Installation of WLAN data links has become commonplace whether within an industrial site, commercial establishment, or home use in both the 2.4 and 5 GHz bands involving 802.11 a, b, and g. Bantam Instruments’ Model 425A wireless LAN spectrum analyzer pushes the limits in frequency range, price and performance in a handheld unit. Measurements can be made in all WLAN bands with coverage extending to 6 GHz without the need for external modules.

A unique capability is not only to measure in dBm and dBmV like other spectrum analyzers, but to directly measure field strength in dBµV/m using the supplied antennas, which cover the entire frequency range of the analyzer. The antenna factors of the antennas are stored in the spectrum analyzer and the corrections are automatically applied. Normally the determination of antenna factor is a chore, involving an expensive setup, but a powerful derivation making use of the Friis transmission formula makes the measurement simple and straightforward (see the sidebar at the end of this article).

The Model 425A is ideal for evaluating WLAN installation sites to identify whether the environment is acceptable for an RF link and to determine which frequency bands and WLAN channels should be used. The next step is to optimize the placement of access points to assure high speed data transfer availability throughout the site. The third is to identify interfering signals and to mitigate them. As a true spectrum analyzer, rather than a WLAN chip set, the Model 425A is invaluable in measuring and identifying interference, as interfering signals can be detected, traced and identified regardless of modulation format and duty cycle.

Coverage is in two frequency ranges: 1.8 to 2.9 GHz and 5.0 to 6.0 GHz. The lower band amply covers the 2.4 GHz ISM (Industrial, Scientific, Medical) WLAN band, plus other bands of interest such as the 1.85 to 1.99 GHz PCS band, which can be of interest for companies depending heavily on cellular telephone accessibility. The 5.0 to 6.0 GHz frequency range covers all of the U-NII (Unlicensed National Information Infrastructure) WLAN bands currently used in the US, in addition to frequency ranges licensed for use in Europe and the Far East. It is expected that in the near future the FCC will authorize these additional 5 GHz frequency ranges for use in the US.

The Model 425A has two basic modes of operation. When an entire frequency range, such as 1.8 to 2.9 GHz or 5.0 to 6.0 GHz, is selected, operation is exactly the same as for a general-purpose spectrum analyzer. Measurements can be made in dBm or dBµV referenced to the input connector, or in dBµV/m using the supplied antenna.

When a WLAN band is selected, the spectrum analyzer is automatically placed in a mode to optimize capture of the spread spectrum signals used in WLAN networks. The preset WLAN bands are 2.4 to 2.5, 5.15 to 5.25, 5.25 to 5.35, 5.49 to 5.71 and 5.725 to 5.825 GHz. The detection mode is PEAK HOLD and a proprietary search algorithm based upon the known WLAN channel frequencies is used to speed capture. A resolution bandwidth (RBW) of 1 MHz is used, which is ideal for rapid capture of WLAN signals as well as achieving the resolution to discern between adjacent WLAN channels. Figure 1 shows a measurement in the 2.4 to 2.5 GHz WLAN band with an access point at channel 11.

Fig. 1 An access point on channel 11.

When in the WLAN measurement mode, a frequency cursor appearing as a solid line at the top of the display identifies the frequency range of each channel. Moving this cursor above the signal of interest identifies the channel number. It is then possible to zoom in to this particular channel using the ZOOM IN soft key for a close up view of this channel only, as shown in Figure 2. Pressing ZOOM OUT returns to the entire WLAN frequency band.

Fig. 2 Zoomed in to channel 11.

In a larger installation, multiple access points are usually used to achieve coverage. Figure 3 is a measurement on an installation using two 5 GHz access points, channels 52 and 60. The Model 425A, which weighs only 1.2 pounds, can be easily moved about to assure that there is at least a minimum signal from an access point at each location.

Fig. 3 Multiple 5 GHz access points.

Once measurements are made, the measurement data can easily be stored in 20 internal memory locations. In addition, 20 measurement setups may be stored so that customized measurement setups can quickly be recalled.

Since it is a true spectrum analyzer, the Model 425A is ideal for identifying interfering signals. Two of the most common in the 2.4 GHz WLAN band are microwave ovens and wireless telephones. Measurements involving WLAN chips would not identify the source, just that the link is marginal or has failed. Figures 4 and 5 show these common interference signals.

Fig. 4 Interference from a microwave oven.

Fig. 5 Interference from a wireless phone.

The Model 425A also has an RS-232 serial interface for communication to an external computer. Software is supplied with the Model 425A to download measurement screen data in the form of a bitmap or tabular data of frequency/amplitude pairs. This PC Enhancement Software package is provided on CD ROM for easy installation.

The Model 425A, shown in Figure 6, is priced at $4400, which includes antennas for both bands. A single band version, the Model 424A, which covers 1.8 to 2.9 GHz, is also available at $2600. If at some point the 5 GHz band is required, the unit can be returned to the factory for upgrade. Additional information may be obtained via e-mail at

Fig. 6 The Model 425A spectrum analyzer.

Bantam Instruments,
Sunnyvale, CA
(408) 736-3030,

A Powerful derivation for measuring antenna factor

Measuring antenna factor of an antenna is normally a chore that requires an expensive test setup where the antenna is placed in a known isotropic field and measurements taken. However, if identical antennas are used for transmission and reception, many unknowns are cancelled out, making the measurement quite straightforward. Start with the Friis transmission formula for determining received power:

Pr = PtGtGr(λ/4πr)2(1–|Γr|2) (1–|Γt|2)

Using this formula, the gain G of an unknown antenna can be determined by having two identical antennas separated by a known distance in the far field.

GdB = 10log(FMHzr)

+ ((dBmr – dBmt)/2)

– 10log(1–|Γ|2) – 13.78

Antenna factor AF can be derived from GdB by the relationship

AFdB = 20log(FMHz) – 29.78 – GdB

Combining the two equations for a direct measurement of antenna factor reveals

AFdB = 10log(FMHz/r) – ((dBmr – dBmt)/2) + 10log(1–|Γ|2) – 16

Antenna factor versus frequency can easily be measured using this formula. It is only necessary to know the power into the transmitting antenna and out of the receiving antenna, plus the match of the antenna. The antennas must be identical and the distance between them must be great enough to be in the far field, where r ≥ 300/FMHz. The displayed corrected measurement on the spectrum analyzer is then

dBµV/m = dBmr + AFdB + 107