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messi-d
Antenna-Theory.com Newbie
Joined: 28 Nov 2019
Posts: 3
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 Posted: Sat Dec 14, 2019 1:51 am Post subject: capacitively fed antennas |
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| why do capacitively fed antennas have higher bandwidth than conventionally fed antennas(direct feed). are there any general guidelines as to what the length, shape and size of the feed must be and how far or close it must from the radiating element? |
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k3nnw
Antenna-Theory.com Newbie
Joined: 23 Mar 2020
Posts: 6
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| Posted: Mon Mar 23, 2020 12:35 pm Post subject: |
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The C is used to tune out the L of the feed. In some cases the length of the feed (e.g. from the centre pin of an SMA connector, via a length of stiff wire to the centre of an air-spaced patch) means a high inductance which can then be compensated for by introducing some series capacitance. The feed impedance of the feed structure thus drops towards zero and the feed point impedance of the patch metalwork itself, and the matched 50R Zo driving it, dominates.
Away from the frequency where the LC series resonance of the feed approaches zero, the rate of change of impedance due to the antenna's own driving point is more rapid than the rate of change of the feed (quality factor) and hence the natural resonance of the antenna is observed (and how it changes with frequency), and not the forced rate of change due to the previously highly inductive feed. |
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