Wi Fi Antenna

7/30/2019 Wi Fi Antenna

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WLAN 802.11b/gBi-quad antenna

Building and testing

Report on reproducing Trevor Marshall's

Bi-Quad Antenna

Antenna specifications: 11dBi, wide band

For original article see: http://trevormarshall.com/biquad.htm


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Antenna wire: lenght 244 mm, diameter 1.2 mm, aluminium

Reflector: circuitboard 123 x 123 mm

Cable: N-female 300mm test-cable MIL-STD RG-58C (semi-rigid)

Complete antenna.

Notes on building:

The center frequency is very sensitive to how you solder the ends of the quad loops to

the coax. After multiple attempts it was determined that even a half a millimeter

difference will move the center frequency by as much as 50 MHz. So be prepared for a

lot of "measure, resolder, measure, resolder ..."


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The feedpoint:

one branch of quad to coax center conductor, and the other to the shield (ground).

The feedpoint is the most ugly place in the antenna. Try to keep the gap as short as

practicable and the crossing lines together in symmetry - however be prepared to

resolder a couple of times to find the best configuration. Note that, to allow for a gap

between the feedpoint, the quad-ring will be distorted somewhat - however try to keep

the quad overall lenght the same 244mm.

When you measure the returnloss/SWR and find that the center frequency is off whereyou want it, you have to make the quad-ring 1mm shorter or longer by adjusting how

you solder the quad ends to the coax. If you find that the returnloss/SWR figure isn't

'sharp' or is distorted then you probably have some non-symmetry at the feedpoint.

Spot on:

Note that the SWR at center frequency isn't actually as important as on

the channels 1 and 11. You want to center to be where it dips those

frequencies the most.


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Test report:

Equipment:

Spectrum Analyser, 3GHz, IFR 2399BReturn Loss Bridge, >40dB 3GHz, Eagle RLB150N5ATest cables and adapter, 0-18GHz, Rosenberger

Test conditions:

The analyser's tracking generator and separate return loss brigde were used to test the

return loss from the test port without the antenna attached. This level was set as zero

reference for the measurement markers. The antenna was suspended with cardboard 1meter above ground with a minimum of 5 meters to adjacent walls and the ceiling. The

antenna was moved around to average out reflections and other anomalies caused by the

less than perfect measuring conditions.

Results:

Mar ker 1: 2412 MHz ( Channel 1) , Ret urn l oss - 23dB = SWR 1. 2



Notes on measuring:

No, you don't need a billion dollar network analyser to do antenna measurements. Very

good results can be achieved with a ordinary spectrum analyser with tracking generatoroption. All you need is a good quality return loss bridge and some test cables and

adapters. At 2.4GHz your equipment needs to be rated for atleast 3GHz operation andthe cables and adapters must have low loss and good SWR at these frequencies. Using

poor cables or adapters in reflection measurements will give you 'better' results than

reality would warrant.


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Remember that in reflection loss measurements you are measuring the reflected energy

from the antenna, which at good SWRs will be very small fraction of the tracking

generators output.

Here's a small table for converting reflection loss and SWR: For example if thereflection loss is -20dB this means 1% of the energy transmitter is reflected back. This

is equivalent to an SWR of 1,222 which is very good.

Ref dB SWR Ref Coff. -1 Ref dB SWR Ref Coff. -1-1 17,391 0,7943 1,26 -21 1,196 0,0079 125,89-2 8,724 0,6310 1,58 -22 1,173 0,0063 158,49

-3 5,848 0,5012 2,00 -23 1,152 0,0050 199,53-4 4,419 0,3981 2,51 -24 1,135 0,0040 251,19-5 3,570 0,3162 3,16 -25 1,119 0,0032 316,23

-6 3,010 0,2512 3,98 -26 1,106 0,0025 398,11-7 2,615 0,1995 5,01 -27 1,094 0,0020 501,19

-8 2,323 0,1585 6,31 -28 1,083 0,0016 630,96-9 2,100 0,1259 7,94 -29 1,074 0,0013 794,33

-10 1,925 0,1000 10,00 -30 1,065 0,0010 1000,00

-11 1,785 0,0794 12,59 -31 1,058 0,0008 1258,93

-12 1,671 0,0631 15,85 -32 1,052 0,0006 1584,89-13 1,577 0,0501 19,95 -33 1,046 0,0005 1995,26

-14 1,499 0,0398 25,12 -34 1,041 0,0004 2511,89-15 1,433 0,0316 31,62 -35 1,036 0,0003 3162,28-16 1,377 0,0251 39,81 -36 1,032 0,0003 3981,07

-17 1,329 0,0200 50,12 -37 1,029 0,0002 5011,87-18 1,288 0,0158 63,10 -38 1,025 0,0002 6309,57

-19 1,253 0,0126 79,43 -39 1,023 0,0001 7943,28

-20 1,222 0,0100 100,00 -40 1,020 0,0001 10000,00( VSWR = 1-SQRT(Reflected/Forward) : 1+(SQRT(Reflected/Forward) )

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