Proposal for a 2.4 GHz Antenna Pattern Test System

Leigh L Klotz, Jr. WA5ZNU

This system tests 2.4 GHz receive antennas and provides a real-time polar azimuthal plot of signal strength and noise floor, enabling rapid comparison of antenna patterns for different antennas.

Test antennas might include commercial 1/4 wave antennas, commercial 5/8 wave antennas, Yagis, patch antennas, home-brew Pringles can waveguides, antennas constructed on-site, etc.

The test system uses common, off-the-shelf hardware and hobby hardware and software to produce real-time azimuth plots of antenna gain for 2.4 GHz antenna systems. It has good immunity to noise sources, and power budget fits well within RF safety guidelines. A laptop web browser display a polar plot a real-time polar plot of signal strength and noise floor as the antenna rotates.

Hardware Description

Receive Test Fixture

The test receive antenna is mounted on a post in the center of a rotating lazy-susan platform which is driven by a center-mounted stepper motor. The use of a stepper motor eliminates the need for a position encoder, and allows the antenna to be in relatively free space; however, manual rotation and a shaft-mounted or roller-wheel position encoder could be used instead.

The antenna is connected by coax to an 802.11 access card, or to access point hardware such as the Linksys WRT54G. The 802.11 hardware driver reports signal strength (RSSI) in dB, and noise floor in dB.

Transmit Test Fixture

The test transmit antenna is an omnidirectional antenna attached to a similar 802.11 device. Standard 802.11 protocol ensures that the signal strength reported by the receiving device is accurate and protects against interference from other spectrum users, and is reported as part of the noise floor.

Software / Microcontroller

A controller such as an Arduino Uno and SparkFun stepper motor controller board rotates the platform. Python software on a Linux laptop provides a data set of rotation angle, signal strength, and noise as an HTTP service. A web browser is used to visualize the data as a polar plot.

Test Antennas

In addition to the above list, we might be able to get to MFJ or another ham manufacturer to donate a Yagi or patch.

Comparison with other work

In contrast to the previously-proposed project using a discrete log-amp detector, the test system has better noise immunity. No special-purpose RF receiving circuit needs to be built. It does not require a continuous-wave transmitted signal, and uses readily-available transmitting equipment.

Below are some similar projects; none is identical, but taken in retrospect, the test system seems to fit well among them.