The aluminum mast and its height do not reflect radar signals very well. When the Stealth Bomber was designed it was determined that radar needs a flat surface in order to get an adequate reflective signal back to the receiver. Consequently, that aluminum mast doesn't fall into that category.
Most radar reflectors provide an excellent, reflective target surface, one that can be seen for miles. The signal is NOT protected by the sailboat's shrouds, nor are they enhanced, which is another of the many myths pertaining to those cables. Today's modern 3G and 4G radar systems, however, has the ability to read the miniscule reflected signal, even from semi-rounded objects such as the boat's mast. However, that mast may show up as being similar to a slim piling on top of another, larger mass that could be mistaken for a tiny island or boulders protruding from the water's surface.
Their are some jurisdictions that mandate radar reflectors, while others merely suggest them. The biggest problem, however, is that there are times when commercial vessels do not have someone monitoring their radar displays, which means that even if you are equipped with a reflector, they other vessel may not be watching the screen. At that point you could end up being a boat pizza.
There are a couple of recommended locations to install the reflector--the back stay, and suspended from the end of a spreader arm--both of which are very effective. And, in the case of radar reflectors, size matters--the larger the reflector, the more powerful the signal reflected back to the transceiver.