Hi Squale and all,
"I am mainly concerned with the Civil Airband (108-137Mhz)"
You may consider a separate antenna for the aero band. You can make a groundplane out of an SO-239 connector and 5 pieces of 1/16" brazing rod cut to 21" each. Seal the connector with a gob of silicone bathtub caulk and tightly wrap the PL-259 with a good quality vinyl tape, beginning below the connector, completely covering it and any exposed threads, back again down to where you started. Finish with one turn not tighly stretched so it doesn't come off and flap around. The coax is run up through ordinary TV antenna mast, the groundplane just sits on top. You may have to wrap the connector and build it up so it fits snug if the antenna won't lie flat by itself.
If you want more signal to the horizon you may consider an omnidirectional gain antenna. That's easy too, Antenna Specialists makes a vehicle mount (ground support crew) mobile antenna for the band having 5dB gain. Just mount it to a home made L bracket, attatch radials and mount it to the mast with a single U clamp. Notching the bracket for the clamp will keep it straight.
The Cushcraft ARX-2 2M antenna will tune the upper portion of the band, just follow the chart. You don't need to go to the bottom of the band and listen to beacons and nav aids.
What we're talking here is resonant antennas which will outperform those wideband scanner antennas by a wide margin.
