Flexible antennas commonly used on portable transceivers have about -5db, “negative gain” compared to a quarter wave whip held at face level. This means that a 5-watt portable VHF with flexible antenna has an effective radiated power of only 1-watt. Placing the portable on your belt produces about -20dB of attenuation, reducing EIRP to 50 milliwatts! UHF results are little better...
An expedient to improve a flexible antenna is to add a some counterpoise wire (19-1/4" long for 2m FM). Use flexible, insulated, stranded wire, AWG16-20 is OK, crimped and soldered to a battery clip, which you attach to the shield of a BNC connector. If your HT has an SMA copnnector then find a suitable ring terminal which will fit over an SMA connector, reducing it's thickness, if necessary to enable you to thread the antenna over it. Reinforce the soldered counterpoise connection with heat shrink to resist flex.
The counterpoise prevents transmitted RF from coupling with your body so that your antenna now performs like a center-fed dipole, instead of an "end-fed dummy load!" The main lobe of the radiation pattern can be "aimed" after a fashion by grasping and pointing the end in the direction where you need a stronger signal.
Some after-market and home-made antennas perform much better than the standard helical. A J-pole antenna constructed of 300-ohm twin-lead rolls up easily and fits into your pocket. When thrown up in a tree, it increases both height and gain. Full-sized, flexible 1/4 wave and telescoping 1/2 wave antennas such as the Larsen or the old AEA "Hotrod" which you still find at hamfests work very well.
A full sized quarter-wave provides unity gain when used with a counterpoise and held at face level. It represents about a 5 dB improvement over a typical short flexible antenna, because most of the effective signal is radiated.
If operating from a vehicle, use a coax jumper to connect your portable to your vehicle mobile antenna to provide a clear RF path outside the vehicle. This overcomes substantial attenuation, about -10dB which results from operating a portable unit from inside a metal vehicle. Always carry suitable adapters so that you can connect your portable transceiver to an outside base or mobile antenna, when one is readily available. A right-angle adapter to the HT reduces strain on the chassis antenna mount.
In marginal operating locations a telescoping, half-wave is may be a better choice. It provides the same unity gain without a ground plane that a 1/4 wave antenna would if it has a ground plane. A 1/2-wave antenna can be pulled up into a tree, dangled out a window, attached to a window pane with suction cups, or be used bicycle or motorcycle mobile, or in city driving on a window clip mount.
In our RACES experience a telescoping half-wave Larson increases useable simplex range of a typical 5 watt, 2-meter portable from about a mile with a typical "stock" flexible antenna in average suburban ground clutter, up to 2-3 miles or more, depending upon terrain. Adding a sime counterpoise to a unity gain antenna enables a portable CERT unit to keep in reliable contact with its neighborhood fire station, within 4-5 miles, if the station is equipped with an elevated, 3 db gain vertical such as a Diamond X50 mounted 40-50 ft. up on the hose tower.
Telescoping antennas are more fragile and work best when stationary or in the open, avoiding side impacts or rough handling. Avoid prolonged mobile use of telescoping antennas on window clip mounts at highway speed, because excessive flexing loosens their internal electrical connections. Never collapse a telescoping antenna by whacking it down with the palm of your hand. Gently pull it down with your fingers. If you note any wobbling or looseness in the sections, replace the antenna.
Flexible antennas are safer when working in close quarters around people and are more durable when performing storm damage assessment, or walking through dense vegetation in wildfire suppression or search and rescue operations. They are a better choice for dual-band transceivers because telescoping antennas are usually mono-band. Most amateur dual-band flexible antennas approximate a 1/4 wave on 2 meters and a 5/8 wave on 70 cm, but are usually optimized for one band and may resonate poorly on the other. How efficient a particular antenna is can be determined only by testing and measuring field strengths. Just sweeping its VSWR on the MFJ analyzer doesn't tell you the whole story, because you don't know what the pattern looks like when it is attached to your, probably compact HT.
In my experience, adding a counterpoise to the original flexible antenna which came with your radio is a good idea. I would also get either a full-sized telescoping quarter-wave, such as the Smiley, or a 1/2-wave Larsen.
Another option is the www.hamstick.com
economy dual-band mobile antenna with magnetic mount, which uses a 3/8-24 base and looks sort of like a cellular antenna. I've used mine on 2 meters, VHF hi-band, 70cm and GMRS and it is wide-banded enough to be serviceable for all. It fits in a briefcase with its mag mount and 18 ft. of coax and with a couple adapters will work with either my agency radio, Marine VHF or hams rigs. I would consider it the best “bang for the buck.”