1. Purpose:

To test the performance of several homemade wireless antenna systems.

- Cantenna  with USB dongle inside
- USB dongle
- Cantenna with built in antenna
- Biquad antenna
- Parabolic Dish antenna
- WLAN omnidirectional indoor antenna
- Cylindrical parabolic reflector
- Access point: Wireless router with an Omnidirectional antenna, Omnidirectional antennas always have vertical polarization.

 

2. Frequency spectrum and modulation methods:

* IEEE 802.11 documents, see below
- DBPSK modulation.
- DQPSK modulation
- OFDM modulation, 802.11g

Detected channels (Europ): channel 1 and 6

Transmited power of the transmitter (omnidirectional): 15 mWatt = 11.76 dBm, a common value

3. Used materials:

Belkin 802.11g wireless network card; chipset: BCM4306/BCM2050
Wireless router: Airvast
Wireless router: Airvast, internal view
WLAN omnidirectional indoor antenna
EZ Connect g wireless SMC USB 2.0 dongle, SMC2862W-G
50 Ohm RG213 coax cable, Loss: 50/100m; (2.4GHz)
RP SMA connector
N plug connector, impedance: 50 Ω
N-type connector (crimp) male
Cantenna 1 Dimensions: Diameter D: 100 mm Length l: 183 mm Built in antenna lenght: 31 mm Antenna offset from the left: 45 mm
Cantenna 2 with USB dongle inside Dimensions: Diameter D: 155 mm Length l: 215 mm Lenght USB dongle inside: 28 mm Offset from the left: 35 mm The effect of Horizontal or vertical placement of the dongle: must be vertical as on the photo.
Cylindrical parabolic reflector with omnidirectional antenna Dimensions: D: 330 mm c: 120 mm
Biquad antenna Dimensions: Lenght of each side: 31 mm Height from plate: 15 mm Groundplane: 123mm/123mm
Parabolic reflector with Biquad antenna Antenna gain: 22 dB
Parabolic antenna with USB Dongle
MSI Bluetooth antenna
Dlink USB antenna modification Model: d-link dwl-122 Remove first the internal antenna. Solder the external antenna. Antenna taken from Wireless Airvast router Antenna gain: 8 dB
Software: Network Stumbler 4.0; GEOGEBRA

4 Test environment:

Distance from Access point (Wireless Router as an Access Point) to the receiver: 45 meter, open air.
Used software: NetStumbler.

5. Measurement and calculation of the coax cable impedance:

The characteristic impedance of a coax cable: Z_c = (138.log (^D/_d)) / √E_r, where d is the diameter of the inner conductor and D is the diameter of the outer conductor: E_r: dielectric constant of the medium (if you don't know take 2.3)
a: inner conductor
b: Dielectric constant (Polyethylene: E_r = 2.3)
c: outer conductor (Tinned copper braid)
d: Bonded aluminum foil
e: Jacket (Polyethylene)

Z_c must be 50 Ω for all antennas, described here

6. Results:

7. Comparison chart of the measurements:

Measurement 1:

A: Bluetooth antenna at the top of the computer
B: Bluetooth antenna in front of the computer
C: Networkcard antenna
D: Bluetooth antenna in the focus point of the Cylindrical parabolic reflector
E: Candongle
F: Cylindrical parabolic reflector
G: Cantenna
H: Cylindrical parabolic reflector new construction

Measurement 2:

A: Biquad antenna
B: Networkcard antenna
C: Bluetooth antenna
D: USB dongle
E: Candongle
F: Cylindrical parabolic reflector
G: Cantenna
H: Cylindrical parabolic reflector with the bluetooth antenna

8. Some calculations

8.1 Power gain calculations:

dBm or dBmW is the power measurement relative to 1 milliwatt, dBm = 10 log P/P₀; when impedances are equal

3 dBm = 10 log P/P₀
or P = 10^0.3.P₀
or P = 2 P₀

10 dBm = 10 log P/P₀
or P = 10¹.P₀
or P = 10 P₀
or P = 10 mWatt

-50 dBm = 10 log P/P₀
or P = 10^-5.P₀
or P = 0.00001 P₀
or P = 10 nWatt

You can also use: dBm/dBW/MilliWatts Calculator

8.2 Power density from an isotropic antenna:

P_D = P_t/4pd²     [Watts/m²]

P_t : transmitted power
d : distance from antenna
Remarks: - this value must be multiplied by the antenna gain G of the antenna (omni directional antenna)
               - dBi = dBd + 2.15

Remarks:
The signal strenght decreases by 6 dB when the distance doubles (outdoor), indoor: 9 dB
 

8.3 Power density received by the antenna:

P_R = P_D.G_R.l²/4p     [Watts]

G_R.l²/4p = effective area of the antenna
G_R = gain of the receiving antenna

8.4 Friis Equation:

P_R = P_t.G_R.l²/(4pd)²     [Watts]
 

8.5 Wavelength calculations:

Wavelength l  = c/f
c: speed of light: 3.10⁸ m/s
Used Radio Frequency for WLAN, f = 2.4 GHz

The lenght of the internal antenna (cantenna) must be: ^l/₄
or 31 mm

The real used carrier frequencies are a little bit higher (see frequency table above), for optimal results take 31 mm for antenna lenght.

 

8.6 Parabolic Dish calculations:

Mathematical formula: y = a (x - p)²+ q
q = offset, take 0
p = horizontally displacement, take 0
New simplified formula: y = a (x)² or y = x²/4f where a= 1/4f ; use this equation to design your own parabola

The focus f of a given parabola = D²/16.c
D: Dish diameter
c: depth of the parabola antenna
 

Example of a Cylindrical parabolic reflector:

y = ax² ; a = 0.05
y = 0.05x²
a = 1/(4f)
f = 1/(4a)
f= 5
Remark for a better result take a= 0.025

Calculation of the needed surface:

y = 0.05x²
Lenght = 2 ₀∫¹³ √(1 + ((0.05x²)')²) dx
Lenght = 0.2 ₀∫¹³ √(100 + x²) dx, to solve this integral use the formule:

Using the definite integral with a = 10, gives lenght = 44 cm

Result: we need a metallic surface with lenght = 44 cm, and height = 30 cm

8.7 Cantenna calculations:

Wavelenght: l = 3.10⁸m/s / 2.4 GHz = 125 mm, in fact a little bit lower.
Built in antenna lenght: 31 mm = l / 4
Antenna offset from the left:

      = 0.25l / √((1 - (l/1.706D)²)

Lenght l of the cantenna must be > 0.75l

9. Useful online calculators:

CantennaCalc v0.2

Conversion from Watts (W) to decibels "milliwatts" (dBm)

EIRP Effective Isotropic Radiated Power calculator

Link Margin

Link Planning for Wireless LAN (WLAN)

RF Calculator Conversion calculator from Watt to dBm and from dBm to Watt

Space loss calculator

Space loss calculator Km

 

10. Some practical calculations:

- Calculation of Free Space Loss = FSL = 10 log (l /4.p.d)² = -100.4 dB for a distance of 1000 m

- Calculation of Maximum distance

  FSL = P_TR + G_TR - P_RC + G_RC -10 dB = 12 + 22 - (-80) + 10 - 20 = 104 dBm

     P_TR = Transmitted Power  (12 dBm)
     G_TR = Antenna gain transmitter  (22 dBi)
     P_RC = Receiver sensitivity (negative value) (-80  dBm)
     G_RC  = Antenna gain receiver  (10 dBi)
     20 dB = Fade margin, dynamic RF parameters which influences the signal

- Maximum distance d:

  ==> (-104 dB) =  10 log (l /4.p.d)²

  ==> d_max = 1500 m

11. Conclusions and remarks:

- Selfmade wireless cantenna is the best for home applications.
- The bigger the diameter of the cantenna, the more access points you will detect at the same time.
- Selectivity is dependent of the diameter of the cantenna.
- Antenna gain is much higher when you will place the antenna in front of the computer or placing a metallic shield behind the receiver!!!
- Glass does not effect the RF signal.
- Humans indeed will cause a signal drop!!
- The USB can dongle is the best if you want to detect more than one access point at the same time
- Place your antenna as high as possible
- Take good care about your connectors, the types listed above give the best results!!!!
 

12. Usefull links:

Antenna Measurements

Cisco Aironet Antenna Reference Guide

Geogebra

Hardware Comparison of some routers

Network Stumbler 4.0 a tool for Windows that allows you to detect Wireless Local Area Networks (WLANs) using 802.11b, 802.11a and 802.11g. Use it for finding locations with poor coverage in your WLAN or detecting other networks that may be causing interference on your network.

RF Power Values

Wlan Antennas
 

13. Biographies/White papers:

IEEE 802.11b White Papers pdf file

IEEE 802.11g White Papers pdf file

IEEE 802.11 white paper pdf file

802.11a Operates in the 5-GHz frequency range (5.125 to 5.85 GHz)

IEEE 802.16 WirelessMAN IEEE 802.16 WirelessMAN Standard for Wireless Metropolitan Area Networks, The IEEE 802.16 Working Group on Broadband Wireless Access Standards