These antennas do work best if you take the time to tune them. If you are building the antenna yourself (easy enough and you can optimize the dimensions for your wingtip) then you must tune it.
SWR bridges work only if you provide RF energy (transmit). A much easier and more accurate way is to use an Antenna Analyzer like this:
http://www.mfjenterprises.com/Product.php?productid=MFJ-259C
If a local ham does not have one to come over and help you, have your EAA chapter buy one. It also comes in real handy for troubleshooting antenna and feed line problems.
I built 5 or 6 wingtip VOR antennas using scrap aluminum and a piece of fiberglass for the gamma match dielectric. I always attach the antenna to a 0.063" piece of aluminum angle that gets bolted onto the outboard rib. This way the wingtip comes off without detaching the antenna.
Rough dimensions for building your own antenna can be found in the AeroElectric book. I make mine so they extend as far as possible into the wingtip. If you make the length an inch or so longer than called for you have something to trim for tuning.
Tuning:
1. Attach the analyzer to the feed point.
2. Determine the resonant frequency (the lowest SWR point).
3. Trim the end of the antenna to raise the resonant frequency to about 110mhz.
4. Adjust the gamma match (the piece with the fiberglass) to further reduce the SWR. You can see which way to go (more capacitance or less capacitance) by loosening the nylon screws that hole the aluminum/fiberglass/aluminum sandwiched together (this reduces capacitance) and see if SWR goes up or down. If it goes down, then make the piece of aluminum that has the feed line going to it slightly smaller and measure again. This is an iterative process but it only takes a few minutes.
5. After the gamma match is set, trim the overall length of the antenna to have minimum SWR at about 113mhz.
At altitude I get VOR signals at 90+miles. I pick up the nearest VOR (5 miles away) with the airplane inside a metal roof hangar.
While you have the analyzer out, connect it to your comm antennas at the radio end of the coax and verify they are working correctly.
Carl