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Coaxial Probe Feed Method

Key Takeaways

  • Coaxial probe feeds are a commonly used technique for microstrip patch antennas.
  • The microstrip patch antenna feeding technique can be broadly classified into contacting and non-contacting methods.
  • The feeding point of the coaxial probe can be anywhere inside the patch to establish an input impedance match. 

Coaxial Probe Feed

The coaxial probe feeding technique is a non-planar contacting type of feed method

Microstrip patch antennas are employed in various communication systems such as WiFi, laptops, mobile phones, multimedia players, etc. Microstrip antennas are reliable, high-performance radiating antennas. Different feeding methods provide RF power to the antennas, including the coaxial probe feed, microstrip line feed, proximity coupled feed, and aperture coupled feed methods.

The microstrip line feeding technique and the coaxial probe feed technique are used to transfer and radiate RF power to the microstrip patch antenna. However, both these contacting-type feeding techniques suffer operational problems when the patch antenna has a thick dielectric substrate. We will discuss these two techniques, but first, let’s briefly review microstrip patch antennas.

Microstrip Patch Antennas

The microstrip patch antenna has a double-sided dielectric substrate patch, a radiating patch on one side, and a ground plane on the other. The figure given below shows the schematic microstrip patch antenna.

Microstrip patch antenna

The feeding technique is a factor that can influence the performance of microstrip antennas. Feeding techniques are used to energize the antenna with RF power and make them radiate power effectively and efficiently. The position of the feed is also significant in impacting radiation efficiency. 

Microstrip Patch Antenna Feeding Techniques

Contacting methods

In the contacting feeding technique, the RF power takes the microstrip line path to reach the patch for radiating power. E.g., the microstrip line feeding technique and coaxial probe feeding technique.

Non-contacting methods

The RF power transfer between the microstrip line and the patch is through electromagnetic field coupling. E.g., aperture-coupled feeding technique and proximity-coupled feeding technique.

Let’s discuss two contacting methods: coaxial probe feeding and microstrip line feeding.

The Coaxial Probe Feeding Technique

The coaxial probe feeding technique is a non-planar contacting type of feed method. The coaxial probe is the contact that establishes the provision for RF power radiation. The coaxial probe is formed by passing an inner conductor throughout the substrate and making metal contact with the patch. 

The coaxial probe feeding technique consists of:

  1. Central probe conductor - This conductor is connected directly to the patch antenna. The central probe conductor feeds the RF power to the antenna. 
  2. Coax - The outer conductor that surrounds the probe is called the coax. The coax is connected to the ground plane to complete the circuit. 

The coaxial probe establishes direct contact with the microstrip patch antenna. The coaxial probe is located where the antenna input impedance is 50 Ω.



  • The feeding point can be anywhere inside the patch to establish an input impedance match
  • Increase in spurious feed radiations and surface waves with the increase in dielectric substrate thickness
  • Limited bandwidth
  • Cross-polarized radiation problems

Microstrip Line Feeding Techniques

The microstrip line feed technique is a method in which the microstrip patch is connected through a conducting strip. The conducting strip brings the advantage of a planar structure, as it can be etched on the same substrate as the patch antenna. The width of the conducting strip is smaller than the patch.



  • Planar structure
  • Difficult to model, as the conductor is inside the dielectric substrate and the coax protrudes out from the ground plane
  • Narrow bandwidth
  • Non-planar structure
  • With the increase in the length of the probe, impedance-matching problems start to appear

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