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.