Thanks for the reply. I think the ebay antenna is a long shot for a number of reasons ( Dorne & Mangolin DMN-42 towel rail type) but we shall see.
I looked at the Bob Archer design for the wingtip and had the following concerns. The design looks optimized for the VHF band for VOR and Localizer with a gamma match. The gamma match will be way off at the 300 Mhz glideslope band so performance is problematic for glideslope. I also looked at the “whisker”
type antenna that have a balun and matching network and work quite well across the two bands. I haven’t seen any beam pattern data for the Archer antenna but would suspect that it would have a R cos theta shape off of the wingtip with a null off the other wingtip. I would also be concerned that the rejection of the vertically polarized wave component is not adequate for either VOR or localizer.
I thought about designing a center fed microstrip phantom slot but the dimensions are quite large and placement would be a big issue. Another big issue is not having access to all the test equipment any more so I feel the need to go with a tested design. The Archer design is appealing - cheap to build, hidden in the wingtip and seems to have a lot of success but the glideslope performance is a question in my mind. If you have data on VSWR and polar signal strength plots for the Archer I would be very interested. Smith chart plots are OK.
Maybe I am just over thinking this issue but better now that after committing to a bad installation.