Possible Antenna issue effecting signal

Hi there,

I have a iPhone 5S on three which was working fine up until yesterday when the network ended up being stuck on searching. I managed to perform an update and this went away and it started to recognise the network, but then I noticed the signal kept going and it would say "No service".

Curious, I lightly tapped the bottom side of the phone and the signal came back. Now when using the phone, the signal drops and I have to tap it to get the signal to come back. My assumption is that the antenna must be loose but before I go opening up, I thought I'd see what you guys suggest here and double check exactly which antenna effects the mobile signal.

It's not longer under warranty as it's 3 years old.

Many thanks!

Stacey