Re: Vertical radiation from horizontal dipole?
- From: Roy Lewallen <w7el@xxxxxxxxx>
- Date: Sun, 20 Aug 2006 16:46:44 -0700
Jim - NN7K wrote:
Think what he may mean is: if you use a Circular polarization , it will receive both horizontal,
and vertical polarization signals, equally well
tho at a decrease of 3 dB in signal , vs. horizontal
to horizontal, or vertical to vertical
polarization. A good way to observe this
optically, for LINEAR polarizations, would
be to find an old pair of sunglasses, useing
polarized lenses. break them in two, and then look
throuh BOTH lens's . As you rotate one, keeping
the other stationary, note the loss of light thru
them. At 90 degrees, it should be almost black!
but at 45, degrees, the degree of darkness (this
is for the stationary lens) will be about the same
if the rotated lense is moved either + or - 45
degrees (the equivalent of circular polarization
in an optic field. Don't know if this explaination
helps, but migh give it a try-- Jim NN7K
Unfortunately, it's not demonstrating circular polarization at all.
Circular polarization isn't the equivalent of 45 degree tilted linear polarization. The polarization of a circularly polarized wave (RF, light, or any other electromagnetic wave) rotates, one revolution per cycle. So over each period, the polarization rotates from vertical, through intermediate angles to horizontal, back to vertical but oriented the other direction, to reverse-oriented horizontal, back to vertical again. A 1 MHz field does this a million times per second.
If you view circularly polarized light through polarized sunglasses, the intensity will be the same regardless of how you rotate the glasses. If you pass circularly polarized light through one polarized lens, the light is linearly polarized on the other side. So rotating the second lens behind it illustrates only cross polarization of linearly polarized waves.
If you have a purely linearly polarized field, say, horizontal, and rotate a dipole in a vertical plane in that field (with the plane oriented so the field is broadside to the dipole), the signal received by the dipole will be maximum when the dipole is horizontal, zero when it's vertical ("cross polarization"), and intermediate values in between. This is the equivalent of the polarized sunglass experiment. But if the impinging field is circularly polarized, the received signal will be the same for any of the dipole orientations. This is because the field is always aligned with the dipole for two instants every cycle (when the antenna response will be maximum), cross-polarized for two instants every cycle (when the antenna response is zero), and at some intermediate relative polarization for the rest of the cycle (when the antenna response will be greater than zero but less than the maximum). And the proportion of each is the same regardless of which position the dipole is rotated to. The 3 dB attenuation relative to a linearly polarized, optimally oriented field is due to the fact that the circularly polarized wave is cross-polarized to various degrees during the cycle and is optimally polarized only for those two instants each cycle. A dual situation exists with a circularly polarized antenna and linearly polarized field: a linearly polarized wave of any orientation is received equally with a right or left handed circularly polarized antenna. Any plane wave can be divided into either vertical and horizontal (or any two orthogonal) linear components, or into right and left handed circular components. Any linearly polarized wave has equal magnitude right and left handed circular components. Any circularly polarized wave has equal magnitude horizontal and vertical linear components. Hence the antenna responses discussed above.
Like a circularly polarized wave, a 45 degree linearly polarized wave also has equal magnitude horizontal and vertical components. But this doesn't make it the same as a circularly polarized wave. The horizontal and vertical components of a 45 degree linearly polarized wave are in time phase or 180 degrees out of phase; those of a circularly polarized wave are 90 degrees relative to each other. This essential difference causes the orientation of the linearly polarized field to stay fixed but the orientation of the circularly polarized field to rotate. Put two crossed dipoles close to each other and feed them in phase or 180 degrees out of phase, and you'll get a 45 degree linearly polarized field broadside to the antenna. Feed them in quadrature (90 degree relative phasing) and you'll get a circularly polarized field broadside to the antenna.
Linear and circular polarization are limiting special cases of the more general elliptical polarization. The polarization of an elliptically polarized field rotates each cycle, but the amplitude can also vary during the cycle. The ratio of the minimum amplitude to the maximum (or vice-versa, depending on the reference) is called the axial ratio. Circular polarization is the special case of elliptical polarization having an axial ratio of one. Linear polarization is the special case where the axial ratio is zero (or infinite, depending on the definition used for axial ratio). A general elliptically polarized wave can have different horizontal and vertical linear polarization components, and different right and left hand circular polarization components.
Roy Lewallen, W7EL
.
- References:
- Vertical radiation from horizontal dipole?
- From: lu6etj
- Re: Vertical radiation from horizontal dipole?
- From: Roy Lewallen
- Re: Vertical radiation from horizontal dipole?
- From: Green Egghead
- Vertical radiation from horizontal dipole?
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