Hi Black and Belter,
Getting real with those gain figures it's unity on 2M and approximately 3dB on 70cM, nothing special. It's a common design made a bit more cheaply than say, a Diamond and frankly I wouldn't touch it with a ten foot SN* because of it's flimsy mechanical construction. Hit a low hanging tree branch and that coil gets a permanent bend and trying to straighten it out will surely detune it.
* SN to SR is the Polish callsign prefix block and a lousy joke.
Now getting down to brass tacks, a quarter wave whip for 2M has nearly the same gain figures and being 20" long and far more rugged that would be my choice if I weren't interested in more gain. Yup, it's 3/4 wave on 70cM which provides a 50 ohm match and approximately 3dB gain over a quarter wave.
Now here I go having fun with Dan again;
"its a 1/2 over 1/4 on 70cm so it does have *some* gain but not a lot and its a 1/4 wave on 2m, so its unity gain."
Uh, if I remember right 1/2+1/4=3/4 but I could be wrong. The 1/4 wave helical delay line makes it a colinear rather than a straight 3/4 wave and that raises the 90 and 180 degree (current) points up the radiator giving it slightly more than 3dB gain while the straight 3/4 wave gives sligtly less. That's why the qualifier "approximately" in my preceeding remarks. There's no point quibbling over a few tenths of a decibel so they're essentially the same.
BTW the ASP667 is the closed coil version of the same antenna so there goes your theory right out the window. (;->) OK OK, don't hit me, the ASP is rugged as all get-out and the spring doesn't mind getting all bent out of shape.