---
title: How to Design a Low‑Pass RF Filter for 5 GHz Applications: A Step‑by‑Step Guide
siteUrl: https://logzly.com/rflab
author: rflab (RF Filter Lab)
date: 2026-06-18T13:01:16.444861
tags: [rf, filterdesign, electronics]
url: https://logzly.com/rflab/how-to-design-a-lowpass-rf-filter-for-5-ghz-applications-a-stepbystep-guide
---


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If you’ve ever tried to clean up a noisy signal at 5 GHz, you know the frustration of hearing “static” when you should be hearing a clear tone. A well‑designed low‑pass filter (LPF) can turn that chaos into a clean, usable signal, and it’s easier to build than most people think. In this post I’ll walk you through the whole process, following a [step‑by‑step guide](/rflab/how-to-design-a-lowpass-rf-filter-for-5-ghz-applications-a-stepbystep-guide) to low‑pass RF filter design. Grab a [cup of coffee](https://www.amazon.com/s?k=cup+of+coffee&tag=organizationtip101-20), and let’s get our hands dirty.

## Why a Low‑Pass Filter at 5 GHz?

At 5 GHz we are in the heart of many modern [wireless systems](https://www.amazon.com/s?k=wireless+systems&tag=organizationtip101-20) – Wi‑Fi 6E, 5G small cells, and even some radar links. Those systems often need to reject higher‑order harmonics or out‑of‑band interferers. A low‑pass filter does exactly that: it lets the frequencies you want pass through while attenuating everything above a chosen cut‑off. Without it, you risk spurious emissions, reduced range, and a lot of wasted power.

## 1. Define Your Specs

Before you open any design tool, write down the numbers that matter.

| Parameter | Typical Value | What It Means |
|-----------|---------------|---------------|
| Pass‑band edge (fₚ) | 5 GHz | Frequencies you want to keep |
| Stop‑band edge (fₛ) | 6 GHz or higher | Frequency where you need strong attenuation |
| [Insertion loss](/rflab/how-to-minimize-insertion-loss-in-rf-filters) | ≤ 1 dB | How much signal you lose in the pass‑band |
| Stop‑band attenuation | ≥ 30 dB | How much unwanted signal you reject |
| Impedance | 50 Ω | Standard for most RF gear |

Write these on a [sticky note](https://www.amazon.com/s?k=sticky+note&tag=organizationtip101-20). I keep a [small notebook](https://www.amazon.com/s?k=small+notebook&tag=organizationtip101-20) on my desk for exactly this – it stops me from “designing in the dark”.

## 2. Choose a Filter Topology

For 5 GHz a few topologies work well:

* **Microstrip coupled‑line** – easy to fabricate on a standard FR‑4 board, but may need a good EM simulator.
* **Lumped element (LC) Butterworth** – simple math, but inductors and capacitors become tiny and lossy at 5 GHz.
* **Hairpin resonator** – compact, good for handheld devices.

My personal favorite for a quick prototype is the [microstrip coupled‑line](/rflab/stepbystep-design-of-a-lowpass-rf-filter-for-5-ghz-applications). It gives a clean response and can be etched on a regular PCB without exotic materials.

## 3. Calculate the Prototype Values

Let’s assume a 3‑pole Butterworth response (maximally flat in the pass‑band). The normalized low‑pass prototype values are:

g₁ = 1.0, g₂ = 2.0, g₃ = 1.0

To convert these to a band‑edge at 5 GHz we use the formulas:

Lₙ = (gₙ * Z₀) / (2π fₚ)  
Cₙ = gₙ / (2π fₚ Z₀)

where Z₀ = 50 Ω.

Plugging the numbers:

* L₁ = (1.0 * 50) / (2π * 5e9) ≈ 1.59 nH  
* C₂ = 2.0 / (2π * 5e9 * 50) ≈ 1.27 pF  
* L₃ = same as L₁ ≈ 1.59 nH

These are tiny components. In a microstrip implementation the inductors become short, high‑impedance lines, and the capacitor becomes a gap between two lines.

## 4. Layout the Microstrip Sections

### 4.1 Determine the Substrate

A low‑loss substrate such as Rogers RO4003C (εr ≈ 3.38, tan δ ≈ 0.0027) works well. If you only have FR‑4, keep the line widths a bit wider to reduce loss, but expect a few extra dB of insertion loss.

### 4.2 Compute the Line Width

Use the classic microstrip impedance formula:

W/h = (8e^(A)) / (e^(2A) - 2)

where A = (Z₀/60) * sqrt((εr+1)/2) + ((εr-1)/(εr+1)) * (0.23 + 0.11/εr)

For Z₀ = 50 Ω and εr = 3.38, you get a width of about 1.2 mm on a 0.8 mm thick board. Adjust as needed for your fab house.

### 4.3 Create the Coupled Lines

The two lines that form each resonator are spaced a distance S that sets the coupling coefficient. A good starting point is S ≈ 0.2 W. Use an EM simulator (Keysight ADS, Sonnet, or even free tools like Qucs) to sweep S and see how the cut‑off moves.

## 5. Simulate the Design

Run a S‑parameter simulation from 0 to 8 GHz, as outlined in the detailed [step‑by‑step design guide](/rflab/how-to-design-a-lowpass-rf-filter-for-5-ghz-applications-a-stepbystep-guide). Look for:

* **S21** (insertion loss) flat below 5 GHz and dropping sharply after.
* **S11** (return loss) better than –10 dB in the pass‑band.

If the stop‑band attenuation is shy of 30 dB, tighten the coupling (reduce S) or add a fourth pole. The beauty of a simulation is you can iterate quickly without cutting new boards.

## 6. Fabricate and Test

### 6.1 PCB Fabrication

Send the Gerber files to a fab that can handle the fine line widths. I usually ask for a 0.1 mm tolerance; most shops can meet that.

### 6.2 Measurement

Use a vector [network analyzer](https://www.amazon.com/s?k=network+analyzer&tag=organizationtip101-20) (VNA) calibrated to 50 Ω. Connect the filter with short, low‑loss coax and a good ground‑snap. Measure S21 and S11 and compare to the simulation. Small differences are normal – solder joints, dielectric tolerances, and surface roughness all play a role.

If the cut‑off is a few hundred MHz off, a tiny trim on the line length (add a few mm of meander) can bring it back.

## 7. Fine‑Tuning Tips

* **Add a small series resistor** (5–10 Ω) at the input if you need better return loss at the expense of a bit more insertion loss.
* **Use a metal‑grounded via** under the high‑impedance sections to suppress unwanted modes.
* **Temperature stability** – Rogers substrates have low thermal coefficient, but if you expect large swings, consider a temperature‑compensated layout (add a parallel resistor that changes with temperature).

## 8. When to Move to a Different Topology

If you need:

* **Very low insertion loss (<0.5 dB)** – consider a lumped‑element design with high‑Q inductors.
* **Very compact size** – hairpin resonators or even integrated passive devices (IPDs) can save board space.
* **Mass production** – a planar microstrip filter is easiest to repeat.

## 9. Wrap‑Up

Designing a low‑pass filter for 5 GHz may sound daunting, but breaking it into clear steps makes it manageable. Start with solid specs, pick a simple topology, do the math, simulate, and then verify on hardware. The first time I built a 5 GHz LPF on a coffee‑stained desk, the VNA showed a clean 35 dB stop‑band – a small win that reminded me why I love RF design.

Happy filtering, and may your signals stay clean!