Topband: antenna coupler help for medium waves?

[sasas asasas]

we have a university official legal radio station. the transmitter is about about 250watt with 50 ohm output at 1431khz medium wave band. my antenna is an inverted-L antenna with (5/16)*wavelenght total lenght (20 meters
vertical and 45 meters horizontal wire 2,5mm).  we have 5 radials (2 of them are l/4 and the other l/8) . The ground system is a 2 meters copper pole vertical tin the ground. the transmitter is 8-9 meters above the ground surface. you can see my system here http://www.tzitzikas.webs.com/1431am.JPG .
i have 3 scematics for antenna coupler system. The first and the second
schematic send me 2 radioamateurs. I calculate the third coupler with 4nec2 software. Which coupler is the best
for my antenna??? do you have to propose another schematic?  the transmitter is only for educational proposes.
i have a variable capacitor 500pf and a variable coil about 50 turns 5cm diameter of turns. 

http://www.sv9cvk.gr/images/KWcoupler/COUPLER.jpg
http://www.tzitzikas.webs.com/match.JPG
http://www.tzitzikas.webs.com/4nec2.JPG

i have read that i need only a series variable capacitor to match the line to antenna. how much capacitance may i need for 1431khz for my antenna???

thank you very match.

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