if you add 10 inches to the bottom, you might only be able to subtract 1/8 inch from the top. anything you do to the shaft has minimal effect on the whip. I build custom antennas all the time. If I had to recommend an antenna for the cascadia, it would probably be about 81 inches long to come even with the top of your trailer. 6 or 7 foot fiberglass whip would work if you are using the brackets that mount down low.
I have to dissagree with you. I had a short (5 inch) shaft wilson 5000. I tuned it and ran it. I decided to put the longer shaft on the bottom to get the load higher and I had to trim about 5 inches from the whip to get the SWR back down to were it was.
Good info right here: http://www.wilsonantenna.com/cutting.php Down at the bottom: (Coil Factor : for W-1000 use : C=41.65 ; for Trucker 5" use: C=52.21 ; for Trucker 10" use C=53.84) The shafts obviously are interchangeable from 5' to 10"...thus this calculation for the whip adjustments/cutting. I've got one of each shaft length but haven't tried the 5000 on my set up yet. Found this link when wondering about how the shaft length affects the whip length.
Have two different SWR meters that I use. Before I put on the 10 inch shaft the SWR was flat across the band. I put on the shaft and had to cut more then 4 inches off the whip to get it back to were it was. I did not change anything else, just the lower shaft. I was trying to find the longer 16 inch shafts but could not find them locally. Trying to get some of the flex out of it by getting the whip shorter.
What was the phase angle of the jumpers you used to hook up the swr meter to the radio and antenna? If they were anything other than 180 degrees for both, then you are not getting an accurate reading with the swr meters since they only indicate flat swr when they see 50 ohms. Swr meters don't care how the 50 ohms is happening as long as you can fool them into seeing it. Which is what your impedance transformer (odd length jumper) is doing. With an antenna analyzer it doesn't matter how long the coax is because swr never changes with cable length.
Correction: any multiple of 180 degrees can be used when using cheap swr meters to obtain a fairly accurate reading.
Current and voltage are only equal every 180 degrees. Where current and voltage both equal zero is where you want to cut the cable to make it null to allow the swr meter to see the actual feedpoint impedance.
All other factors being equal, the top-loaded antenna will have a lower takeoff angle than the center-loaded antenna.
