Partially Collapse Telescoping Antenna to Change Effective Length
300 MHz has a wavelength of precisely 1 metre and, a quarter wave dipole (a telescopic antenna for example), would need to be 25 cm long at 300 MHz. At 170 MHz it would need to be \$\frac{300}{170}\$ times longer at 44 cm.


  Is there something special about telescoping antennas such that they
  only work when fully extended, or can I partially retract


You can retract to the length you want but I wouldn't expect it to be that good if you had to retract it to just a few cm because it's effective diameter would be significant but I don't see it would be a problem for you usage. Try it and see.


  Many sites with instructions for a V-Dipole antenna for this purpose
  indicate that a 53cm half-wave antenna works well.


Hmm! They probably work fine - it's not so critical for a receiver as it is for a transmitter.