Here's a nifty and simple portable ground plane antenna for VHF and UHF.
Scavenge around a loading dock or receiving area for some used steel bailing straps. Cut two pieces, each a half-wave for the frequency band for which the antenna will be used. Lay the two pieces on a reasonably flat surface to make an X. Then place a magnetic-mount vertical antenna in the center.
I have used this antenna on two meters in motel rooms and in the woods with a 50-watt FM tranceiver.
This antenna started out with placing the mag-mount antenna on convenient metal objects in motel rooms. Air conditioners and metal door frames worked after a fashion, but the antenna was frequently hard to tune and sometimes I had to move it around to get it to load up. The baling straps made a good reliable artificial ground with the antenna on a table or bed. In the woods, I have placed the ground plane on an overturned metal pail and on a metal camp stove. The extra metal is not necessary for hamming, but it keeps the mag-mount from tipping over.