On 05/06/12 00:30, Dr. David Kirkby wrote:

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

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@rubidium.dyndns.org

This is a subject I have some familiarity with.  A helix antenna which is
right hand for receive is also right hand for transmit.  Think of it this
way.  If you have a bolt with a nut on it and you turn the nut to the right
it will move along the bolt away from you.  If you turn the bolt around so
you are looking at the other end and turn the nut to the right it will move
away from you.  For your transmit antenna the waves are moving away from you
and turning to the right.  For the other guy's receive antenna the waves are
turning to the right and moving away from you.  It's the same if you think
of yourself as the receive guy and the other guy as transmitting.

Regards.

Max.  K 4 O DS.

Email: max@maxsmusicplace.com

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----- Original Message -----
From: "Dr. David Kirkby" david.kirkby@onetel.net
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Monday, June 04, 2012 5:30 PM
Subject: [time-nuts] Antenna question about RHCP/LHCP I'm sure a time-nutcan
answer

Not quite.

"The definition of right-hand circular polarization, as standardized by
the IRE... is as follows:  For an observer looking in the direction of
propagation, the rotation of the electric-field vector in a transverse
plane is clockwise." - Jasik, "Antenna Engineering Handbook", First
Edition,  p17-3

A right-hand circularly polarized antenna both transmits and receives
RHCP.  What is confusing people is a reflection of a RHCP wave is a
LHCP wave.

-Chuck Harris

Magnus Danielson wrote:

Well, they could be consistent.

Most of those photos show only two sizes of helix-type antennas.  The
larger diameter (probably lower frequency) are quadrifilar helix designs,
and they are uniformly "left hand thread" helixes.  (I assume that everyone
agrees on what a left-hand thread looks like, no matter how they label
circular polarization).  The more numerous smaller diameter antennas are
multi-turn one-element helixes, and they always seem to be "right hand
thread" in all of the photos.  The smaller antennas are almost certainly
for L1.

The complication is the "Block-IIR-M-SV-2S" photo.  But it has three
sizes of antennas visible.  The largest are left-hand-thread quadhelix as
before, and thus likely close to the same physical dimensions as the large
antennas in the other photos.  The mid-size ones are multi-element
multi-turn helixes that look a lot like the quadhelixes in design except
that the ends are left open.  And they are about 2/3 the diameter of the
quadhelixes, much larger than the simple helix antennas in the previous
group, so probably for a different frequency.  Then there are the smallest
antennas, which appear to be a single-element helix with many many turns -
but these are about 1/3 the diameter of the large quadhelixes, and thus
these are likely the L1 antennas.  And, if I look closely, these small
helixes do appear to be right-hand-thread wound.

 Dave

On Mon, Jun 4, 2012 at 8:24 PM, Tom Van Baak tvb@leapsecond.com wrote:

On 05/06/12 04:51, Dave Martindale wrote:

Well, the traditional Block-II antennas had an inner and and outer ring,
at it was made so on purpose to create a antenna-lobe such that it would
direct more power towards the edge of the globe than straight down, such
that the distance difference and hence the damping is first degree
compensated such that the signal strength depending on azimuth is more even.

I could dig up the reference if I where at home, but I recall it since I
think it is kind of neat engineering.

Good that you guys set me straight on the orientation-stuff.

Cheers,
Magnus

On 5 June 2012 01:12, Dave Martindale dave.martindale@gmail.com wrote:

This is a funny topic. No matter where see it discussed, there are
people with different views on it. I looked on the edaforum

http://www.edaboard.com/forum26.html

and found a thread (can't find it now unfortunatey), where someone was
adament they needed to be one way (I forget whether both RHCP or
RHCP+LHCP), and someone else was adament a colleague nearly lost his
job after making that mistake. I think there was about a 50:50 mix of
views on the topic

I think I might have to make 3 axial mode helical antennas and test
this myself. If I wind two in one way, and one in another, it should
be possible to determine if the strongest signal is received with them
wound the same way, or wound the opposite way.

I don't know much about helix antenna design, but I know there are two
modes - "normal" and "axial". I have some software able to design
either, as well as some complex types. Sticking a frequency of 300 MHz
(so lambda is a convenient 1 m), I get:

Normal mode: helix diamater 0.05 lambda, helix spacing 0.05 lambda.
1.818 turns (no option to set gain).
Axial mode: helix diameter 0.318 lambda, helix spacing 0.222 lambda,
3.341 turns for 10 dBi gain, 6.667 for 13 dBi, 13.302 turns for 16 dBi
and 26.541 for 19 dBi and 33.413 turns for 20 dBi.

So the diameter of the helix does not tell you much on its own, as
there are two different types, one of which has a very different
diameter to the other.

Note the axial mode helix has a diameter of 0.318 lambda, so a
circumference of Pi*0.318 = 1.0 lambda. I think there is some story
that when Krauss invented this antenna, he made the first one with a
circumference of 1 wavelength, and more by luck than anything else,
got it right first time.

(Note, this software is designed to create a model for a 3D
electromagnetic simulator, so the results are not perfect, and one is
expected to tweak the design using the 3D electromagetic simulator).

Dave