Art Unwin, KB9MZ wrote:
"What other options do I have for increasing Q other than silver plating
of the copper?"

Art has access to yhe 1955 edition of Terman`s "Electronics and Radio
Engineering", I believe. On page 32 Terman writes:
"In designing single-layer coils, the highest Q in proportion to size is
obtained when the length of the winding is somewhat less than the
diameter of the coil."

It appears Art went in the right direction by increasing the coil
diameter to 12 inches from 4 inches. Nonetheless, his coil is 35 inches
long.

If Art doesn`t want to use a high permeability core, and his coil
already has the required inductance, it seems fewer turns on a larger
diameter form would have a higher Q, so the length of the coil can be
less than the diameter of the coil for the same inductance.

It is good to space the turns by about the diameter of the conductor, or
slightly less. Insulation can be lossy and tends to rise in loss by the
cube of the frequency, so Terman warns about cotton or enamel covered
wire and insulating material used in coil forms at high frequencies. See
page 35 in his 1955 edition.

The existing coil can be measured for inductance, if it is right, and a
coil calculator or program can be consulted to get a coil with better
proportions.

To tune a circuit, a variable capacitor may maintain a better Q than a
variable inductance, (variometer) but at 160 meters, permeability tuning
of the coil should be practical if the power level isn`t too high.

Best regards, Richard Harrison, KB5WZI