Ken, KG0WX wrote:
'Is it possible to make a mobile 10m antenna that is physically shorter
than a 1/4 wave whip yet will perform better than a 1/4 wave whip?"

In general, the full-sized 1/4-wave whip is a very good compromise
mobile antenna because it can be self-resonant, and it is almost
omnidirectional in azimuth. Physically, slightly shorter than a CB
antenna, it is readily available by trimming a CB antenna to resonance,
literally, or by using a low-loss variable capacitor in series.

It is possible to use an antenna less than 1/4wavelengrh against ground
as an efficient radiator but it requires care to minimize loss.
Shortening the radiator reduces its vertical directivity. If gain is the
performance measure, vertical directivity lost by shortening must be
replaced, perhaps by horizontal directivity. Gain from horizontal
directivity needs to cover not only lost vertical directivity, but
losses caused by loading too-short elements in the array. These
elements may produce horizontal directivity where none exists with a
sole avertical element.

In the September 1973 "QST" is "A Bite Size Beam". This article refers
the reader to an earlier March 1973 QST article by Sevick, "The W2FMI
Ground-Mounted Short Vertical".

Summary: Can you do it? Yes. Is it practical? Maybe.

Best regards, Richard Harrison, KB5WZI