I'm wondering how quarter wave antennas mimic half-wave antennas by using the ground plane.

I haven't touched these subjects in a bit, but here are my thoughts:

How does the mirrored return current not result in a voltage change for reference of the other half of the wave? Why are the voltages not just shunted to ground, especially on a cell phone? I assume cell phones are designed with bypass capacitors to limit the noise on the ground plane.

  • 1
    \$\begingroup\$ @eurditass, this is a long answer i do not have time to write currently. I will not that cell phones normally use a different antenna. Inverted-F antennas are more often used. This is mostly for size on PCB \$\endgroup\$
    – Kortuk
    Jun 13 '11 at 17:37
  • \$\begingroup\$ Can you rephrase the first sentence? \$\endgroup\$
    – endolith
    Jun 13 '11 at 19:02

The last cell phones I opened up had a loaded bent dipole antennas or J-pole looking antennas. Of course there are tons of antenna styles possible. Cell frequencies are from 900-1900 MHz, so 1/4 wave is 1.55-3.3". Without loading, the antenna would be longer than the phone.

Looking at the document Kortuk linked, it looks like they would be Inverted-F. Not sure how those vary from J-poles, as they look almost exactly the same. It might be that dual frequency phones can have Inverted-Fs that work as dipoles and the lower frequency. RF is sort of like a voodoo art. :)

If the antenna is balanced, you should not get reflected power so the noise induced in the ground will be low, just the power required for the signal. You can't bypass the antenna with caps, or you will "short out" the antenna at RF frequencies. This is no different than noise that digital chip would induce on ground by driving a fairly large load with fast digital pulses. The filtering needs to be done where it matters, at the power pins of the sensitive parts of the circuit.


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