Neil,
By now, you have probably figured out that the 'Hustler'
antenna as a portable isn't exactly the best idea without
a very good ground system. But, it will work with a
counterpoise. Not well, but it will work. Something in
the near 'neighborhood' of a 1/4 wave for the counterpoise's
length should make the transmitter 'see' a load it can work
into (depending on how that counterpoise is 'laid out').
There are other options for a portable antenna that may
work better than an extremely shortened mobile antenna...
'Doc