Chapter 73: ESP-NOW Range Optimization

Chapter Objectives

By the end of this chapter, you will be able to:

• Understand the key radio frequency (RF) factors that determine the range of ESP-NOW communication.
• Learn how to configure Wi-Fi transmit power (Tx Power) on ESP32 devices to influence range.
• Understand the impact of different Wi-Fi PHY layers and data rates on range and reliability.
• Appreciate the critical role of antenna type, design, placement, and orientation in maximizing range.
• Identify common environmental factors (obstacles, interference) that affect ESP-NOW range.
• Implement basic range testing methodologies for ESP-NOW applications.
• Recognize best practices and trade-offs when optimizing ESP-NOW for distance.

Introduction

ESP-NOW is prized for its low latency and ease of use in direct device-to-device communication. While it can achieve impressive distances in ideal conditions (hundreds of meters in open air), the practical range in real-world applications can vary significantly. Factors like transmit power, antenna quality, the surrounding environment, and even the data rate of underlying Wi-Fi acknowledgments play a part.

Optimizing the range of ESP-NOW often means pushing the boundaries of what the 2.4 GHz Wi-Fi radio on the ESP32 can achieve. This could be critical for applications like remote environmental sensors, agricultural monitoring systems, or long-distance remote controls where devices are spread out.

This chapter will guide you through the various elements that influence ESP-NOW’s effective communication distance. We will explore configurable parameters within ESP-IDF, discuss hardware considerations like antennas, and examine how the physical environment interacts with your RF signals, all with the goal of helping you design more robust and longer-range ESP-NOW solutions.

Theory

Fundamental Factors Affecting RF Range

The range of any wireless communication system, including ESP-NOW (which uses Wi-Fi radio hardware), is governed by fundamental RF principles. The key elements are summarized by the Friis transmission equation and related concepts:

1. Transmit Power (Tx Power):
   • The amount of power emitted by the transmitter’s antenna. Higher power generally means the signal can travel further before attenuating below the receiver’s sensitivity threshold.
   • Measured in dBm (decibels relative to one milliwatt) or mW.
   • ESP32 allows configuration of Tx power, but there are regulatory limits (EIRP – Effective Isotropic Radiated Power) that must be respected.
2. Receiver Sensitivity:
   • The minimum signal strength the receiver can detect and successfully demodulate data from.
   • A more sensitive receiver (i.e., a lower dBm value, e.g., -97 dBm is better than -85 dBm) can pick up weaker signals, thus increasing range.
   • This is largely an intrinsic characteristic of the radio hardware.
3. Antenna Gain:
   • A measure of how well an antenna converts input power into radio waves headed in a specified direction (for transmitting) or how well it captures radio waves from a specified direction (for receiving).
   • Measured in dBi (decibels relative to an isotropic antenna). Higher gain antennas focus RF energy, increasing signal strength in their preferred direction(s) at the expense of others.
   • Both transmitter and receiver antenna gains contribute to the link budget.
4. Path Loss:
   • The reduction in power density (attenuation) of an RF signal as it propagates through space.
   • Free Space Path Loss (FSPL): Attenuation in an ideal, unobstructed environment. Increases with distance and frequency.
   • Obstructions: Physical objects (walls, buildings, foliage, people) absorb, reflect, refract, and scatter RF signals, causing additional attenuation. Different materials affect signals differently (e.g., metal and concrete are significant barriers).
   • Multipath Fading: Signals can take multiple paths from transmitter to receiver (reflections). These paths can interfere constructively or destructively at the receiver, causing rapid fluctuations in signal strength.
5. Interference (Noise Floor):
   • Unwanted RF signals in the operating frequency band (2.4 GHz for Wi-Fi/ESP-NOW) from other devices (other Wi-Fi networks, Bluetooth, microwave ovens, Zigbee, etc.).
   • This raises the “noise floor.” The desired signal must be sufficiently stronger than the noise floor for reliable reception (Signal-to-Noise Ratio – SNR). High interference reduces effective range.
6. Data Rate and Modulation:
   • Generally, lower data rates and more robust modulation schemes are less susceptible to noise and can be decoded at lower signal strengths, thus achieving longer range. Higher data rates require a better SNR and typically have shorter ranges for the same Tx power.
   • ESP-NOW itself sends its data packets typically using a robust, low Wi-Fi data rate (e.g., MCS0, which corresponds to 6.5 Mbps for 20MHz channel width in 802.11n, or 1 Mbps for 802.11b). The acknowledgments (ACKs) sent back by the receiver also use a specific Wi-Fi rate.

%%{init: {"fontFamily": "Open Sans"}}%%
graph TD
    subgraph Sender
        TXP["<b>Tx Power</b><br>(Transmitter Output Power)"]
        TXANT["<b>Tx Antenna Gain</b><br>(Directivity)"]
    end

    subgraph Path
        PL["<b>Path Loss</b><br>(Free Space, Obstructions,<br>Multipath Fading)"]
    end

    subgraph Receiver
        RXANT["<b>Rx Antenna Gain</b><br>(Directivity)"]
        RXSENS["<b>Rx Sensitivity</b><br>(Minimum Detectable Signal)"]
    end

    subgraph Link Condition
        LC{"<b>Successful Link?</b>"}
    end

    NM["<b>Noise Margin</b><br>(Signal strength above sensitivity<br>and noise floor for reliability)"]
    NF["<b>Noise Floor</b><br>(Interference)"]
    
    TXP -- "[+] dB" --> TXANT
    TXANT -- "[-] dB (Loss)" --> PL
    PL -- "[+] dB" --> RXANT
    RXANT -- "Signal Arrives At Receiver" --> RXSENS

    RXSENS -.-> LC
    NM -.-> LC
    NF -.-> RXSENS


    classDef default fill:#DBEAFE,stroke:#2563EB,stroke-width:1px,color:#1E40AF;
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    %% This diagram focuses on the components and their relationships.
    %% A caption could state: "For a successful link, the received signal strength (Tx Power + Tx Antenna Gain - Path Loss + Rx Antenna Gain) must be greater than the Receiver Sensitivity plus a required Noise Margin."

ESP-NOW Specific Range Optimization Techniques

• Configuring Transmit Power:
  • ESP-IDF allows you to configure the Wi-Fi transmit power.
  • esp_wifi_set_max_tx_power(int8_t power): Sets the maximum Tx power the Wi-Fi driver will attempt to use. The value is in quarter-dBm units (0.25 dBm). For example, a value of 80 corresponds to 20 dBm. The valid range is typically from 8 (2 dBm) to 84 (21 dBm) for many ESP32s, but consult the datasheet for your specific chip, as hardware limits apply. The actual power might also be capped by regulatory domain settings or calibration data.
  • esp_wifi_set_tx_power(int8_t power): An older API, often esp_wifi_set_max_tx_power is preferred for more global control.
  • Impact: Increasing Tx power can directly increase range, but it also increases power consumption and must comply with local regulations (EIRP limits). Setting it too high might not always be beneficial if it causes more interference or if the receiver is easily saturated (less common for ESP-NOW ranges).

• Wi-Fi Protocol and Data Rate (Indirect Influence):
  • While ESP-NOW data packets are sent at a fixed, robust rate (like Wi-Fi MCS0), the overall Wi-Fi configuration of the device can influence the radio environment and ACK packet characteristics.
  • esp_wifi_set_protocol(wifi_interface_t ifx, uint8_t protocol_bitmap): This function allows you to restrict the Wi-Fi protocols used by an interface (e.g., STA or AP).
    • protocol_bitmap: Can be a combination of WIFI_PROTOCOL_11B, WIFI_PROTOCOL_11G, WIFI_PROTOCOL_11N, WIFI_PROTOCOL_LR (if supported by the chip).
    • By setting the protocol to only WIFI_PROTOCOL_11B, you force the Wi-Fi interface to use 802.11b data rates (1, 2, 5.5, 11 Mbps). These rates are generally more robust and have better receiver sensitivity characteristics than higher 802.11g/n rates. This could improve the reliability of ESP-NOW ACKs in challenging RF conditions, thereby improving the effective range where successful two-way ESP-NOW communication occurs.
    • Note: The ESP-NOW data packets themselves are already sent at a low PHY rate. Forcing 802.11b primarily affects the rates used for Wi-Fi management frames and potentially the ACKs for ESP-NOW packets.

• Antenna Considerations (Crucial):
  • Type:
    • PCB Trace Antenna (MIFA – Meandered Inverted-F Antenna): Common on many ESP32 modules (e.g., ESP32-WROOM-32). Cost-effective, compact, but performance is highly dependent on PCB layout, ground plane, and enclosure. Typically omnidirectional in the PCB plane.
    • Chip Antenna: Small ceramic component. Performance also depends on layout and ground plane.
    • External Antenna (via U.FL/IPEX connector or SMA connector): Allows connection of various external antennas (dipoles, patch antennas, Yagi, etc.). This offers the most flexibility for range optimization.
      • A simple dipole antenna (e.g., “rubber ducky” type) can offer better performance than a PCB antenna if well-matched and positioned.
      • Directional antennas (Yagi, patch) can significantly increase range in a specific direction by focusing RF energy, but reduce coverage in other directions.
  • Placement:
    • Keep antennas away from metal objects, large components, and ground planes (unless the antenna is designed to use it, like a patch).
    • Avoid enclosing the antenna in a metal case. Plastic enclosures are generally fine but can have some attenuation.
    • For PCB antennas, the module placement within the final product and the product’s enclosure design are critical. Follow manufacturer guidelines for keep-out areas around the antenna.
  • Orientation: Antenna polarization matters. For best results, both sender and receiver antennas should have the same polarization (e.g., both vertical or both horizontal). Mismatched polarization can cause significant signal loss (e.g., >20 dB).
  • Impedance Matching: The antenna must be impedance-matched to the radio’s output (typically 50 Ohms). Mismatches lead to reflected power and reduced efficiency. This is more of a hardware design concern.

• Channel Selection:
  • The 2.4 GHz band has multiple Wi-Fi channels. Some channels might experience more interference than others in a given environment.
  • While ESP-NOW devices must be on the same channel, you can experiment with different channels (1, 6, 11 are often recommended in North America as they are non-overlapping for 20MHz bandwidth) to find one with less interference.
  • Use esp_wifi_set_channel(uint8_t primary, wifi_second_chan_t second) to set the channel for ESP-NOW-only devices or an ESP32 in SoftAP mode. If the ESP32 is a STA, the channel is dictated by the AP.
• Environmental Factors:
  • Line-of-Sight (LoS): Always provides the best range.
  • Obstructions: Minimize obstructions between devices. Understand the type of materials – wood and drywall are less attenuating than brick, concrete, or metal.
  • Fresnel Zone: This is an elliptical region between antennas. Obstructions within ~60% of the first Fresnel zone radius can significantly degrade the signal. For longer distances, this zone can be surprisingly large, meaning even ground proximity can be an issue.
  • Height: Elevating antennas can help clear the Fresnel zone from ground obstructions and reduce ground reflections.

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graph TD
    A[/"<b>Start: Define Range</b><br><b>& Reliability Needs</b>"/] --> B{"Hardware Available?<br>(ESP32s, Antennas)"};
    B -- Yes --> C["Initial Setup:<br>Basic ESP-NOW Code<br>(Sender/Receiver, Same Channel)"];
    B -- No --> B_END[/"<b>End: Acquire</b><br><b>Necessary Hardware</b>"/];
    C --> D{Initial Range Test:<br>RSSI & Send Status};
    D --> E{Range Adequate?};
    E -- Yes --> F[/"<b>End: Success!</b><br><b>Monitor Stability</b>"/];
    E -- No --> G["Optimize Antennas:<br>1. <b>Type</b> (PCB, External Dipole, Yagi)<br>2. <b>Placement</b> (Clearance, No Metal)<br>3. <b>Orientation</b> (Matching Polarization)"];
    G --> H["Adjust Tx Power:<br><code style='font-family:monospace; background-color: #E5E7EB; padding: 1px 3px; border-radius:3px; color: #374151'>esp_wifi_set_max_tx_power()</code><br>(Balance Range vs. Power Consumption & Regulations)"];
    H --> I["Consider Wi-Fi Protocol for ACKs:<br>Force 802.11b for Robustness?<br><code style='font-family:monospace; background-color: #E5E7EB; padding: 1px 3px; border-radius:3px; color: #374151'>esp_wifi_set_protocol(..., WIFI_PROTOCOL_11B)</code>"];
    I --> J["Analyze Environment:<br>1. Identify & Mitigate <b>Obstructions</b><br>2. Check for <b>Interference</b> (2.4GHz Noise)<br>3. Test Different <b>Wi-Fi Channels</b>"];
    J --> K["Systematic Test & Measure:<br>- Log RSSI vs. Distance<br>- Monitor Send Callbacks<br>- Vary One Parameter at a Time"];
    K --> E;

    classDef startend fill:#EDE9FE,stroke:#5B21B6,stroke-width:2px,color:#5B21B6;
    classDef process fill:#DBEAFE,stroke:#2563EB,stroke-width:1px,color:#1E40AF;
    classDef decision fill:#FEF3C7,stroke:#D97706,stroke-width:1px,color:#92400E;
    classDef success fill:#D1FAE5,stroke:#059669,stroke-width:2px,color:#065F46;
    classDef check fill:#FEE2E2,stroke:#DC2626,stroke-width:1px,color:#991B1B;

    class A,F,B_END startend;
    class C,D,G,H,I,J,K process;
    class B,E decision;

Practical Examples

Prerequisites:

• At least two ESP32 boards.
• VS Code with the Espressif IDF Extension.
• ESP-IDF v5.x.
• (Optional but recommended for some tests) ESP32 modules with U.FL connectors and external antennas.

Example 1: Setting Maximum Transmit Power

This example shows how to attempt to set the Wi-Fi transmit power to its maximum configured value.

Code Snippet (in app_main or a Wi-Fi initialization function):

#include "esp_wifi.h"
#include "esp_log.h"

#define RANGE_OPT_TAG "RANGE_OPT"

// In your Wi-Fi initialization, after esp_wifi_start()
// ...
// ESP_ERROR_CHECK(esp_wifi_start()); // Wi-Fi must be started

// Attempt to set maximum configured Tx power
// The value is in 0.25dBm units. e.g., 80 = 20dBm.
// Check chip datasheet for actual supported min/max hardware values.
// Common range for ESP32 is 8 (2dBm) to 84 (21dBm), but may vary.
// Let's try to set it to a high value, e.g., 20dBm (80 * 0.25dBm)
int8_t tx_power_val = 80; // Target 20 dBm
esp_err_t err = esp_wifi_set_max_tx_power(tx_power_val);

if (err == ESP_OK) {
    ESP_LOGI(RANGE_OPT_TAG, "Successfully requested max Tx power to be set around %d (0.25dBm units).", tx_power_val);
} else {
    ESP_LOGE(RANGE_OPT_TAG, "Failed to set max Tx power, error: %s", esp_err_to_name(err));
}

// You can also get the current Tx power (may not reflect max if not transmitting)
int8_t current_power;
err = esp_wifi_get_max_tx_power(&current_power);
if (err == ESP_OK) {
    ESP_LOGI(RANGE_OPT_TAG, "Current configured max Tx power: %d (0.25dBm units)", current_power);
}
// ...

Build, Flash, Observe:

• Flash this to an ESP32. Observe the log output. The esp_wifi_set_max_tx_power function requests the stack to use up to this power. The actual power used can also be influenced by regulatory settings and internal power control algorithms.
• This setting applies to all Wi-Fi transmissions, including ESP-NOW.

Warning: Always adhere to local radio regulations regarding maximum permissible transmit power (EIRP). High Tx power also increases current consumption.

Example 2: Forcing 802.11b Protocol (Potentially for Robustness)

This example shows how to configure the Wi-Fi interface to primarily use 802.11b protocols, which are generally more robust over distance than 802.11g/n. This might help with the reliability of ESP-NOW ACKs.

Code Snippet (in Wi-Fi initialization, before esp_wifi_start()):

#include "esp_wifi.h"
#include "esp_log.h"

#define RANGE_OPT_TAG "RANGE_OPT"

// In your Wi-Fi initialization function, after esp_wifi_init() and before esp_wifi_start()
// ...
// ESP_ERROR_CHECK(esp_wifi_init(&cfg));
// ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA)); // Or WIFI_MODE_AP

ESP_LOGI(RANGE_OPT_TAG, "Setting Wi-Fi protocol to 802.11b only.");
// For STA interface
esp_err_t err = esp_wifi_set_protocol(ESP_IF_WIFI_STA, WIFI_PROTOCOL_11B);
// For AP interface, if using AP mode for ESP-NOW
// esp_err_t err = esp_wifi_set_protocol(ESP_IF_WIFI_AP, WIFI_PROTOCOL_11B); 

if (err == ESP_OK) {
    ESP_LOGI(RANGE_OPT_TAG, "Successfully set protocol to 802.11b.");
} else {
    ESP_LOGE(RANGE_OPT_TAG, "Failed to set protocol, error: %s", esp_err_to_name(err));
}

// ESP_ERROR_CHECK(esp_wifi_start());
// ...

Build, Flash, Observe:

• Apply this to both sender and receiver ESP-NOW devices.
• Conduct range tests (see Example 3) and compare with the default protocol settings (which usually include 11b, 11g, 11n). You might observe more stable connections at the edge of the range, though throughput for any standard Wi-Fi connections (if active) will be limited to 11 Mbps.
• The effect on ESP-NOW range itself (which already uses low data rates for its packets) is primarily through improved ACK reliability in noisy environments.

Example 3: Basic ESP-NOW Range Test (RSSI Reporting)

This sets up a sender and a receiver. The receiver prints the RSSI (Received Signal Strength Indicator) of incoming ESP-NOW messages, which is a good proxy for signal quality and range.

Sender Code (main/range_sender_main.c):

Use a simple ESP-NOW sender like in Chapter 70, Example 1. It should periodically send a counter or a fixed message.

Receiver Code (main/range_receiver_main.c):

Modify the ESP-NOW receiver from Chapter 70, Example 2, to focus on RSSI.

#include <stdio.h>
#include <string.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_system.h"
#include "esp_log.h"
#include "nvs_flash.h"
#include "esp_event.h"
#include "esp_wifi.h"
#include "esp_now.h"

#define RX_TAG "RANGE_RX"
#define WIFI_CHANNEL 1 // Match sender's channel

// Receive callback
static void app_esp_now_recv_cb(const esp_now_recv_info_t *recv_info, const uint8_t *data, int len) {
    if (recv_info == NULL || data == NULL || len <= 0) {
        ESP_LOGE(RX_TAG, "Receive CB error");
        return;
    }
    const uint8_t *mac_addr = recv_info->src_addr;
    // Access RSSI from the new recv_info structure (ESP-IDF v4.3+)
    // For older ESP-IDF, rx_ctrl might be part of a different structure or obtained differently.
    // esp_wifi_internal_rx_ctrl_t *rx_ctrl = (esp_wifi_internal_rx_ctrl_t*)recv_info->rx_ctrl; // This casting might not be needed/correct for all versions.
    // The recv_info->rx_ctrl already provides rssi.
    
    ESP_LOGI(RX_TAG, "Received %d bytes from " MACSTR ", RSSI: %d dBm", len, MAC2STR(mac_addr), recv_info->rx_ctrl->rssi);

    // Optional: Print data
    // char buffer[len + 1];
    // memcpy(buffer, data, len);
    // buffer[len] = '\0';
    // ESP_LOGI(RX_TAG, "Data: %s", buffer);
}

static void wifi_init_for_esp_now(void) {
    ESP_ERROR_CHECK(esp_netif_init());
    ESP_ERROR_CHECK(esp_event_loop_create_default());
    wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
    ESP_ERROR_CHECK(esp_wifi_init(&cfg));
    ESP_ERROR_CHECK(esp_wifi_set_storage(WIFI_STORAGE_RAM));
    ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA));
    ESP_ERROR_CHECK(esp_wifi_start());
    ESP_LOGI(RX_TAG, "Setting Wi-Fi channel to %d for ESP-NOW", WIFI_CHANNEL);
    ESP_ERROR_CHECK(esp_wifi_set_channel(WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE));
}

void app_main(void) {
    ESP_ERROR_CHECK(nvs_flash_init());
    wifi_init_for_esp_now();

    ESP_LOGI(RX_TAG, "Initializing ESP-NOW...");
    ESP_ERROR_CHECK(esp_now_init());
    ESP_LOGI(RX_TAG, "Registering ESP-NOW receive callback...");
    ESP_ERROR_CHECK(esp_now_register_recv_cb(app_esp_now_recv_cb));

    ESP_LOGI(RX_TAG, "ESP-NOW Receiver ready. Waiting for messages...");
}

Testing Procedure:

1. Flash sender and receiver.
2. Power both devices (e.g., with USB power banks for mobility).
3. Open the serial monitor for the receiver.
4. Start with devices close to each other. Observe the RSSI values (e.g., -30 to -50 dBm).
5. Slowly increase the distance between the sender and receiver.
6. Note how the RSSI values decrease (become more negative, e.g., -60, -70, -80 dBm).
7. Observe the point where messages become intermittent or stop being received. This is the approximate edge of your range for the current setup. The send_cb on the sender will also start reporting failures.
8. Experiment with different orientations, Tx power settings (on sender), and environments (indoors, outdoors, with/without obstacles).

Variant Notes

• Transmit Power Capabilities:
  • Different ESP32 variants and modules might have slightly different maximum hardware Tx power limits. Always consult the specific datasheet. The esp_wifi_set_max_tx_power() function will respect these hardware limits.
  • For example, the original ESP32 series often supports up to ~20-21 dBm. Newer chips like ESP32-C3, S3, C6, H2 also have configurable Tx power, typically in a similar range.
• Antenna Configurations: This is a major differentiator.
  • ESP32 (Original), ESP32-S2, ESP32-S3: Many modules come with PCB trace antennas (e.g., ESP32-WROOM-32, ESP32-S3-WROOM-1). Some modules offer U.FL connectors for external antennas (e.g., ESP32-WROVER series, some WROOM variants with ‘U’ suffix). Using an external, well-placed antenna will almost always yield better range than a PCB antenna.
  • ESP32-C3, ESP32-C6, ESP32-H2: These often come in smaller form factors, also typically with PCB antennas or options for external antennas on specific modules. The antenna design integrated into the module is key.
• Wi-Fi Long Range (LR) Mode (WIFI_PROTOCOL_LR):
  • The WIFI_PROTOCOL_LR flag for esp_wifi_set_protocol() is mentioned in ESP-IDF. This typically refers to using 802.11b rates or specific vendor LR modes if supported.
  • ESP32 (Original): Supports 802.11b.
  • ESP32-S2, S3, C3: Support 802.11b/g/n. Setting WIFI_PROTOCOL_11B is the primary way to force lower data rates for range.
  • ESP32-C6, ESP32-H2: In addition to 802.11b/g/n/ax (C6) or b/g/n (H2) for Wi-Fi, these chips also have 802.15.4 radios for Thread/Zigbee, which have their own range characteristics and are distinct from ESP-NOW over Wi-Fi. For ESP-NOW, their Wi-Fi radio’s 802.11b mode would be the “long-range” Wi-Fi option. The WIFI_PROTOCOL_LR might enable specific 11b features for extended range if the hardware and IDF version support it beyond just selecting 11b rates. Check the ESP-IDF documentation for the specific chip for details on WIFI_PROTOCOL_LR actual implementation.
  Important: ESP-NOW itself does not change its fundamental packet structure or modulation for “long range.” The benefit of WIFI_PROTOCOL_11B or WIFI_PROTOCOL_LR is primarily in making the underlying Wi-Fi acknowledgments more robust and potentially extending the distance at which these ACKs can be successfully exchanged.

Common Mistakes & Troubleshooting Tips

Exercises

1. Tx Power vs. Range Test:
   • Using the sender/receiver setup from Example 3:
     • Systematically vary the Tx power on the sender using esp_wifi_set_max_tx_power() (e.g., try values corresponding to 10 dBm, 15 dBm, 20 dBm).
     • For each power level, measure the maximum reliable range (where RSSI is acceptable and send success rate is high).
     • Plot Tx power vs. observed range. Note the point of diminishing returns.
2. Antenna Comparison (if hardware allows):
   • If you have ESP32 modules with U.FL connectors:
     • Test range with the on-board PCB antenna (if the module allows switching or if you have a module with only PCB antenna as a baseline).
     • Test range with a standard external dipole antenna (e.g., 2-3 dBi).
     • Test range with a higher gain external antenna (e.g., 5-9 dBi omnidirectional or a small directional panel antenna if available).
     • Compare RSSI and maximum distance achieved for each antenna type, keeping Tx power constant.
3. Impact of Obstructions:
   • Set up the ESP-NOW sender and receiver at a fixed, moderate distance where you get a good RSSI (e.g., -60 dBm) with line-of-sight.
   • Introduce different obstructions between them one by one: a wooden door, a person standing in the path, a thin sheet of metal (briefly and carefully), a brick wall (if testing outdoors/safely).
   • Observe and record the drop in RSSI for each type of obstruction. This helps build an intuition for how different materials affect 2.4 GHz signals.

Summary

• ESP-NOW range is influenced by Tx power, receiver sensitivity, antenna performance, path loss, interference, and the underlying Wi-Fi data rates used for ACKs.
• Tx power can be configured using esp_wifi_set_max_tx_power(), but must adhere to regulatory limits.
• Antenna type, placement, and orientation are critically important for maximizing range. External antennas often provide significant improvements.
• Forcing the Wi-Fi protocol to 802.11b (WIFI_PROTOCOL_11B) can sometimes improve robustness and effective range in challenging RF conditions by making ACKs more reliable.
• Minimizing obstructions and operating on less congested Wi-Fi channels can significantly enhance range.
• Always consider both ends of the communication link for optimization.
• All Wi-Fi capable ESP32 variants support these range optimization techniques through common ESP-IDF APIs.

Further Reading

• ESP-IDF Programming Guide – Wi-Fi API:
  • Transmit Power: Search for esp_wifi_set_max_tx_power and esp_wifi_get_max_tx_power in https://docs.espressif.com/projects/esp-idf/en/v5.4/esp32/api-reference/network/esp_wifi.html
  • Protocol Setting: Search for esp_wifi_set_protocol.
• Antenna Theory and Design (General RF Resources):
  • ARRL Antenna Book (for radio amateurs, but excellent general theory).
  • Online tutorials on antenna basics, dBi, VSWR, and impedance matching.
• Espressif Hardware Design Guidelines: (Available on Espressif’s website) These often contain crucial information about PCB layout for modules with on-board antennas, including keep-out areas and ground plane considerations. For example, ESP32 Hardware Design Guidelines.