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Can anyone answer this question :

I have a 4 element yagi antenna which is horizontally polarized. Let's say it has a front to back ratio of 20 dB. Does that front to back ratio apply to received signals just for horizontal polarization, or for received signals of any polarization ? For a vertically polarized received signal, would only the parts of the yagi which have a vertical component, for example the diameter of the tubing of the elements, be involved in reception ? Then when looking towards the yagi from behind, for a vertical wavefront which is really thin, the yagi would look like say 4 x 19 mm high thin slivers of antenna in a row spaced by the distance between the elements, which doesn't have any front to back ratio at all ...

My brain hurts.

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  • $\begingroup$ In practical HF operation over real ground, this is a non-issue. According to the ARRL Antenna Book, the ionosphere tends to cause arriving waves to be elliptically polarized; that is, there are vertically and horizontally polarized components that change with ionospheric and path conditions. As answered previously, the arriving plane wave does not have a "thickness" because it was not generated by a coherent source. $\endgroup$ – Brian K1LI Oct 29 '18 at 16:40
  • $\begingroup$ Hi Brian thanks for the reply, i should have been more specific, the question is directed at received signals with a single non-changing polarization in free space. $\endgroup$ – Andrew Oct 30 '18 at 4:03
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No.

It can't. The cross-polarization loss of a linear antenna (such as the dipole elements in your Yagi-Uda antenna) is infinite.

Hence, you won't see really vertically polarized signals at all with a horizontal antenna. So, there's undefined forward/backward gain.

For a vertically polarized received signal, would only the parts of the yagi which have a vertical component, for example the diameter of the tubing of the elements, be involved in reception ?

They wouldn't, unless they are wrongly dimensioned from the start; they should be too small to "rotate" anything (because that would be bad for horizontal polarization), so anything that's vertically polarized stays vertically polarized, and doesn't cause any current in the antenna feed.

The yagi would look like say 4 x 19 mm high thin slivers of antenna in a row spaced by the distance between the elements, which doesn't have any front to back ratio at all

'xactly! :)

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  • $\begingroup$ My understanding is that the cross polarization loss of the linear antenna you mention is only infinite if the antenna presents zero surface area in the opposite plane and that in reality all horizontally polarized yagi antennas for example have some vertical component because the elements have some vertical height and are not infinitely thin in the vertical direction. $\endgroup$ – Andrew Oct 30 '18 at 4:08
  • $\begingroup$ I don't understand your second comment. What i mean is that vertically polarized signals can only induce a voltage in the parts of an antenna which have a surface area or physical presence if you like in the vertical plane. $\endgroup$ – Andrew Oct 30 '18 at 4:12
  • $\begingroup$ I think that for the third bit, this would be true if the wavefront was really thin and the height of it's wavelength. I was just trying to make a point there. I think in reality the wavefront isn't thin but more like a wall approaching the antenna. This is where i start getting confused. $\endgroup$ – Andrew Oct 30 '18 at 4:15
  • $\begingroup$ a wavefront hasn't got a thickness – I really don't understand your point here. Think of it this way: the polarization property says in which direction the electrical field lines go. Only along these field lines, a potential difference and hence a current in a conductor can be produced. If you put your conductor in parallel to the field lines, you get maximum current. Put it perpendicularly, and you get 0 current. Full stop – there's nothing with thickness here, it's all about direction only. $\endgroup$ – Marcus Müller Oct 30 '18 at 9:23
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The pattern of any antenna can be divided into Vertical and Horizontal components (these are not the only choice sometimes we use LHCP and RHCP, etc). For V and H we're looking at the Theta (up/down) and Phi (left/right) electric fields.

(we analyse antennas as transmitters, its easier, and they work the same when receiving)

Now the yagi will create some V and some H radiation on every direction, but the radiation is mostly H and mostly in front. For a well designed yagi, if we call the front Hpol radiation intensity 0 dB, then I estimate: sidelobes of -10 dB. Exactly sideways -15. Backlobes -15. Exactly behind it there is a hole, -20 dB. This is an azimuth cut, moving around in the Theta=0 plane. So far so good.

Now plot the Vpol radiation. The amount of this is strongly dependent on the feed and matching, how well balanced the coax is, whether the mast resonates at that frequency. I am going to guess for a typical Hpol yagi installed on a metal mast and Gamma fed, the Vpol radiation will be -15 dB in front, compared to the Hpol in front. Going around the antenna though, I don't expect it to change much. As the mast, cable etc are radiating, it will be roughly omnidirectional.

As an aside, If the yagi were floating in free space with only a tiny transmitter connected to the feedpoint, the Vpol radiation would be much less, perhaps -40 dB compared to Hpol.

So to answer your question - well first the front to back ratio needs to be carefully defined to make sense. I would start with this: The ratio of total power radiated to the back, to total power radiated to the front. Because when you look at F/B ratio, you're interested in the rejection of all signals from the back, not just horizontal ones. Perhaps there is a more complete definition out there.
It doesn't help the amateur to talk about the Vpol F/B ratio, but you could certainly define it and calculate it. For an Hpol antenna, Vpol F/B might be close to 0 dB.

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  • $\begingroup$ Thanks Tomnexus, that all makes complete sense to me. $\endgroup$ – Andrew Oct 30 '18 at 5:14
  • $\begingroup$ I think the definition of front to back ratio is often not clearly defined. I have read that the front to back ratio is the ratio of the maximum forward gain to that one point in the exact opposite direction ie : rotated 180 deg. But to me it makes more sense to define it as you say for transmit as total power radiated to the front compared to the total power radiated to the back. So you can't determine front to back ratio with two measurements. $\endgroup$ – Andrew Oct 30 '18 at 5:26
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    $\begingroup$ You could define 'Maximum Front to Back Ratio' as the maximum forward gain compared to the minimum reverse gain, those two values possibly being anywhere in their half of the pattern. $\endgroup$ – Andrew Oct 30 '18 at 5:32
  • $\begingroup$ Oooh, hang on. I meant Total power = V power + H power. You could go on a separate course and define Front to Back Sector... This is what cellular companies use - they're not so much interested in the Phi=0 to Phi=180 ratio, as the peak radiation anywhere in the back sector. This is much more strict, and more useful. So the definition is very important. But F/B is usually just the two points. $\endgroup$ – tomnexus Oct 30 '18 at 5:33

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