Antonio Vernucci wrote:

Instead of this, why not use identical feedline lengths into a Tee and
position one cross polarised antenna 0.25WL in front of the other, to
obtain
the 90-deg phase shift?

David, VK3BDK

Doing as you propose is possible, and would produce the same circular
polarization on the antenna boresight.

But moving away from the boresight, the circularity of polarization
would degrade (into elliptical polarization) faster than if instead
using a quarter-wavelength delay on one of the two feedlines.

73

Tony I0JX

Interesting..
I just ran some NEC models with a pair of crossed dipoles. Two cases:
1) where the two are a quarter wave apart and fed in phase
2) where the two are in the same place and fed 1/4 wave apart

here's the axial ratios
angle spaced phased
0 0.99895 0.99373
5 0.99259 0.99173
10 0.9686 0.9774
15 0.92991 0.95122
20 0.87836 0.91519
25 0.81619 0.87053
30 0.74579 0.8185
35 0.66954 0.7604
40 0.58964 0.69752
45 0.50804 0.6311
50 0.4265 0.56226
55 0.34658 0.49196
60 0.26988 0.42098
65 0.19811 0.34989
70 0.13339 0.27906
75 0.07827 0.20868
80 0.03576 0.13878
85 0.00889 0.06928
90 0 0

Not a huge difference.. you have to be 45 degrees away before it's 10%
different. If you were using directional antennas, the gain would be
way down by the time you were that far off boresight, so it would be a
non-issue.


I suspect that other factors would be bigger in importance.

Not a huge difference.. you have to be 45 degrees away before it's 10%
different. If you were using directional antennas, the gain would be way down
by the time you were that far off boresight, so it would be a non-issue.


I suspect that other factors would be bigger in importance.

I agree that, with a high gain antennas, it makes almost no difference at angles
within the antenna main lobe. I could make more difference with regard to the
susceptibility to strong interfering signals coming far away from the boresight,
but this may be quite a theoretical case.

It could be more important in the case of two crossed dipoles (turnstile)
typically used with low-orbiting satellites, but even in that case differences
may be hardly noticeable.

In any case it is good to keep that in mind.

73

Tony I0JX

I suspect that other factors would be bigger in importance.

BTW, if you work at JPL, please give my 73 to Eric Archer N6CV who was at my QTH
at least ten times!

73

Tony I0JX

On Sat, 2 Jan 2010 09:08:16 -0800 (PST), mr1956
wrote:

I am trying to stack two Yagi antennas to form a ground station to
receive GPS data from an experimental rocket.
(...)

The topic of model rocket telemetry communications was previously
covered in this thread:
http://groups.google.com/group/rec.radio.amateur.antenna/browse_thread/thread/c158b265a041f9b6/
While it doesn't answer your question about antenna stacking and
combining, it may offer some insight.

There was also this thread:
http://groups.google.com/group/alt.internet.wireless/browse_thread/thread/108522ee81505069
which involved model airplane control and tracking at 2.4GHz which
might be of interest.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558