The 165-175 foot length (longer than a quarter wave) raises the radiation
resistance of the antenna to somewhere close to 50 ohms but, at the same
time increases the inductive reactance. The capacitor is used to cancel the
inductive reactance when the total length goes beyond a quarter wavelength.
I use the same technique by making my "quarter wave verticals" longer than a
quarter wave to bring their radiation resistance up to 50 ohms and then I
tune out the additional inductive reactance by using series capacitance. I
live in a salt water environment and do not like using expensive variable
capacitors that are exposed to the elements. I use pieces of coax as a
simple inexpensive substitute for variable capacitors. RG-8 runs about 30
pf per foot. I "tune" the coax capacitor with wire cutters and seal the
exposed end with epoxy when I get the length right. The trick is not to
snip too much off at one time!
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