If you are changing the size and length of the metal receiving elements of the home built antennas you are changing the operating frequency of the antennas. Because of the 20 foot high dirt mound and the weak signals at your location you Will need to get a factory built tv antenna. You are lucky because all of the receivable Real digital tv channels are UHF channels. You will NEED to get the antenna up to the peak of the roof , I suggest a peak of the roof eave mount
http://www.sadoun.com/Sat/Products/Perfect/PVEM1.gif , No Holes In The Roof with this mount. Be sure and answer these questions--> How many floors high is the apt. building?? Is your apt a end unit?? If not a end unit then where is your apt located?? How many apt's make up the building??