Antenna And "Wireless" RF Testing 300 MHz To 40 GHz

Test Capability Overview...

  • Antenna Gain (or directivity) patterns in 2D polar cuts or full 3D spherical from 300 MHz to 40 GHz (there are many examples below, and very detailed example reports here), plotting software HERE
  • Effective Isotropic Radiated Power EIRP patterns in 2D polar cuts or full 3D spherical
  • Total Radiated Power TRP for standards like the PTCRB requirements for cellular transceivers
  • Antenna Efficiency from 3D plots of either TRP or gain
  • Radiated Spurious Emissions RSE from 30 MHz to 18 GHz (such as FCC Part 15)
  • Harmonics of your transmitter frequencies can be measured quickly with automation software
  • Troubleshooting with specialized probes and tools, such as a 5 foot screen inside the anechoic chamber which can show analyzer levels and limit lines in real time
  • Size: To be tested spherically, items should be less than 24 x 24 inches with a depth of 24 inches or less (table top-items for polar cuts can be larger and heavier), and GSM/CDMA cellular products have notes here

Costs... "How Much ?"

With most labs starting their quotations at several thousand dollars, no wonder everyone wants to know "how much" ... before going any further. Our typical antenna evaluation costs are $450 . We have prepared  this page which prices out four common test program options, and you can easily and accurately estimate costs for yourself, without any lengthy email dialog or waiting for call-backs on quotations from salesmen. (We have no salesmen !)  But unless you are an antenna expert, we urge you to familiarize yourself with the details and terminology below, before you jump straight to the prices!  We also have a wide range of example reports here. 

All About RF "Pattern Testing"

The following notes have been created to help our customers understand the various types of RF far field pattern testing at available at RTCE. Typical example results and graphs are also shown below (all images are "click to enlarge"). Lots of photographs are also included to show the typical test setups and the "Antenna Under Test" (AUT) or "Device Under Test" (DUT) mounting possibilities. Plotting software is availabe HERE.

Also, RTCE performs many other regulatory type tests such as ESD, surge, RF immunity, unintentional emissions (radiated or conducted) and design consulting. However, the focus of this page is strictly on our “pattern” measurement capabilities.

Introduction

In RTCE’s lab, “pattern” measurements of radiated RF signals are performed in a 3 meter fully anechoic chamber. A fully anechoic chamber is one where all interior surfaces (floor, ceiling, walls, door, etc.) are treated with RF absorber. RTCE’s chamber is lined with 10,500 pounds of ferrite tile which absorbs RF from 25 to 1000 MHz, and topped with 18 inch carbon pyramids which absorb RF from 400 MHz to 40 GHz. This environment simulates “quiet free space” with an absence of reflections and ambient RF signals. Please note: many “EMI/EMC” test chambers used for FCC work are semi-anechoic, where the chamber floor is not absorber covered, intentionally giving a full RF reflection. Such chambers are not useful for “pattern” work due to their floor reflections.

Coordinates

RTCE performs measurements according to Phi axis rolling and Theta stepping per the “Great Circle Cut System”. The figure below shows the Theta axis (turntable or azimuth control) and the Phi axis “roll positioner" (or elevation control). The positioners are fiber-optically controlled, with a maximum weight rating of 600 pounds on the Theta axis, and 40 pounds on the Phi axis. (Important ... A test sample over 10 pounds will need prior arrangements to optimize mounting for spherical testing.) Rate of turn is 3 RPM / 20 seconds with a positioning accuracy of better than 1 degree. Photos of the laboratory (later in this document) show the mounting arrangements for various DUTs. The receiving antennas cover 30 MHz to 40 GHz. For increased throughput speed, the range of 300 MHz to 40 GHz uses quad ridge guide horns to quickly take separate vertically and horizontally polarized measurements.

 

Measurements are logged in tables of Theta and Phi spherical coordinates, per IEEE Std 149-1979, “Test Procedures for Antennas”. The standard IEEE antenna coordinate system is illustrated below.

The spherical coordinates relate to the Cartesian axes as follows:

  • Z Axis is Theta=0 & Phi=any angle
  • Y axis is Theta=90 & Phi=90
  • X axis is Theta=90 & Phi=0
  • XZ Cut (Turntable Cut) is Theta=0 to 360 & Phi=0, or Theta=0 to 180 & Phi=0, then Theta=180 to 0 & Phi=180
  • YZ Cut is Theta=0 to 360 & Phi=90, or Theta=0 to 180 & Phi=90 then Theta=180 to 0 & Phi=270
  • XY Cut (Roll Cut) is Theta=90 & Phi=0 to 360

Alternatively, “patterns” measured in a single plane (or “cut”) can be plotted in polar format, such as the measurements below (from Example Results # 7). Polar plots can have several lines or "rings" such as "Total, Vertical, and Horizontal". 

When polar patterns are measured over a range of frequencies, the data is usually presented in a family of plots, or maybe a few over-laid patterns. When interested in changes over frequency, the user is forced to flip through many plots. An innovative visualization tool developed at RTCE in 2010 is a "polar cylinder", which stacks up individual polar plots in ascending frequency, to form a cylinder, where changes in pattern vs. frequency are more easily visualized. For example this Log Periodic Dipole Array can be seen to develop a front facing pattern as frequency increases. More of these graphs can be found here:  Advanced Plots

 

 

Two Types of RF Field Measurements: Gain or EIRP

A far field “pattern” can be created by one of two methods, Gain or EIRP.

Antenna Gain (dBi)

The method preferred for testing passive antennas is gain measurement. This is done by exciting an antenna under test with a CW or swept RF signal, and measuring its gain (in dB isotropic / dBi) via the substitution method. The substitution method involves setting up a reference antenna and a radiated path, then normalizing the path loss to 0 dB. Exchanging the reference antenna for the antenna under test, will then measure gain relative to the reference antenna. Simply adding the reference antenna's gain (in dBi) to these measurements then yields the unknown antenna’s gain in dBi. RTCE’s inventory of calibrated reference antennas can be either commercial reference dipoles (300 – 1700 MHz), or calibrated broadband horns (300 to 8000 MHz, 700 MHz to 18 GHz, or 18 to 40 GHz). For example, using the calibrated 700 MHz to 18 GHz reference horn would allow measuring an unknown antenna’s gain at any number of frequencies over this very wide range, simultaneously at each physical Theta/Phi position with a VNA, in one test run. Tests DO NOT have to be repeated at each individual frequency, as with tuned dipole substitution.

The following "click to enlarge" figure shows gain (in dBi) of a reference dipole at 840 MHz. Distortion in the ideal dipole “doughnut” pattern can easily be seen in the -Z axis, and is due to feed line radiation. (The feed line extends from the dipole in the –Z axis in this case.) Even though this antenna has a sophisticated tunable balun to decouple the feed line, dipoles are not the perfect reference antenna many people think they are! When measured in a "Satimo" type arch, this feedline radiation would be missed in the chamber's "blind spot" (at the base of the arch). This plot contains 1860 vertical and 1860 horizontal gain measurements, each combined for "total gain". The spheroid below is plotted in 5 degree increments, and actually has 2700 grid points. Perhaps you can see why it is foolish to think that dipoles have exactly 2.14 dBi of gain!

Effective Isotropic Radiated Power (EIRP)

The second “pattern” method is to measure a DUT’s EIRP via pre-calibrated path losses. In this method the “pattern” shows EIRP in many directions, while the DUT is transmitting “live”. This is typical of a small transceiver with an integrated antenna. The plot below is EIRP of a 2 Watt (+33dBm) GSM cellular modem, at a quick 15 degree resolution. The device is supposed to be omnidirectional, but the “pattern” shows otherwise. Also, this device has an FCC mandated maximum EIRP of 4 Watts (+36dBm), which can be seen as exceeded in the “-X axis” direction of the resulting plot. Total radiated power (TRP) can also be calculated by integrating over the sphere. Knowing TRP along with conducted launched power and antenna mismatch, allows antenna efficiencies to be calculated. Many modern “wireless” products seek ultra small antennas, and efficiencies can sometimes be very poor (10 to 20%).

 

Regarding angular resolution

If you are used to looking at electromagnetic simulator outputs, then 1 or 2 degree resolution may seem normal. This may be practical in the test lab for polar “cuts”, where only 360 physical positions are required for 1 degree resolution. However, on the surface of a sphere, the number of measurement points goes up with the inverse square of resolution. The examples below illustrate this nicely (more examples are here). To gauge how much resolution you really need, please look over the following illustrations of a horn antenna’s gain, at various resolution levels. All plots are "click to enlarge"

30 Degree Resolution (91 point spherical grid), very basic 3D visualization
20 Degree Resolution (190 point spherical grid), quick test time, minimum usable grid size for visualization
15 Degree Resolution (325 point spherical grid), a good balance between time and accuracy, especially for low directivity patterns (i.e "omnidirectional transmitters or antennas", with enough points for total power integration (used in efficiency or TRP calculations)
10 Degree Resolution (703 point spherical grid), longer test times, but very good for visualization, with enough points for more accurate total power integration (used in efficiency or TRP calculations)
5 Degree Resolution (2701 point spherical grid), longest test time, with very detailed plots which are great for complex or high-directivity patterns. These are also preferred for publication or antenna marketing. At frequencies above 10 GHz, directional antennas (especially horns with gain >15 dBi) can have features too closely spaced to be resolved even at 5 degrees.

Example Results File

This page contains many detailed examples of our test results on public domain antenna samples. In the raw data tabs of the spreadsheets you will find the tabulated gain (or EIRP) along with the following codes:

  • M – Measured Value
  • I##i## - Interpolated between index numbers ## and ##
  • D# - Duplicated value from index ## (usually the same physical point)

The numerical results are simply a table of antenna gains, usually in several hundred directions, at a hundred or more frequencies.

Time And Cost Estimates

RTCE uses custom created automation software (written in Agilent VEE) and modern RF instrumentation to minimize test time. In addition, the following practices maximize timely throughput:

  • Quad ridged (vertical/horizontal) horns covering the 300 MHz to 40 GHz range allow two orthogonal measurements (usually vertical and horizontal) within mill-seconds of each other for each positioner direction
  • High speed Elevation over Azimuth turntable with a 20 second revolution time on all axis
  • Measurement algorithms that duplicate or interpolate measurements on points of the spherical grid close to the Zenith and Nader, which are angularly close to each other (or overlap), compared to points on the “equator” (spherical grid points vs. measurement points in the table above)
  • Broadband reference antennas, which allow hundreds of individual frequencies to be patterned in one test run

This page contains our rates, and enough information for most people to estimate testings costs without the need for a quotation.

 

Laboratory Photos

The following table contains photographs of the RTCE laboratory and some of the equipment associated with “pattern testing”...

Close-up view of an Anritsu MP615B 470-1700 MHz reference dipole mounted on the roll axis, ready for substitution method antenna gain measurements.
RTCE's open-boundary quad-ridged 300 MHz to 6 GHz test horn (with an aperture of approximately 4 square feet).
Glenn Robb standing in the fixed antenna end of the RTCE chamber, with the quad-ridged test horn shown above.   
RTCE's quad-ridged 2 GHz to 18 GHz test horn.
RTCE's quad-ridged 18 GHz to 40 GHz test horn.
RTCE's quad-ridged 18 GHz to 40 GHz test horn mounted, showing the Miteq LNA, and Agilent K-connector relay, ready for Ku/K/Ka band pattern testing at 3 meters.
RTCE's double-ridged 18 GHz to 40 GHz calibrated reference horn.
RTCE measured antenna pattern of a WR-28 waveguide sector horn at 38 GHz, which performed well, yielding a measured -3dB beam-width of 8 x 96 degrees. More data can be found in Example 9.

A video which animates the Phi axis of this plot can be seen here on YouTube.
A Plexiglas mounting plate used to mount most small "DUTs" (Device Under Test). All items in the photograph are non-metallic, including the entire roll-axis mechanism and support tower. Blocks of low permitivity RF foam (Er ~ 1.1) are normally used as spacers.
The roll axis (Phi) mounting flange, with four 1/2 holes (on a 6" x 6" grid). The axis of rotation for the turntable (Theta) is exactly 12" in front of this Phi axis surface).
A standard 2x2 foot 1/2" drywall plate is always available for hosting wall-mount DUTs. The DUT shown has two polarization diversity directional antennas, which are tuned for maximum front-to-rear ratio when mounted to drywall.
RTCE's mobile antenna standard ground plane mounted to the spherical positioner for 3D antenna testing. This 1 meter diameter ground plane with a continuous 2 inch diameter rolled edge is always on hand for standard testing of mobile (automotive) mount antennas. This geometry is the de facto standard for mobile antennas, and is specifically required by Satellite Digital Audio Radio Systems (SDARS) such as, Sirius Service, or XM Radio Service, and many OEM automotive and cellular manufactures.    
The rear side of the vertical horizontal open-boundary double-ridge guide horn antennas, preamp and HP relay. For pattern work at higher angular resolutions (10 degrees or less), quad-ridge horns are used to remove the spacing errors introduced by separate V and H horns. A large collection of high pass filters are also used when measuring harmonics of higher powered transmitter devices.
RTCE's Rohde & Schwarz FSP40 spectrum analyzer.
RTCE's Rohde & Schwarz ZVK Vector Network Analyzer (10 MHz - 40 GHz).
RTCE's antenna, pre-amp, and positioner controllers.
Another antenna under test (300 to 400 MHz LPDA).   
Another antenna under test (RHCP L-band tapered helix).  Shown with Glenn Robb.  
A 915 MHz band pass filter designed by RTCE in Sonnet.
A 836 MHz (cellular) band stop filter designed by RTCE, and hand tuned for the enclosure effects.
Microstrip coupon used to characterize a board manufacturer's product for RTCE designs in Sonnet and HFSS, and measured with our 40 GHz TDR software.
   
   
   
   
   
   
   

Service Summary

Antenna Gain Patterns
EIRP, TRP, Efficiency
RF Design and Debug
RSE and Harmonics
Failure Mitigation
GSM/CDMA Cellular