I was about to make one of those types of antennas years ago, but a
Ham friend took me tothe local Ham shop and the guy at the counter put
together the right parts for a 5/8th wave antenna with a magnetic base
for about $80.
Works great, I receive gliders when those with 1/4 wave antennas don't
even hear them. It's about 5 ft high and removable which is a geat
advantage over the bolted to the bumper kind. SWR at 123.4 came in at
1.1 or less with 125.0 only 1.2 or so. It seems to combine the best of
all worlds in sensitivity, range of freqs and handling.

Give it a try,

Wayne