Sky technical question

Not quite, it's because the dish isn't passive like an aerial it has electronics in the LNB that are powered by the dish. That's where the polarity is switched and in some cases (I'm not sure about the ones in use for Freesat/Sky) there's also a switch between high band and low band frequencies. So when you change channels it's not just the tuner that selects the right frequency, it has to send a signal up to the dish to switch stuff

With terrestrial TV the transmissions from a particular site are either H or V and are in a certain frequency range so the aerial is chosen and installed for a particular combination, it's just a passive bit of metal. You can split/amplify the signal between multiple tuners.

The same is true of cable - no LNB, all the frequencies come down the wire together and it is split. here for example I have one bit of one co-ax feeding 4 tuners: 3 in a V+box and one cable modem

It might be useful if she has a seperate FM hi-fi tuner (or a DAB one if it's a DAB aerial) although often communal systems have those odd halo aerials that don't really work that well


I doubt a landlord, just like most of the public, would know what 'Freeview+' is. As pointed out by others, the installation is no different to a normal terrestrial receiver so it makes no difference


Is she using an installation that was specifically put in for her or the one built into the building? The latter will rarely be set up for Sky+ for the reasons already discussed.

My sister had a similar issue at her old flat when she got Sky - the flats had a multi-LNB dish but each flat only got one feed from it!

What is the reason why an LNB can't just send down the whole band and both polarisations, like a terrestrial aerial does?


That's really the same thing although it lets you record more than two things. The problem is I suppose that services are fairly disparate across transponders and the chances of you only wanting to record 2 things on the same transponder are unlikely.

It sounds like MythTV is like those Psion Wavefinders, with the correct software you could record everything on a particular DAB MUX at the same time to separate mp3 files. Not particually useful

Slightly more complicated than this and there are different ways of handling it.

Terrestrial TV is broadcast with both horizontal and vertical polarisation. However all of the channels from a given transmitter are broadcast with the same polarisation, so you can have a fixed polarisation aerial. The aerial can be passive (or have some amplification) but the signal it receives is what is passed down the cable to the receiver. (Signals of around 470 to 860MHz in the UHF Band IV and V used for analogue and digital TV). Therefore all tuners can be fed from the same aerial feed - as it has the entire band from that transmitter.

AIUI transmitters using the same frequencies use alternative polarisation to reduce long distance interference even further. So if you want to receive local TV and TV from a distance away, you may need two aerials - with different polarisations, though you'd not want to mix them together - you'd feed them to separate tuners (or use some very neatly designed filters IF they were suitably far apart in frequency terms). (In the UK we also have a band plan that groups analogue TV stations quite close together in the spectrum - so that you can have an aerial optimised for a particular part of the UHF TV band) When the BBC were planning for the next generation of Freeview, they looked at using both polarisations at once (to deliver a lot more bandwith) - but as this would require different aerials to be fitted, it wasn't incorporated into the DVB-T2 standard.

Satellite TV is different. Both polarisations (horizontal and vertical in Europe - though in others parts of the world left/right hand circular polarisation is used - as it was for European DBS services like BSB) are used, and you need to be able to switch between them on many platforms (*) to be able to get all of the services. Satellite TV also operates at around 11GHz - that's a hugely higher frequency than UHF TV. You can't send it down a cable - it would degrade too quickly. So instead of that the LNB (the bit at the focus of the dish) uses techniques to translate the satellite frequency band down to something that can be carried down a coax cable. However because most satellites still have a wider band than can be accommodated down long cable runs, the LNB can be remotely switched between shifting two different frequency bands down the cable. The receiver tuner switches between H and V polarisation and Hi/Lo frequency bands to select the 1/4 of the available band it needs to tune the service.

This means that a standard, European, universal LNB actually can send down 4 different sets of signals - Hi/H, Hi/V, Lo/H and Lo/V. So conventionally you need a separate feed from an LNB to each tuner, or from a multiswitch that can switch between 4 permanent feeds of the 4 bands.

Unicable changes this, by instead of sending the whole of each band down at a time, it only sends one transponder's width down. The LNB (or device in between) actually mixes a whole bunch of "selected" transponders into one signal, with each tuner having it's own dedicated slot. As long as there are enough slots for every tuner, they can all share a single cable. Also - fibre LNBs are now appearing where all 4 combinations are sent down a fibre optic cable (which has much higher bandwith and goes far longer distances without signal degradation)

(*) Some platforms in some territories, not the UK, broadcast all of the services for a given platform in the same 1/4 of the band (i.e. all Hi/H for instance), so that you can send the same LNB feed to every tuner, as the LNB never needs to be switched on a channel-by-channel basis.

Yes, that's exactly why it's done. The orientation of the aerials need to be the same orientation, a vertical aerial trying to receive horizontal transmissions won't get much if anything. I don't think it's totally zero signal, but it's enough to avoid interference. Incidently, a radio signal has both an electrical and an magnetic element, this is why the ferrite rod in a MW/LW tuner is horizontal in your radio whereas the transmitter aerial will be vertical.

The general rule in the UK with TV is that main transmitters are horizontal and relays vertical, although there are exceptions. With radio things are different as very few people listen with fixed aerials on their wall. When the BBC started broadcasting in VHF/FM they did it in horizontal polarisation, then came along ILR which used mixed from launch which was far better for reception in cars and on portable sets. So from the late 70's they re-engineered their network to be mixed too, with aerials at 45 degrees, the big disadvantage was that the power needed to be higher to give the equal signal strength at both polarisation. That is why the circular 'halo' aerials are pointless.... they are an omni directional horizontal aerial and there are no horizontal transmissions to pick up. A single dipole vertical aerial is omni-directional

Thanks for the very detailed explanations!


It's not just "old" new builds either. I moved in to a brand new flat in September 2008 which only had a single satellite feed.

The one I'm moving in to was built in 2005 so I don't hold much hope. I am pretty sure there is a faceplate in the master bedroom though, so I could run a line of coax downstairs in to the main room. Would be a faff though.

It's worth adding to this how things work in distribution systems in flats.

There will be a quatro LNB on the dish, giving outputs of H-Lo, H-Hi, V-Lo, and V-Hi (incidentally, in a professional context they're often referred to as X and Y instead of H and V). All four are fed into a distribution switch box (called a multiswitch), which effectively pretends to be an LNB but with many outputs: it receives the volts from the receiver, and switches out the appropriate band to each output.

For blocks of flats that have been cabled up for PVRs, then each flat will just receive two outputs from a multiswitch box.

In some cases, the distribution feed will also have other aerial feeds added to it - and then something called a diplexer splits it back out again. That's how the faceplate in my living room has sockets for TV, FM, DAB, and two satellite feeds.

<rant>
Except the second satellite feed still doesn't work. When I moved in a year ago, they'd already draped the new cables down the side of the building; a few months later, someone abseiled down to properly attach them to the wall (and gave me a bit of a fright, because I'm not used to seeing people outside my window: it's on the third floor); a few months after that, someone turned up unannounced to drill a hole in the wall and fit the dual-feed faceplate; but they've still not connected the new cable to the headend, apparently because they need to wait until they've done all the flats in the block.

It might not work though, as I mentioned above I know someone who tried just that: one tuner connection into the bedroom socket and the other into the lounge. It turned out that they were the same dish connection just split into two and so the two tuners fought against each other

Yep - until recently that was the only solution available. Unicable, as I mentioned, is another viable system now. It can either use a specialised LNB (for smaller installs) or specialised multiswitches, but rather than having to route every tuner feed back to the multi-switch, you can route one feed to multiple residences, and allocate each residence a number of "slots" on the unicable system. (This can also potentially support multi-satellite set-ups as well - though multiswitches already do)

(I'm trying to work out the best way of implementing a system to receive Astra 28.2E/Hotbird 28.5E, Astra 19.2E, Hotbird 13.0E, Sirius 4.8E, Thor 0.8W and Atlantic Bird 5W. Looks like a Toroidal dish, and lots of Quad LNBs with Diseqc switches... I don't want to go motorised as I want to be able to view and record from different satellites at the same time...)

In Spain it's common to see properties having two aerials of opposing polarities pointing in the same direction (usually those ugly Televes wideband jobs). I've heard two theories, one that it's so they can use adjacent channels by alternating the polarisation, and the other that analogue transmissions are all horizontal, and when DTT was introduced those transmissions are all vertical.

I think it was true for both those reasons. The DTT allocations in Spain were indeed adjacent to analogue channels (as they were/are here in many cases) but there was concern about the stronger analogue signals 'deafening' DTT receivers.
Not unfounded, here the early OnDigital boxes (particularly the Philips ones) suffered from just that.

All academic now, DSO in Spain is complete. I think post DSO DTT uses HP for main stations, VP for relays (much like here).

Well it appears to have recorded *some* overlap, as there's more than 115 minutes of BBC HD Wimbledon coverage but it has "Part Rec" the BBC One HD coverage due to a schedule change apparently. I didn't bother sitting through it all to see what happened.

I'll test it with some shorter programmes soon.