Handful of Concepts - Portable Signal.

"A small ariel which plugs into your phone allowing signal
on the tube."

If you've ever experienced London tube transport before, you know how annoying it can get when your half way through a conversation with somebody - whether that is on the phone or texting, whatever the case; it is always interrupted with 'Hold on, i'll have to call you back in 20 mins, just going onto the tube'. This overtime becomes extremely annoying. So I have thought of a solution which would get by this life's obstacle - having a portable ariel no bigger than your small finger which connects into your phone via the AUX (Headphone) port. It would then emit a signal strong enough to hold a conversation whilst you are down underground, putting an end to those persistent disruptions.
 This could also work as a general ariel, if you have no signal in the middle of no where - just plug this bad boy in and hey presto, you've got signal.