In the microwave band here are multi-element detectors, but at longer wavelengths the telescope is a single pixel.
Yes it does take a while to build up an image, but radio pictures aren't usually very large - not the millions of pixels of an optical/IR image.
One big advantage of radio telescopes is that you can combine telescopes 1000s of km apart to create an image with the resolution of a single dish that large ( you can just about do this in the optical with 100m separation now)
You know about the interference pattern you get with two slit? If you picture the two telescopes as the two slits and you interfere (electrically) the signals to form the fringe pattern. You can then calculate the shape of the light source - a single point will produce the classic fringe pattern, 2 close points will produce a slightly different pattern etc.
edit: Building an optical 'radio' telescope - the original English solution and the rather more impressive Teutonic result (if you have a lot more $$$)
(I'm not exactly sure of this, comments greatly appreciated)
Radio waves have a longer wavelength (3-4 m) than cell phone waves(fractions of a meter). Thus these are easy to block. They can travel around buildings &c, but can't enter cars because the windows are tiny. Cell phone waves are small enough to enter the windows of a car.
Radio waves have large antennae due to their wavelength. So reorienting them makes a difference as the effective size of the antenna facing the wave changes drastically. A cellphone jas a tiny antenna. Reorienting it makes little difference. Imagine making the antenna shorter and shorter till it becomes a ball. Reorienting that makes no difference at all. And a cellphone does change quality on rotation, but its less evident. There may be some smoothing circuitry involved.
Edit: Antennae need to be with the same order of magnitude as the radiation as they need to resonate. See http://enwp.org/Antenna_(radio)#Resonant_antennas . A short antenna cannot effectively acheive resonant frequency of a long wave.
Best Answer
The idea behind the quarter wavelength antenna is that it is self-resonant: it is "tuned". You can however use an antenna of any size to pick off some electromagnetic energy - and you can tune the antenna by adding some inductance in series (or inductance and capacitance). The reason that you tune an antenna is simply this: you want it to have real impedance, which happens exactly at resonance. When this happens, then all the power that is incident on the antenna ends up going into the electronics; when the impedance is complex, the current either lags or leads the voltage, and this results in reflection of the power (and less power going into the amplifier).
You can't get around the fact that you are "capturing" energy from less area in space, so it will be less efficient. On the other hand having an antenna that is too long doesn't buy you anything as the signal from one part of the antenna would cancel the signal from another.
This is one of the beautiful things about the Yagi antenna: the multiple elements are spaced in such a way that they cause constructive interference along a particular axis, giving you both gain and directionality - in essence you are capturing energy from a larger volume of space.
Little anecdote: from wikipedia article on Yagi-Uda antenna
I have heard it said that the inventor was executed during WW-II by the Japanese because his invention "helped the enemy". I can't find a reference to support that. Apparently he wasn't even the inventor - it was Uda. But Yagi published it, patented it, and even sold the rights to Marconi (before the war). It would be karma if he ended up paying the ultimate price for stiffing the real inventor... "No, it wasn't me! But Professor, your name is on the patent... off with his head." In fact it is apocryphal - a bit more searching reveals that he died in 1976.