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If you open up a rubber duck antenna, it usually has a wire inside, plus a crimped on piece of metal, like this: antenna1 enter image description here

Other antennas, like those on an RC receiver do not have this second metal part: antenna2

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?

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

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

According to the link:

Since the transmitter is transmitting the RF as a sinusoidal wave, it needs both poles as it alternates between positive and negative voltage for improved efficiency. If the transmitter only had a monopole, roughly half the power would be transmitted. Lost efficiency...

This is the "practical" answer that I was looking for (if it's correct).

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