why is the height of the 3dr 915 mhz antenna the same as the 400ish mhz antenna? (3dr telemetry)

I'm curious why the antenna provided with the 915 mhz 3dr telemetry radios is apparently the same length as the 400ish mhz radios.

I'm also curious if anyone has 'repurposed' one of the 'nice and short' 915 mhz antennas for a 900ish mhz video antenna (instead of using the 915 mhz range for telemetry)

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The antenna's electrical length is what's important.  The physical length is less important.  My guess is that the 900MHz antenna is a straight piece of wire about 9cm long.  (One quarter wavelength).  The 400MHz antenna is electrically equivalent to a 17cm straight wire, but probably a helical wound to be physically shorter.  They are not interchangeable regardless of the physical similarity.

Reffering to first answer just below:

As far as I know just the opposite (almost) and the antennas supplied with my "exact clone" radios are simply short fragments of wire. The range is ca 20m :)) . Maybe with 3DR originals its different case....

The people on polish fpv forum wrote that supplied antennas are only not to burn radio when connected, not for serious work.

I started with prolonging air module antenna (with just another wire) to ca 70cm (half wave) and looks like the distance is much better, even with stock antenna in ground module. 

During week-end I'll connect HK RMilec 433MHz antenna (many good opinions!) on the ground station and will be ready for real range test...

I was hoping someone with an SRM had actually 'tested' the various antennas provided (both 3DR and clone) to see if they were actually (semi) tuned, or were there just to not burn out the radio . . .

I would go for the later. Nobody seriously expects much from bundled antennas with any sort of tx or rx. Its almost an expected unwritten rule that you will need to buy or make better ones.

1/4 wave, 1/2 wave etc. 1/4 wave 915MHz will be close to a 1/2 wave 433MHz. Check IBCrazy stuff in RCGroups.

Yes the supplied antennas are crap if you want to go any serious distance.

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