Don't know if can explain this the best way, but in the
late '50's, early 60's there was a 6 meter antenna,
that was a YAGI design, but with a TWIST (Literally).
The Reflector was Horizontal, the 1st Director was
VERTICAL, and the other elements were "Skewed" so as
to imitate a helix , over the length of the boom
(some 25-35 FEET).

A normal helix, if memory serves, is wound 1/4 wavelength
to obtain circular polarization-- and the same for
satellite antennas, cross polarized , and offset by 1/4
wave, (and fed either 90 , or 270 degrees out of phase
from each other . For Right hand, or Left hand circular.

My question is 2 fold: 1) Did the Long Design really
generate Circular polarization (-3dB down in ALL polarizations)
or was this tilting at windmills.
and 2), for Circular polarization, does the constraint of
1/4 wavelength need to be applied to a (given) antenna?

Reason for this is an antenna that will pretty much
avoid polarity shifts, during band openings, or am I
Halucinating again?? Your thoughts Please
Jim NN7K

If you do a bit of analysis with a modeling program, I think you'll find
that if you generate a circularly polarized signal, it'll become nearly
linearly polarized once it reflects from the ground. And it's nearly
impossible, and often undesirable, to prevent ground reflection at HF.
There might be a way to generate a signal that's circularly polarized
after reflection, but I don't know how to do it.

Roy Lewallen, W7EL

Jim-NN7K wrote:
Don't know if can explain this the best way, but in the
late '50's, early 60's there was a 6 meter antenna,
that was a YAGI design, but with a TWIST (Literally).
The Reflector was Horizontal, the 1st Director was
VERTICAL, and the other elements were "Skewed" so as
to imitate a helix , over the length of the boom
(some 25-35 FEET).

A normal helix, if memory serves, is wound 1/4 wavelength
to obtain circular polarization-- and the same for
satellite antennas, cross polarized , and offset by 1/4
wave, (and fed either 90 , or 270 degrees out of phase
from each other . For Right hand, or Left hand circular.

My question is 2 fold: 1) Did the Long Design really
generate Circular polarization (-3dB down in ALL polarizations)
or was this tilting at windmills.
and 2), for Circular polarization, does the constraint of
1/4 wavelength need to be applied to a (given) antenna?

Reason for this is an antenna that will pretty much
avoid polarity shifts, during band openings, or am I
Halucinating again?? Your thoughts Please
Jim NN7K

That kinda the same conclusion I came to, Roy.
Asked on the VHF reflector, too-- the antenna was
apparently made by Telerex, and called the "Spiralray".
But tho it was touted as one that would work with
BOTH polarizations, I always wondered IF it worked-
and IF so, WHY no one ever duplicated it!
Like SOME old memories about radio, guess this one is
(Like TVI in the AM days, on Ch#2) best FORGOTTEN!!

No Free Lunch--(sigh)!

Tnx es 73, JIM



Roy Lewallen wrote:
If you do a bit of analysis with a modeling program, I think you'll find
that if you generate a circularly polarized signal, it'll become nearly
linearly polarized once it reflects from the ground. And it's nearly
impossible, and often undesirable, to prevent ground reflection at HF.
There might be a way to generate a signal that's circularly polarized
after reflection, but I don't know how to do it.

Roy Lewallen, W7EL

Jim-NN7K wrote:
Don't know if can explain this the best way, but in the
late '50's, early 60's there was a 6 meter antenna,
that was a YAGI design, but with a TWIST (Literally).
The Reflector was Horizontal, the 1st Director was
VERTICAL, and the other elements were "Skewed" so as
to imitate a helix , over the length of the boom
(some 25-35 FEET).

A normal helix, if memory serves, is wound 1/4 wavelength
to obtain circular polarization-- and the same for
satellite antennas, cross polarized , and offset by 1/4
wave, (and fed either 90 , or 270 degrees out of phase
from each other . For Right hand, or Left hand circular.

My question is 2 fold: 1) Did the Long Design really
generate Circular polarization (-3dB down in ALL polarizations)
or was this tilting at windmills.
and 2), for Circular polarization, does the constraint of
1/4 wavelength need to be applied to a (given) antenna?

Reason for this is an antenna that will pretty much
avoid polarity shifts, during band openings, or am I
Halucinating again?? Your thoughts Please
Jim NN7K

"Roy Lewallen" wrote
If you do a bit of analysis with a modeling program, I think you'll find
that if you generate a circularly polarized signal, it'll become nearly
linearly polarized once it reflects from the ground. ... There might
be a way to generate a signal that's circularly polarized after
reflection, but I don't know how to do it.
________

Roy, won't a c-pol signal remain c-pol after a low-angle terrain reflection,
except that its rotation sense is reversed? (The magnitudes of the v-pol
and h-pol reflection components are nearly the same, but there is a
180-degree phase reversal in the v-pol reflection with respect to the h-pol
reflection.)

This has been applied with good results in analog TV broadcasts using c-pol,
because a c-pol receiving antenna rejects reflections of the transmitted
signal -- which effectively reduces the multipath "ghosts" seen on a TV set
when linearly polarized receive antennas are used.

RF

Richard Fry wrote:
"Roy Lewallen" wrote
If you do a bit of analysis with a modeling program, I think you'll
find that if you generate a circularly polarized signal, it'll become
nearly linearly polarized once it reflects from the ground. ... There
might
be a way to generate a signal that's circularly polarized after
reflection, but I don't know how to do it.
________

Roy, won't a c-pol signal remain c-pol after a low-angle terrain
reflection, except that its rotation sense is reversed? (The magnitudes
of the v-pol and h-pol reflection components are nearly the same, but
there is a 180-degree phase reversal in the v-pol reflection with
respect to the h-pol reflection.)

This has been applied with good results in analog TV broadcasts using
c-pol, because a c-pol receiving antenna rejects reflections of the
transmitted signal -- which effectively reduces the multipath "ghosts"
seen on a TV set when linearly polarized receive antennas are used.

A head-on reflection to a flat surface results in the polarization sense
reversal you mention. But reflection at a shallow angle doesn't. To see
why, get the EZNEC demo program and run separate elevation plots of a
dipole and a vertical, save the first plot, then superimpose it on the
second. Begin with a perfect ground. You'll see that the field from the
horizontal antenna is zero at zero elevation angle, while the field from
the vertical is maximum. So if you reflect a circularly polarized signal
from a perfect ground at a very low angle, you'll end up with a
vertically (linearly) polarized field. The horizontal component
disappears (that is, the field disappears each time it rotates to
horizontal) because the reflection is equal in magnitude to and out of
phase with the direct signal, so the two sum to zero at a distant point.
If you look at a higher angle where the horizontal and vertical fields
are equal, that's an angle at which you'd maintain circular polarization
if those same two antennas were spaced and phased for circular
polarization. At the elevation angle of the next null in the horizontal
field, you'll again get a purely vertically polarized field. The
conditions change when the ground has a finite conductivity, but you can
use the same general process to understand what happens.

EZNEC+ or NEC-2 users can model particular antennas and directly see
what happens to the polarization circularity. In EZNEC+, click on the
Desc Options line, Plot and Fields tabs, and choose one of the circular
polarization choices for Fields To Plot. Then run a calculation and look
at the plot. You get the polarization circularity by clicking the FF Tab
button to show the pattern data in tabular form. The AxR column shows
the axial ratio - the ratio of major to minor axis in dB. 0 dB
represents perfect circularity, 99.99 dB means the field is completely
linear, and in between represents various degrees of elliptical
polarization. Begin with an antenna in free space and verify its
polarization circularity in some direction. Then elevate the antenna,
add a ground, and note the effect. A simple test antenna is a pair of
crossed dipoles, very close but not touching, fed in quadrature, which
will generate fairly good circular polarization broadside to the plane
containing the antennas.

Roy Lewallen, W7EL

"Roy Lewallen" wrote
A head-on reflection to a flat surface results in the polarization sense
reversal you mention. But reflection at a shallow angle doesn't. (etc)
__________

Thanks for the reply. I was recalling the experimental results that RCA
gathered in fixed and mobile tests in the Chicago loop when Ch 7 (ABC)
installed an RCA c-pol transmit antenna on Sears Tower there in the 1970s.
The ghost-reduction for same-sense c-pol tx and rx antennas was clearly
evident, and quite remarkable.

The link below leads to a scan of several graphs showing why this was true
for those near-in tests. RCA published a paper about it, which I will try
to find and provide a link.

http://i62.photobucket.com/albums/h8...eflections.jpg

RF

Richard Fry wrote:
"Roy Lewallen" wrote
A head-on reflection to a flat surface results in the polarization sense
reversal you mention. But reflection at a shallow angle doesn't. (etc)
__________

Thanks for the reply. I was recalling the experimental results that RCA
gathered in fixed and mobile tests in the Chicago loop when Ch 7 (ABC)
installed an RCA c-pol transmit antenna on Sears Tower there in the 1970s.
The ghost-reduction for same-sense c-pol tx and rx antennas was clearly
evident, and quite remarkable.

The link below leads to a scan of several graphs showing why this was true
for those near-in tests. RCA published a paper about it, which I will try
to find and provide a link.

http://i62.photobucket.com/albums/h8...eflections.jpg

Sorry, I can't get enough out of these graphs to come to any
conclusions. But the equations for reflection coefficient as a function
of polarization and ground conductivity and permittivity are well known
(cf. Kraus, _Antennas_) and are used by NEC-2 and EZNEC. So conclusions
based on reflection coefficients should be the same. Perhaps the
multipath reflections involved in TV broadcasting are primarily from
surfaces more or less normal to the radiated signal.

Roy Lewallen, W7EL