Circular Polarisation

> What can I do to receive right and left circular polarised channels
> using a Universal LNB?

Probably nothing whatsoever. You need to read up on how a depolarizer works. Then you'll see that the latter will take the H & V rotating components of a circularly polarized signal, and reinforce them by combining them into one of two linear planes (depending upon whether LC or RC). The LNB can then discriminate between each orientation, and also benefit from a 3dB increase in signal. However, the depolarizer also has an effect on linearly polarized signals. If the dielectric material is positioned so as to minimize this, then the circularly polarized signals will be received at +/- 45 degrees to the H & V, and hence cannot be picked up by the (fixed) H or V probes of a Universal LNB. OTOH, if the material is positioned so as to present the circularly polarized signals AT H or V, then it will affect the linearly-polarized signals by creating an effective circulalry polarized signal, which will present at +/- 45 deg (and moreover be reduced by 3dB (= fifty percent!). Thus applying a depolarizer to a Universal LNB is a non-starter; it is ONLY applicable to an LNB with a separate polarizer (magentic or mechanical) which can select other angles apart from strictly horizontal or vertical.

> I have heard something about cutting a plastic kitchen board and placing it
> in front of a feed of LNB.
> I would rather use Your professional advice before I cut my wife's favourite
> chopping board.

I don't know where this story about using any old piece of dielectric has arisen over the years, but it keeps on being trotted out. I've worked professionally (in very high powered radar applications) with circular polarization, and can tell you that it's not just "a bit of chopping board". The permittivity of the material has to be carefully chosen, and from this the propagation velocity calculated (vis-a-vis that in air), to permit the derivation of the exact dimensions of the piece of material: these dimensions have to be accurate to within a percentage point or two of a quarter-wavelength, which you must remember is of the order of 6mm: so the block of Teflon needs to be machined to an accuracy of better than 0.1mm --- hacking a chopping board with a Kitchen Devil will NOT be very productive.

> Will it affect the reception of H&V?

See above: with a Universal, you could receive EITHER linearly-polarized OR circularly-polarized signals, but not both. (Of course, you *can* receive circularly-polarized signals without any depolarizer at all, but then lose 3dB of the signal) > Any suggestions deeply appreciated. Well, I'm going to assume that you were really hoping to get an improvement in signals from Thor I, since that seems to be the favorite pursuit of people such as yourself in the UK. In which case, DON'T BOTHER. Apparently, due to the shape of the beam from Thor I, by the time a dribble of signal reaches these shores, the majority of the circular polarization appears to have gone: the signal is to all intents and purposes almost linearly polarized (actually, elliptically). So a mechanical or magnetic polarizer on a traditional LNB could achieve much more, by orienting to the major axis of the elliptical signal. However, you DO need a big dish to even contemplate receiving Thor I; only in the eastern extremities of Kent, East Anglia, and Scotland is reception possible on something of reasonable size (most reports are of 1.2m dishes, although a favoured few have succeeded with 1.0m, or even 90cm). By the time one moves westward at all, the signal strength dives rapidly, and the minimum dish size increases: I see NOT ONE SMIDGEON of signal with a 1.2m dish in Oxfordshire. So realistically you will need a 1.8m dish or even bigger.

Thanks to for this explanation.
-- Brian {Hamilton Kelly}
"The fundamental design flaws [of the products of the Micros^WSirius Cybernetics Corporation] are completely hidden by their superficial design flaws." SLATFATF 1983 --- wasn't Douglas Adams prescient?:-)

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