# Why do rubber duck antennas have this metal part, while other antennas do not?

If you open up a rubber duck antenna, it usually has a wire inside, plus a crimped on piece of metal, like this:

Other antennas, like those on an RC receiver do not have this second metal part:

Both of these antennas are 2.4ghz, why does one require this metal part where the other does not?

Also, if the metal tube forms part of a dipole, then how is it electrically different than the shield it's covering? I think they're connected. Wouldn't it do nothing that the shield isn't already doing?

• A rubber duck cannot form a dipole. It can form a monopole. – Andy aka May 21 '18 at 21:38

The cutaway picture looks like a coaxial sleeve antenna. Section 3 in the linked reference discusses this type of antenna.
edit: also see figure a on page 11 (assymetrically-fed sleeve dipole)

• Nice reference. – Jack Creasey May 21 '18 at 22:03
• That helps, but what I really want to know is what practical difference this makes. – Drew May 21 '18 at 22:28

I found a source, that seems to answer my question.

https://www.rchelicopterfun.com/rc-antenna.html

The "whisker" type antenna is a linear monopole. The sleeved antenna is an "assymetrically-fed sleeve dipole" (as AlmostDone pointed out).