Dipoles, Monopoles and Loop Antennas

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Last updated 5:29 PM on 5/27/26
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8 Terms

1
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Directivity of a short dipole

1.5 or 1.76 dBi

2
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Half wave dipole

  • Widely used

  • High radiation efficiency

  • Easily matched to standard transmission lines

  • λ/2 dipole is a resonant structure and supports a sinusoidal current distribution

  • Radiation pattern can be calculated by assuming it is made up of lots of small dipoles and integrating along length

  • Ideal has a significant reactive component

    • 𝑍𝐴 ≈ (73 + 𝑗42)Ω

    • If the length is reduced to 0.48λ, the reactive component becomes zero

<ul><li><p>Widely used</p></li><li><p>High radiation efficiency</p></li><li><p>Easily matched to standard transmission lines</p></li><li><p>λ/2 dipole is a resonant structure and supports a sinusoidal current distribution</p></li><li><p>Radiation pattern can be calculated by assuming it is made up of lots of small dipoles and integrating along length</p></li><li><p>Ideal has a significant reactive component</p><ul><li><p><span>𝑍<sub><sup>𝐴 </sup></sub>≈ (73 + 𝑗42)Ω</span></p></li><li><p><span>If the length is reduced to 0.48</span>λ, the reactive component becomes zero</p></li></ul></li></ul><p></p>
3
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Directivity of a Half Wave Antenna

Dmax = 1.64 or 2.15 dBi

4
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Balanced to Unbalanced Transformer

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5
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Monopole Antenna

  • Simple antenna

  • Half of a dipole with “missing” half replaced by a ground plane

  • Un-balanced so easily matched to co-ax transmission lines

  • Only radiates in one hemisphere so directivity is twice that of a dipole

<ul><li><p>Simple antenna</p></li><li><p>Half of a dipole with “missing” half replaced by a ground plane</p></li><li><p>Un-balanced so easily matched to co-ax transmission lines</p></li><li><p>Only radiates in one hemisphere so directivity is twice that of a dipole</p></li></ul><p></p>
6
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Directivity of a Monopole Antenna

Dmax = 3.28 or 5.15 dBi

7
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Radiation Resistance of a Monopole

36.6

8
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Loop Antennas

  • Simple and cheap

  • Common at lower frequencies (<1GHz)

  • Usually circular loops but can have many shapes

  • Small loops (r<λ/10) have a radiation pattern similar to that of a dipole but with E & H fields interchanged.

  • Radiation resistance can be increased by using a multi-turn loop which increases by N2

<ul><li><p>Simple and cheap</p></li><li><p>Common at lower frequencies (&lt;1GHz)</p></li><li><p>Usually circular loops but can have many shapes</p></li><li><p>Small loops (r&lt;λ/10) have a radiation pattern similar to that of a dipole but with E &amp; H fields interchanged.</p></li><li><p>Radiation resistance can be increased by using a multi-turn loop which increases by N<sup>2</sup></p></li></ul><p></p>