First, for any antenna, by changing the length of the feed line, you 
move the point along the voltage curve of the feed line at which you 
are measuring the SWR when that measurement is taken at the back of 
the radio, rather than at the antenna feedpoint. So, you are basically 
fooling the transmitter.

The sloper antennas from W8AMZ, Alpha Delta, or CableHelp, are 
shortened by what is in reality a loading coil. Some of the companies 
call it by other names, but it is basically a loading coil.
The Alpha Delta twin sloper model is a full size quarter wave on 80, 
and 40, but has a coil at the end of the 40 meter leg, with about 20 
more feet of wire beyond the coil which forms the 160 meter element.
Any quarter wave sloper, regardless of whether it is shortened by 
loading coils or not, must work against a counterpoise in order to 
tune and perform properly. They typically are designed to be hung from 
a tower, with the ground side of the coax attached to the tower, a few 
feet below the top of the structure. The tower, along with any metal 
antenna such as a tri-band beam that is on top of it, form the 
counterpoise. The beam then acts like a capacitance hat to help load 
the counterpoise end of the system. Thus, any sloper is greatly 
affected by whatever is, or is not above it. This is why the 
instructions say that you must run a wire from the feed point to 
ground if you are using the sloper with nothing above it, or if you 
hang it from a tree.

Basically, it is a quarter wave vertical that is leaning, and that is 
fed at the top, rather than at the bottom, with the tower or 
counterpoise wire replacing the radials that would normally be at the 
base of the vertical if it were being fed at the bottom.

In tom's case, I would consider adding a capacitance hat to the top of 
the mast. . A capacitance hat looks somewhat like mounting a bicycle 
wheel, complete with the spoaks, horozontally at the top of the mast. 
You can buy a capacitance hat from any number of sources, or build one 
with a trip to Home Depot.

Since his slopers are both physically short, any addition to the top 
of the mast will make it necessary to re-tune the antenna.

Mike Duke, K5XU