Most technicians start off with a HT so they don’t think about the most important part of the transmit system…the antenna. You can use an external antenna to get: more gain, a better line of sight to the station you are transmitting to or simply for better reception.
Remember the 3db gain rule. If you have a 5 watt radio and a 3 db gain antenna you have 10 watts Effective Radiated Power! (We are not going to count losses of cable and connectors) This can make the difference between having a noisy signal into a repeater and being full quieting.
On the receive side, if you are operating inside of a vehicle it is like being in a can. Yes signal will get in but not a lot. Putting the antenna on the roof of the vehicle will give you some height gain, clear path signal gain and antenna gain.
If you are at home and use an antenna on the roof you will have a noticeable change. Even a simple J Pole will be like night and day. For those who cannot put an antenna outside, put it in the attic. For those who like covert things there are ways of disguising antennae on vent pipes and other places. The best place to put something is right out in the open where no one would suspect it is an antenna!
Buy good cable for connecting your antenna to your radio. One of the best right now for minimal signal loss on VHF/UHF is RG 8X. The most popular for premade mount is RG58. If you buy the cable assembly you are good to go. Those who want to make their own cables, please get the correct connector adapters that will fit the cable you select.
Last note- most HTs do not have a normal connector on them. (BNC) so you will need an adapter. Make sure you get the correct one. I have already seen one radio mangled by a user who forced the wrong adapter onto the antenna connector.