Yes, you need an aerial to get Freeview through your TV. Check your current one to see if it's working properly – if not, it's best to get advice from a qualified installer.
Yes, Freeview carries HD channels. The same aerial that receives the standard def channels also picks up the HD ones. No special 'HD' version required. Your TV or receiver box will need a HD compatible tuner though (DVB-T2 tuner). Most TVs above 23" sold within the past decade should fit the mould, but there's bound to be the odd exception somewhere. Check your manual.
As for aerials, the correct type for your home might be different to what I use, or what others in this forum use. Much depends on the signal strength (the field strength) where you live.
Have a look at your neighbour roofs if the aerials are visible. If you know that someone is picking up Freeview with something that looks like it has been up 50 years because of how corroded it looks, then chances are that the field strength is fairly healthy. If that's the case, then a Log Periodic will probably do just fine. Where you're seeing lots of aerials on tall masts - 20ft of so - or satellite dishes rather than aerials, then reception may be trickier. Forum member @ericmark will tell you what it's like living in the Welsh hills and valleys and how that affects Freeviw reception.
If your neighbors have rooftop aerials for Freeview it’s likely all you need for a similar setup. All you need to do is make sure that your TV has a DVB-T2 tuner, and you’ll get HD channels. That Freeview’s postcode tool can also give you a signal coverage check for your area!
Your neighbours may not always have it right most of mine point their aerial off towards Ipswich which is many miles away while we have a transmitter less than a mile from us in the opposite direction.
So check location of your nearest transmitter.
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