boleh.

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Added on June 29, 2012, 11:18 am
first, talk about theory>

1) higher frequency band = carry more data = higher speed 
lower frequency band = carry less data = lower speed

(pratically true, as what u saw)
2) higher frequency band = weaker indoor penetration signal /no far enough
lower frequency band = better indoor penetration signal / further than higher frequency band

(practically what being seen is sth on the contrary, just what u said, 900MHz even worsen signal)
the ideal band still 2100MHz for 3G, especially we now already on high speed HSPA+ or DC=HSPA+ network, i don't think ppl wanna pay expensive 3G $$ fees but using lower speed band. improve 2100MHz reception power with external antenna 'to compete' with others rather than 'surrender' to lower speed band.

external omni antenna for 3g? it may return u with reversed expected result.

since u already have basics, consider this scenario:

where is signal source? 3g signal source is come from 3g base station. where is 3g base station? 3g base station sit at high rise building, probably not at same floor with you (same horizontal line).

how higher dbi omni antenna works? omni antenna coverage area is a donut shape (vertically not 360 degree), not a ball shape (vertically 360 degree). The higher dbi of omni antenna = the flatten of the donut size, ie the horizontal space is extended, but the vertical space is dropping.

now matching the above 2 into a single picture, what can be seen?

if signal from base station at high rise building already getting weak near your spot, using a higher gain external omni antenna (work in a flat donut coverage shape) e.g 10dbi may lead you to even lower signal level compared with internal built-in 2dbi omni antenna since 2dbi omni has widen vertical area. so using external 3g omni antenna is no guaranteed to get improved result, it got high chances of showing lower signal level, depends on circumtances. so till today i still don't take any external 3g omni antenna stock bcoz no guaranteed result.

still, the directional antenna the ideal choice for these wireless technology (3G, wimax, LTE) where the signal source is come from high rise building. It cannot be apple to apple comparison to home wifi scenario.

for smaller size, and multi band purpose, u can consider the directional panel with up to 10dbi effect. the stock can be ordered.

the basics:

1) a prescribed "antenna core" responsible to provide high gain for a certain frequency band.
(for dipole rubber duck antenna unwrap the antenna ruber skin will see the antenna core inside; bare skeleton antenna then can seeing the core straight away)

2) a dual-band high gain antenna will thus consisted of more than one prescribed antenna core (different size / different shape), each responsible to provide high gain for different frequency band(s).

3) so if a omni antenna core is seen in same design from top to bottom (no different core shape/size), then it is actually providing high gain for single band only, not multiband.
A lot of product listed in international commerce site is actually providing false statement, particularly products from cn/hk sellers', they are well known of doing this all the time to fool average users.