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WLAN Survey antenna

Description of the receive only antenna FRARS members use for WLAN surveying.

Written by: G4RFR ( FRARS ) Friday, 29 November 2002

Click for full size imageAt a UK ham radio rally this year (2002), a number of ex-commercial antennna's were being sold off at a very low price - mainly because the specification and application was not known.

After opening the antenna, I discovered that in fact they are very suited to wireless lan applications around the 2.45Ghz mark.

The actual antenna is marked 540001 Rev A and has "receive antenna" written on the box.

The antenna element in these units is marked "Micro Pulse Inc. 31006 REV B". After chatting to the manufacturer, the frequency of design was obtained - 2483-2500 Mhz.

It would appear the these antenna's were designed for LEO satellite receive operations due to the nature of the omni-directional, circular polarity antenna. In fact, after investigation at the UK Radio-communications agency's page, the 2483.5-2500 MHz band in Region 1 (space-to-Earth) are also allocated to the radiodetermination-satellite service on a secondary basis. I.E. Radio Location service.

The antenna feeds a 3 stage ceramic filter centered on 2490 with a bandwidth of 16Mhz. At first, it would appear to be a little to narrow for WLAN use, but looking at the output on a spectrum analyser, one can easily see that it provides an excellant amount of gain over the 2400Mhz to 2500Mhz band.

The filter feeds a single gasfet stage, followed by 1 MMIC. Power is fed up the coax in the original design, but I have modified it to allow a seperate power feed.

Of course, this antenna is excellant for other S-Band applications, like Helicopter video, Video sender detection, S-Band satellite beacons and of course the 13cm ham band.

The use of this antenna is recommended for any of the License Free ISM band applications where greater receive sensitivity is required.

An initial test with this antenna indicated we could detect our test Access Point in a metal clad building, from over 1Km away. Mobile WLAN assessments can be productive in locating sources of interference in the band, and also helping us to avoid heavily used WLAN channels.

In the current setup, this antenna is fed with 12V, and it's RF output is connected to the modified Compaq WL100 pcmcia card. Using the Linux wireless lan auditing tool 'Kismet' with this antenna provides very good results. As Kismet is a passive sniffing tool, transmit / receive switching is of no concern.

Links:

  • More FRARS Wireless LAN articles [Click here]

  • Attachments: (Click to view/download):
    antenna1.JPG boxant1.jpg boxant2.jpg boxpic1.jpg
    M540001B.pdf undant1.jpg undant2.jpg undant3.jpg
       
    undant4.jpg undant5.jpg    

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