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If your Yes TV by Fetch box is having problems tuning into Free to Air channels then it could be two issues, the box itself or reception issues. I have had a Fetch TV box that had faulty tuners in the UHF band, so all channels broadcast in this band would not get picked up while all the VHF channels worked fine. The simple test is to try it via the TV only, except I would run the antenna cable directly from the wall to the TV and do a tune to see if you get all channels, this is your control test. If you do then great, it could be the box or you could have a weak signal. If you can't get all channels it might be the signal.
Before you go any further go to this website (http://myswitch.digitalready.gov.au/) and type in your address. It will show you what the broadcast signal is like around your area and also show you which direction your antenna should be facing. Take some notes and go outside and have a look to see it is set up right. Off the top off my head I think you can go into the Fetch TV settings under channels and see the signal strength so this is another way to see how much signal the box is getting. Your TV might have a similar function so you can check that too.
Now onto signals to explain why sometimes you get channel dropouts. In analogue TV as the signal got weaker you got a fuzzy picture, graininess, ghosting, etc and it got worse the weaker it got. In digital TV you get a perfect picture and it stays that way as the signal gets weaker until such a point that it just fails, you might get digital glitches if it is borderline, but the picture/audio just stops when it is too weak. So it is a case of on or off where as analogue was variable all the way. Where the reception fails as the signal gets weaker varies with each device and the sensitivity of the tuners.
Easy, lets move on. Let's say your antenna has 100% signal strength, if your house has two TV sockets then each socket gets 50% of the signal as it is split. In one of those sockets you plug in the Fetch TV, it has 3 tuners so now each tuners gets 16.66% of the signal, plus it has the bypass port for the TV so in reality it is probably closer to 12.5%, 3 tuners and the TV. Now lets say your aerial is not set up right, you have a dodgy socket on the wall or you live in a low signal area and you only receive a signal strength of 50%. You can see where this is heading.... By the way I should mention that it doesn't usually matter if something is plugged in or switched on at a socket for your signal to be split. So if you have 4 TV sockets in your house and only 1 TV, it still only gets approx 25% of the signal. Now the quality of all the connections from the antenna to the TV matter, the antenna connection, the cable, the socket, any splitters you have, it can all influence the strength of the signal. Bad earthing can also cause interference in the antenna cable so this could account for losing channels on the TV when the box is switched on if you are already getting a not so optimal signal. I hope you are still with me.
A few more things to go. Powered antenna boosters are usually a waste of time and are better installed by a pro who will test the strength and quality of your signal first. Basically the reason is if you have a rubbish signal and you amplify it, you have a amplified rubbish signal. A booster cannot increase the quality of the signal (full stop). If you read forums a lot I have lost count how many people write they went out and bought one and it did nothing. I am not surprised.
Onto antennas, DO NOT listen to the marketing spin about digital antennas. There is no such thing!! The signal is broadcast via radio frequency transmission within the same band limits of VHF and UHF that it always has been broadcast on. Except that before it was broadcast in analogue and now it is broadcast in a digitally compressed signal so you can get more bandwidth (hence higher definition and better picture) within the same frequency. All you need is an antenna that is tuned to receive the full range of bands in VHF/UHF for Australian broadcasts, which is most antenna sold in the last 40+ years. Back in the day before SBS some people had VHF only antennas so in this case you would need an upgrade, but if you received all the channels before digital there is no reason you cannot get them now if your signal quality is good. The only change that was needed between going from analogue broadcast to digital was the requirement of a new decoder to decode the digital broadcast, hence why people needed a set top box for old TV's until all new TV's had it built in. The whole need a "digital ready" antenna was just clever marketing to sell antennas. Though I must add that in some cases new antennas are needed for people as you might have an old antenna with low gain which was fine for analogue, but a new higher gain antenna might be needed to give you sufficient signal to get good digital reception (if you are at that on/off point I mentioned above and live in a poor reception area).
So what to do... go to the website I said above, check the signal for your area, if it is listed as good, take note of the direction, go outside and check your antenna (compare it to every other antenna on your neighbours roofs, make sure it matches, take note if it is sagging or you can see a wire or something lose (DO NOT climb onto your roof if it is wet, they are very slippery), check how many sockets you have in your house, do you need them all? (they must be disconnected at the splitter if to be removed), check the wall socket the device is plugged into (if it is only an aerial socket remove it and check the connection behind it, if there is a power socket on it also do not remove it, leave it to a pro), check you cable (swap it if you have another), unplug the fetch TV and try just the TV directly, if you have an old set top box try that also just to confirm everything. If the device is able to check the signal quality/strength via the device settings, if they are low you may need a pro. If after all this you still cannot get channels, then save the money you'd spend on a booster and put it towards a reputable antenna person to come and check your signal strength and quality, you may need to replace your antenna with one that has a higher gain if you are in a bad reception area.