FWIW, my experience with a PROPERLY INSTALLED Archer Nav antenna is that it?s performance is on par for VOR, LOC and GS with externally mounted traditional antennas. I freely admit that I have a limited number of places that I?ve flown an ILS into with an Archer nav antenna, and it?s certainly possible that local conditions might cause issues.
Many years ago I talked with Bob Archer himself and he was adamant that following the install directions was critical for max performance, particularly grounding and wingtip wire routing. Many times when people talk about their particular installation details, their explanation includes statements like ?instead of...? or ?Just like the directions said, except...?.