I have tuned in my new Samsung HD UEES5500 TV and all of the available SD digital channels clearly appear, but not the HD channels which constantly "break up".

Could it be a problem with the terrestrial aerial, rather than the TV itself? The signal strength according to the TV is about 50%.

How can I resolve this problem?

Thank you.

50% signal strength is just on the safe side of marginal but, more importantly, what is the signal quality?
Advice here
http://www.bbc.co.uk/reception/digitaltv/freeview.shtml
http://www.frequencycast.co.uk/freeviewadvice.html

What type of aerial it/where's it located/how old is it?

Another thing worth checking, although fairly uncommon, is the position of HDMI leads near aerial cables

50% is meaningless, those strength scales are arbitary. Mine is 100% quality no-breakup with ~15% of 'signal strength'. agc comes in around 80%-90% 'signal strength'.

The fact that one multiplex is breaking up means some sort of aerial system problem not the TV as such.

Outside aerials get corroded connections, water in the cable, sunlight, weather damage etc.

If your aerial is in the loft it's obviously easier to check on it being the correct type, and maybe adjust the direction, position etc.

Have you got a phone nearby. If my phone is near my TV it will knock out the mux that Sky news is on the others are fine.Could be it affects just the HD mux on your TV

HD channels are almost pointless with an aerial. Get a satellite dish

Why ?

I have both, if anything the Freeview-HD pictures tend to look just a tad better (probably down to 1080p25 support)

Satellite adds just one extra free HD channel at the current time with a fixed dish (NHKWORLD TV) unless you have a Sky box which will give you CH5 HD with a card.

Ahem.

Not everyone is allowed a sat dish in rented or tied accommodation. Nor cable. Aerials are still the only method some are able to use to receive TV pictures.

Must just be from where I am then lol. Aerial signal here is awful and no matter where my aerial is placed, whenever I receive a text or something i always lose signal for a few seconds

A satellite dish has made a whole load of difference for me

It did for me for years, now my local transmitter has increased it's power post dso I get rock solid Freeview. Very surprised if you can't do the same. What does this site say about reception at your location after entering your postcode. (Tick the detailed view box)

http://www.digitaluk.co.uk/postcodechecker/

Are you sure the TV has tuned the HD channels in properly? If your aerial can 'see' more than one transmitter the TV may be using the wrong frequencies.

Put your postcode into the following url to find out details of your local transmitters.

http://www.wolfbane.com/cgi-bin/tvd.exe?DX=L&HT=10&O

What a stupid and pointless comment. There's nothing wrong with HD via an aerial. I too had problems with HD reception but mine was caused by to much signal which caused the pictures to break up all the time.