You only need two to result in field cancellation
to prevent radiation. However, that is only the
case in free space. Any near objects or objects
in electrical contact may distort
the field from one of the elements differently
from the other, so resulting in less-than-perfect
cancellation, especially in the case of ground radials.

"Dan Richardson" wrote in message
...
On 10 Jan 2005 08:48:37 -0800, wrote:
I have seen some with 4 elements, some with 5. Also is the spacing
between them important?
The ¼-wave groundplane was developed by George Brown 1938. Here's a
partial quote from his book:
"... In our initial experiments we found that only two horizontal rods
(ground rods) functioned as well as four. Many people from the
Broadcast Sales organization came by to view our tests and they always
expressed doubts as to the ability to radiate uniformly when only two
ground rods were used. To quiet them, we used four ground rods for a
while, thus stilling the criticism. When the antenna became really
popular, we did not dare confess to our ruse."
There you have it from the inventor of the antenna.