I don't want to discourage the use of the archer antenna, particularly on the nav side by what I've posted, Dale. I hate hair sticking out on aircraft, probably more than most. Using a BNC feedthough on the end rib should reduce EMI well below the point of it not being a problem. That problem solvable.
However, the big problem using an Archer for a com antenna is still there. Polarity. A/C communication now a days are vertical polarized. FAA ground facilities used to use a kind or twisted turnstile antenna that was OK for any polarization, but I think that has mostly gone away. It is very difficult challenge to launch a vertically polarized 120 MHz signal from an object only 6 inches thick. Canting the antenna to get some vertical current as Bob Turner suggested could help, but not as much as the geometry would indicate. Much of the vertical current will be cancelled by the opposite vertical current induced in the structure, leaving very little radiated.
Re RG400 vs RG58, I see little performance advantage to the RG400 - a db or so. It is more heat tolerant, though. And if you are trapped in a space capsule with 100% oxygen, it won't make as much toxic fumes if it burns, and other advantages which may not be important to gen aviation.
I always liked the luxury, pre-GPS, of picking up the GS 40 or 50 miles out, hitting top of decent and riding it down. To do this, the GS has to be working very well. In my opinion, the surest bet would be a separate GS antenna. There are several posts on making a separate GS antenna, which isn't a big deal. Further, I have simulated the archer antenna for quite a while now, and though I don't know how accurate my model is, it does not do GS freqs too well. I know Bob Turner has said that antennas usually work pretty well on 3rd harmonic freqs, which is correct, but the Archer has a built-in matching network which can diminish this - so maybe not the best for GS distance, though many are happy with the Archer as a source for the GS.
3D of an Archer model which might be of interest..
I would like to mention for those with some background in electronics, there is a great antenna tool available on Ebay, etc called a NanoVNA. I bought a Vector Network Analyzer a few year ago for the better part of a grand. But these handy little rascals are only ~50 dollars. You'll need some BNC adapters as well - and maybe some good reading glasses
. More to say. Too much thread drift.
Ron