Private Jets For Sale

You must be getting sick of by me now, but I need to ask another question:

Why do aircraft manufacturers like Boeing/Airbus always install the no. 1 VHF communications system antenna on the the top of the aircraft's fuselage?

Is it simply becuase certain other surfaces eg. the tail section, are more prone to damage or destruction, and so it is better to have an antenna on the fuselage as a precautionary measure?

For the airport Norwich, UK (ICAO: EGSH), the aeronautical chart for an ILS approach states:

AD Elevation: 117ft
THR Elevation: 106ft
OBSTACLE Elevations: 757 AMSL (651 above THR)

I understand that AD stands for Airworthiness Directive but how does that relate to [AD] elevation?

Also, is the obstacles elevation the maximum height of a surrounding building / object in close proximity to the airport?
i.e: the tallest building near the airport is 757 above mean sea level, which is 651 (757-106) above the threshold elevation?

Thanks for your answers in advance.
Thanks Joseph P.. what was unclear is now clear (Y)

1.A pilot, removed, reinstalled, and again removed an aircraft’s turn coordinator. This is a violation of?
2.An FAA inspection has determined that an aircraft that had recently received a 100 hour inspection is unairworthy. In fact an Airworthiness Directive (AD) that would have caught the unairworthy condition was also “signed –off” by the A&P mechanic. This is a violation of?
3.The Director of Operations and a pilot of a Part 121 operator underreported the flight time of a specific route in order to stay within the flight crew duty requirements. This is a violation of?
4.A Part 135 operator operated an aircraft three times after welding an oil scavenger line instead of replacing it as required by the manufacturer. This is a violation of?
5.A mechanic installed unapproved parts and used unapproved methods in a repair of a helicopter. This is a violation of?

its in FAR part 43 but FAR part 43. what? need answer for each question

Do fighter jets and helicopters have a special device like a GPS??
Like if a Naval aviator and an Airforce pilot are in the air, how does the NAVAL aviator know that a 'friendly' is approaching him? is there some kind of technology that makes him conform this on his radar or does he just have to trust that the Airforce pilot is not lying on the radio??? (thats alot of trust if you ask me)
Does the Coast Guard, NAVY, Army and Airfocre pilots have special flight codes that they use in conversations? and if so how does everyone memorize them?

Also, the NAVY, Marines and Airforce are the only branches with fighter jets; Do they help out and participate with one another or is there a sought of mild rivalry since all branches are well trained and not dependent on one another??