The spacing between the wires on the horizontal top section is simply
dictated by the fractional wavelength and the degree of efficiency required
versus the mechanical and economical constraints of having a very close
spacing and more wires to give the same total width.  Think of it in terms
of a part of a Faraday cage. If the spacing is small relative to the
elevation then it's fine.  Shorting the far ends should not make much
difference.
73,
Roger, G2AJV.