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One explanation I found is that a signal reflected from ionosphere is always circularly polarized. Thus it can be received by both vertically and horizontally polarized antennas (naturally, with 3 dB attenuation). However I don't trust the source very much and thus decided to ask for a second opinion.

Is this explanation accurate or is the truth a little more complicated?

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    $\begingroup$ The loss between linear and circular polarization is 3 dB, not 6 dB. Think of it this way: a circular polarization is both horizontal and vertical, combined. Received with just a vertical antenna, only half (-3 dB) of the power is received, and the horizontal part is wasted. $\endgroup$ – Phil Frost - W8II Jan 22 at 16:03
  • $\begingroup$ I believe it's -3 dB (10 * log10(1/2) ) in terms of amplitude and -6 dB (20 * log10(1/2) ) in terms of power. $\endgroup$ – Aleksander Alekseev - R2AUK Jan 22 at 16:20
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    $\begingroup$ Decibels are always power ratios, by convention. The "20" factor in the decibel calculation comes from the fact that power is proportional to voltage squared (assuming a fixed impedance), and 2*log(x) = log(x^2). The difference is -3 dB either way, and if you want to consider that as "half the power" or "1/sqrt(2) of the voltage" is up to you. $\endgroup$ – Phil Frost - W8II Jan 22 at 16:52
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Why can I receive a vertically polarized signal on a horizontally polarized antenna on HF and vice versa?

Who says you can?

Cross-polarization is a common source of path loss even on HF. LZ1AQ has some experiments which show as much as a 25 dB difference between horizontal and vertical polarization. 25 dB is quite a lot, enough to make the difference between 100% copy and unintelligible.

In practice, ionospheric propagation changes rapidly and unpredictably, and the polarization might be rotated 90 degrees one instant, and 30 seconds later, not rotated at all. As such, trying to match the other station's polarization is a futile game of chance. So it's not so much that the problems with opposite polarization don't exist on HF, but rather they are just unavoidable.

Unless of course you have both horizontally and vertically polarized antennas to choose from, and some mechanism to combine them dynamically in the best way in the moment. This is called diversity reception, and it can greatly increase the robustness of a receiver. It could be as simple as a switch between two antennas, or two separate receivers and antennas driving different ears on headphones, or a pair of phase-coherent receivers and a dynamic algorithm that finds the best combination for each moment.

Why is the fading only 25 dB, and not much more? Theoretically, the coupling between cross-polarized antennas should be zero, but in practice there's always a bit of overlap. Polarization is not a binary choice between "horizontal" or "vertical", but can be any angle. The coupling between antennas is proportional to the cosine of the difference between the angles, so there's infinite loss only when the polarization is exactly 90 degrees apart.

Furthermore, most paths, and especially ionospheric paths, are not truly just one path: they are the combination of many paths. It is very unlikely all possible paths at any given time will be cross-polarized.

Finally, all real antennas are somewhat sensitive to the opposite polarization though imperfections in their design. For example, a horizontal dipole often has a vertical feedline. Although not an ideal behavior, the antenna will be somewhat sensitive to common-mode currents on this feedline which will be vertically polarized.

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    $\begingroup$ Phil - Did you meant to say "Who says you CAN'T?" $\endgroup$ – mike65535 Jan 22 at 18:45
  • $\begingroup$ @mike65535 no, I'm pretty sure the point Phil is making is that on an ideal polarized antenna, you CAN'T. So, the claim that OP can receive on a h-pol antenna a v-pol signal is simply wrong – either the antenna isn't 100% h-pol, or the signal not 100% v-pol, or as he explains, both. $\endgroup$ – Marcus Müller Jan 22 at 21:51
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    $\begingroup$ Can i point out also that a physical horizontal antenna for example is not infinitely thin in the vertical plane because the wire or elements have a height, and as such it has some vertical component which will respond to vertical signals. The reverse applies for a physical vertical antenna. Also, not all signals received are via skip, reception can be via ground waves on HF, and these signals probably have mostly the original polarization $\endgroup$ – Andrew Jan 23 at 4:59
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Reflections from the ionosphere are randomly distorted from the original polarization - but not circular. Polarization matters for ground waves but not for most sky waves.

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