Lous Varney G5RV invented the G5RV antenna, he was a CW man and wanted an antenna that would perform well on 20m it was not designed for other bands but it was found to be resonant on other bands and with the use of an ATU could be brought within the tunable range of the Transmitter. The classic length is 102′ with a ladder line tuned stub of 34′ originally the ladder line would have been open wire as its very low loss, however now it tends to use Ladder Line. There are also half sized versions available.
If you are going to use the G5RV as a multiband antenna the end of the tuning stub would be a good place to use a AMU otherwise you can use a standard coaxial cable to run greater lengths. However you will need to insert a choke balun between the tuned stub and the Coax otherwise your coax becomes part of the tuned antenna, this in turn causes you greater problems. Whilst this antenna will tune on many bands I will re-iterate that this antenna was designed for 20m CW. There is also no reason to buy one, they are more fun to build.