I attempted this morning to make a 5/8 wave ground plane antenna for 70cm out of some solid copper mains cable and a chassis mount SO-239 connector. I tried two designs for the radiating element with a coil at the bottom, but both gave "High SWR" from the FT-817 and were so far away from a match that there seemed no hope of getting anywhere. My SWR analyzer doesn't go above 200MHz so that wasn't any help in finding out what was wrong.
After these abortive attempts I decided to straighten out the coil and make a 3/4 wave ground plane instead. This gave a perfect match with barely perceptible reflected power being shown on the SWR meter.
The four radials are each 165mm (6.5in.) long and the vertical element 483mm (19in.) The radials need to be horizontal for the best match, not slanting down as they usually are for a quarter wave vertical.
Hopefully I will be able to receive satellite downlinks with this. There are no 70cm repeaters in this area and no 70cm activity that I know of so it will not be much use for anything else.