HF mobile antennas can be a good project for the mechanically inclined--it is possible for the serious amateur to build antennas as efficient as commercial versions. Increasingly popular with the advent of POTA and the renewed interest in portable/mobile operations, a solid mobile antenna can open up a whole new world of operating beyond the old chair in the basement.

HF mobile antennas commonly make use of a car's metallic body to serve as the ground plane for a textbook shortened vertical antenna. With many cars having plastic or composite bodies for construction, it is worth doing a magnet test on the vehicle before installing a vertical and loading it up with 100 watts. If a car body is not metal it is not the end of the world, it just means that further thought needs to be made to find a way to give the antenna a good ground, or swapping the vertical with an antenna that does not require a good ground place such as a dipole or tuned loop. When there is more space, such as on the top of an RV or a trailer, there are far more options for better performing antennas. 

• A Monobander for Going Mobile on 17 meter
  QST May 2013, pp. 30-33
  A simple 17 meter mobile antenna project. Design can easily be modified to cover other bands instead by changing the main element length or the coil.
• A Deluxe RV 5-Band Antenna
  QST October 1980, pp. 38-40
  Mount a fold over Hustler 4BTV on your RV for some serious 5 band HF mobiling.
• Another Look at an Old Subject: The Bug Catcher
  QST December 1980, pp. 30-32
  A build it yourself 80- though 10-meter loaded vertical.
• A $20 HF Mobile Antenna
  QST April 2000, pp. 33-35
  A few hours of fun with PVC and wire and you’ve got yourself a respectable mobile antenna for 20- through 6-meters.
• The Alpha Special
  QST July 1969, pp. 26-29
  (Although this multiband horizontal antenna was designed for a 1960’s full size station wagon and not practical for a 21^st century wagon, it should work well and look pleasing on mini-van. - Ed.)
• Antennas for Travel Trailers and Campers
  QST March 1969, pp. 34-38
  Some antenna mounting solutions for recreational vehicles.