Question: For a normal 5 watt handy working in VHF or UHF freq with 50 ohm antenna , is it possible to use a same length coax cable instead of a stub antenna, so as to work like a crude antenna. Will this only result in extra capacitance/ VSWR damaging the system ?

Answer: 
The electrically shortened antennas often found on HTs are not simple pieces of wire, but coils:

[![enter image description here](https://i.stack.imgur.com/b1blz.jpg)](https://commons.wikimedia.org/wiki/File:Uhf_cb_wit_rubber_ducky_exposed.jpg#/media/File:Uhf_cb_wit_rubber_ducky_exposed.jpg)  

*By Shootthedevgru at English Wikipedia, [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0 "Creative Commons Attribution-Share Alike 3.0")*

The coil adds inductance over the length of the antenna, making it appear electrically longer than it is physically. Without the inductance, the antenna would need to be approximately a quarter-wavelength long.

A length of coax of the same physical length would not have this inductance, so would not be a good replacement.

Besides the issue of length, using a length of coax, with the coax connector attached the usual way, screwed into the antenna connector on the HT isn't an antenna at all since the shield would surround the center conductor which would otherwise be the antenna. It's just a feedline, with no antenna on the end.
youtube has a video of a piece of coax with the last 19" being separated between the center, pulling it out of the sheath. poor mans dipole. seemed to work
<http://bit.ly/2scIYD7>