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Measuring Wifi Radiation Properties with a MegiQ RMS

EB001_TPLink_Rotation_Orientations

This application note describes a case study of the power and radiation pattern measurements on a Wifi Access Point. It shows several ways of using the RMS-0660 to measure antenna radiation and power properties.

"Using two completely different methods, with only 0.4dB difference, the radiated and conducted results were nearly identical."

MegiQ RMS

EB001 MegiQ RMSThe RMS system is a turnkey system for measuring radiated properties of wireless devices and antennas. It consists of a turn table, dual polarized measurement antenna, dual channel receiver and PC software. There are several RMS types with frequency ranges between 400MHz and 6GHz, covering most popular communication bands of wireless and IoT devices.

EB001 RMS Operating ModesThe RMS allows measuring radiation patterns of wireless devices that transmit their own signal, as well as measuring standalone antennas by feeding a signal from the built-in RMS generator.

Both methods were used to reconcile the results of the two measurements.

TP-Link Access Point

We used an older model TP-Link access point TL-WR641G that supports 802.11b (11Mb/s), 802.11g (54Mb/s) and 802.11g-superspeed (108Mb/s).

The ERP radiation patterns and TRP of the AP were measured while it was transmitting.

In addition, we also measured the Antenna Gain patterns and TIG of the AP’s antenna by feeding an RMS generator signal to the antenna. After that the conducted transmit power was measured with a spectrum analyzer directly at the internal feedpoint to the antenna.

Signals to measure

Ideally a DUT sends a continuous signal that can be measured. Some devices and chips can be put in a continuous signal test mode but this usually requires access to the firmware or a communication tester.

If we want to measure an off the shelf device we have to make due with the signals that it sends by itself.

Beacon and Data signals

An access point sends a beacon signal at regular intervals containing the identity of the network (SSID), and we can measure these transmissions.

The disadvantage of this beacon signal is that it has a low duty cycle of about 5%, so that the measurement takes a longer time.

The access point transmits a lot more continuous when software like ‘Iperf’ is used to send large data packets continuously. This causes a transmission signal with 40 to 60 percent duty cycle.

Setting up Iperf

Iperf (www.iperf.fr) is a program that is commonly used to force continuous data streams, usually to measure data throughput. Iperf runs on a host (server) computer to send data and on a client to request data packets from the host. Iperf is available for many operating systems including Android and IOS.

A host computer connects to the AP under test by a network cable and Iperf is started as a server with the client’s IP address to listen for.

A wireless client (laptop, smart phone) is connected over the air to the AP and Iperf is started as a client with the server’s IP address to connect to. As soon as the client connects to the server a continuous data packet stream is started and we can measure the packet transmissions.

Radiated measurements

Signal Search

The RMS has a signal search utility to determine the center frequency, bandwidth and duty cycle of the signal. The duty cycle is important to establish a measuring time during rotations.

This shows the bandwidth and duty cycle of the 802.11g beacon signal: 

EB001 802 11g Beacon Signal

This shows the bandwidth and duty cycle of the 802.11g signal during data transmission:

EB001 802 11g Data Signal

The RMS determines the bandwidth edges between which the total energery of the signal has dropped 10dB below the maximum of the total energy.

DUT positioning for rotation.

The radiation patterns are measured in three rotations around the X, Y and Z axis of the AP. This shows the orthogonal positioning of the AP for each rotation.

EB001 TPLink Rotation Orientations

Radiation Patterns

These are the rotation patterns of the 802.11g beacon and data transmission signals: 

EB001 802 11G Beacon Pattern

EB001 802 11G Data Pattern

Because of its low duty cycle the beacon was measured at 10 degree intervals, taking 9 minutes for each rotation.

The data transmission signal was measured at 5 degree intervals, taking 5 minutes per rotation.

Despite the lower duty cycle and fewer rotation data points for the beacon signal, the final Total Radiated Power (TRP) values are within 0.2 dB the same.

The results of the 802.11b signals were very similar but they were 1.4dB lower.

Antenna measurements

We then set out to measure the antenna by itself, by feeding it with the generator output of the RMS. The antenna was disconnected internally from the transmitter and soldered a UFL connector to the antenna feed.

First the Return Loss of the antenna was measured with a VNA at the feed point that normally comes from the transmitter output. The return loss was excellent at below -20dB:

EB001 TPLink Antenna Return Loss

The loss of the long feed cable, including UFL adapter, was also measured with a VNA and the S parameters were imported into the RMS program so it can compensate for the loss in the coax feed cable.

After connecting the antenna UFL to the coax feed from the transmitter the 3D pattern was measured at 5 degree steps:

EB001 TPLink Antenna Pattern

The radiation patterns are nearly identical to the transmission patterns.

The Total Isotropic Gain (TIG) of the antenna is very high at -0.1dB, or 96.8% efficiency. This value is probably a bit on the high side but the conclusion is that the TP-Link has an excellent antenna. It is probably a sleeve dipole that does not depend on the PCB as ground plane.

Conducted output power

The conducted output from the AP’s transceiver circuit was measured with a spectrum analyzer.

For the 11g mode the output power was 20.9dBm. For 11b this was 1.4dB lower, exactly the same difference as what we found in the radiated measurements.

Adding it all up

We can compare the results from the radiated TRP measurements and the conducted power and gain measurements.

 

Measured TRP

Calculated TRP - Beacon

Difference

Mode

TRP

Data
dBm

TRP Beacon
dBm

TX power

Cond.
dBm

TIG Antenna
dBi

TRP

Calc.
dBm

 


dB

802.11b

20.0

19.7

19.5

-0.1

19.4

0.3

802.11g

21.4

21.2

20.9

-0.1

20.8

0.4

802.11g+

21.4

         

Using two completely different methods, with only 0.4dB difference, the radiated and conducted results were nearly identical.

Cable routing

One possible source of errors during radiated measurements is difference in routing of cables to the device.

EB001 Cable RoutingWhen the DUT has cable connections like power supply, USB, sensors etc it is important to keep the routing of those cables relative to the DUT as much the same as possible between the X, Y and Z rotation measurements.

This is especially true for devices with an antenna that depends on the PCB ground (most antennas do), and even more if the PCB ground is small compared to the wavelength of operation.

In those devices it is likely that the cables become part of the radiation solution, so the routing relative to the device affects the radiation patterns.

When a coax is used to feed a test signal to the device’s antenna it may be beneficial to route it through a ferrite core near the DUT to prevent ground currents through the cable that would distort the measurement.

Measuring different devices

The RMS is capable of measuring almost any kind of RF device as long as it operates in the frequency range of the RMS.

It is preferable to measure a device in its ‘native’ mode, through its own functionality and transmit circuit. This requires some means of activating the transmitter, preferably in a continuous or semi continuous transmit mode.

For Wifi Access Point measurements it is relatively straightforward to get a ‘measurable’ signal. If the device is a Wifi data client it is in most cases also possible to use Iperf to force a data stream.

Bluetooth devices may have a data or voice stream that can be forced to semi-continuous transmission by activating their function.

GSM, CDMA and LTE devices have a variable transmit power that is controlled by the base station they connect to. This is not good for measuring radiation because the transmit power is unknown and can even vary during the measurement.

Some modules for these applications allow activating specific test modes through AT commands. If this is not possible a communication tester is required to activate a test mode at a specific power.

If there is no possibility to activate transmission streams or test modes, the antenna can be contacted internally to feed it from the RMS generator output.

Conclusions

The RMS is well suited to measure the radiated properties of a variety of wireless devices with good accuracy.

Several RMS modes of operation give it the flexibility to use different configurations and use cases.

With an RMS system as part of an RF lab, the high availability of antenna pattern and device radiation measurements will greatly support efficent wireless development and verification. This will improve the quality of the final products.

 

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Sunday, 21 April 2024

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