in the opposite direction.  A
right-handed spiral is converted to a left-handed one only by reflecting it
in a mirror.

Try this: pick up two identical bolts.  Think of the bolt heads as the feed
end of the antenna, with the threads being the helical element.  Rotate the
two bolts so they are aligned on the same axis, but facing each other.
Note that the threaded portions of both bolts spiral the same way.  So two
identical antennas will work fine as a transmit/receive pair over an
open-space path.

But if you bounce a RHCP signal off some passive reflector, the signal
becomes LHCP (or vise versa), and the transmit and receive antennas need to
be mirror images of each other.

Dave

On Mon, Jun 4, 2012 at 7:26 PM, Magnus Danielson <magnus at rubidium.dyndns.org
> wrote:

> 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<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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