There was a small article in SPRAT, the G-QRP club magazine this quarter that is both devilishly simple and also appears to work well.
The addition of a 1/4wave counterpoise to a handheld does improve the performance of the rubber duck quite noticeably. This consists of nothing more than 1/4 wavelength of wire connected to the earthy side of the antenna connector and is allowed to simply hang down. The article suggests an 1 to 1.5 S point improvement.
I’ve tried it on my VX170 from home using GB3CS as a reference signal. It’s about S3-S4 using the rubber duck and it’s S5 with a counterpoise. Switching to a 1/4wave telescopic antenna and no counterpoise results in S4-5 as the reading is bouncing about. Using the 1/4wave antenna and counterpoise resulted in the signal spending most of its time around S6.
Myself and Brian G4ZRP used such a counterpoise along with a telescopic 1/4wave on my FT817 from Fair Snape Fell and Longridge Fell the other week as someone (me) forgot the proper 2m antenna.
For the time it takes to make and deploy and for the cost and weight penalty this is really something that everyone who uses a handheld should have with them. Easy to fit when needed and if you have a softcase for your handheld you can coil up the wire and slip inside the case so it’s always with you.