Here's an idea: Instead of alligator clipping the wire to the antenna,
loosely wrap about 6 - 8 turns of the wire around the whip so it
inductively couples to the antenna. If the radio is indeed
overloading this could solve the problem; not as much signal is
transferred to the whip as with a direct connection. I've done this
with portables and have had success, your mileage may vary.
HK

The above idea has worked for me in the past.
These radios are very sensitive so they can be used with the whip, I might
be wrong but I dont think it has anything to do with the resonance frequency
of the wire, I would say its more the length of the wire that is the problem
when it comes to the smaller portable radios.