I currently use a CB-Type Mobile antenna, referred to as "DV-27s", which can
easily be modified for the Ham-Radio bands of 10m to 20m. (Check the photo on the right).
The original CB Antenna is quite cheap (around 5 Euros) and rather common in Europe, I
bought two of these antennas including the base mount and a magnetic mount for the car on
a flea market for 10 Euros all together.
One of the two antennas I mounted to the fire ladder on the roof of my appartement, inlcuding some additional radials, the other one I use mobil/portable with the magnetic mount on the car.
Modifying this antenna for Ham-Radio is easy. The original antenna has a short metal rod inserted at the tip, which enables fine-tuning of the SWR by adjusting it's length. There is a little screw mount, so that you can easily take off the original rod, and insert others of variabel length. For the Ham bands, these lenghts are: