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HF Skywave Polarization – Not What You Think

hf skywave polarization

All shortwave signals arriving at your receiver have elliptical polarization. HF skywave polarization is simply not what you think it is.

Most of our HF antennas have horizontal or vertical polarization. Polarization refers to the orientation of the E-field, as shown above. So, a vertical antenna orients the E-field up and down, while a beam or dipole does it side to side, above left.

But the moment your radio signal enters the ionosphere, it starts to spin on its axis. And, it’s still spinning when it gets to your receiving antenna. Simply put, HF skywave polarization is elliptical, sometimes circular.

We rarely notice this effect, because the loss between a circular versus linear polarization is only 3 dB, or about half the signal power. That’s about half an S-unit, and on shortwave, not worth worrying about. Now, you have probably heard the polarization mismatch can be as high as 20-40 dB loss. But that only happens for direct waves between horizontal vs vertical antennas, or between right versus left hand circular polarization.

What sets your E-field vector spinning is its interaction with the Earth’s magnetic field in an ionospheric plasma. This effect is most prominent when the radio signal path is perpendicular to a magnetic field line of force. Most skywave signals cross many magnetic lines of force which can increase or decrease their spin. More on this later.

So, your bottom line is that neither vertical nor horizontal polarized antennas have any advantage when it comes to HF skywave propagation. It’s all elliptical.

HF Skywave Polarization – Other Factors

Now, antenna polarization can make a difference. Vertical antennas tend to have lower angles of radiation if there is a good ground plane. This increases skip distance. Directional horizontal antennas, at sufficient height, can increase signal strength and reduce interference.

But when it comes down to transmitting or receiving skywave signals, antenna polarization in itself does not matter. HF skywave polarization is always elliptical. This is simply the result of physics caused by magnetic fields in a plasma.

One comment

  1. Zaba says:

    Happy New Year, John! And thanks for reminding about this very important feature of HF-propagation. Actually there is just too little experimentation with the polarizations, except lately for NVIS on the low bands, with marked differences with clockwise vs. anti-clockwise phasing and dependences on the time of day…

    Actually a X-configured yagi on the higher bands makes quite an interesting option, but one has to realize that the ground reflection has an overwhelming effect on the polarization pattern. Apart for additional effects around the Brewster angle,
    for high-up antennas the rotation inverts with every elevation lobe, whose maxima are peaking at different elevation for the horizontal vs. vertical component, and so
    varies the ellipticity of the combined vectors as well…

    Anyway, if one feels that “everything” has been explored, then that will open a new world… Also it is interesting to compare the signals of two antennas when ‘added’ electronically or by listening through two separate channels and letting one’s brain combine the spectrum… Cheers/73, Zaba OH1ZAA

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