Antenna: Wireless LAN Waveguide Type

The model of wave guide tube is the same construction with the theoritical principals in coaxial adapters.

This antenna is constructed of cylindrical can and a N connector with centre tap lengthened. Point the open end of the can to the ISP station and begin surfing. Some cable needed between the antenna and wireless LAN card. Lets take a look to cabling details. 

The diameter is about 100 mm at 2.4 GHz band. It can be  made from coffee tin or else. The diameter of the can must be 90 - 10 mm, side and bottom should be smoothen.

Dimensioning

*Creech Lambda is replaced to L letter.

The best length for the can may be 3/4 Lg or more. The centre tap of  the N connector is lengthened with 4 mm dia brass rod to Lo/4. Lo It depends on the nominal frequency. Lo = 122 mm @ 2.45 GHz so Lo / 4 = 31 mm.

Lg depends on dia of the tube, here are some values: 

For a N-connector needed a 12 mm dia hole which is Lg / 4 from closed end. According the flange of the connector, also needed four 3.5 mm in hole. The inner tap of the N connector is lengthened to Lo/4 or 31 mm with brass rod about 4 mm dia. The length of the tap is not needed to be very accurate. I am sure that several lengths from 25 to 40 mm is not a big deal although the impedance of the antenna is depending of the tap length. It's good idea to drill a 3mm hole axially to the end of the brass rod where the centre tap of the connector goes tightly, thus the rod soldering becomes very firm.

N connector is fixed with four 3 mm screws so that bolts are pushed from inside of the tube and nuts screwed outside, so there becomes inside so less as possible extra taps, which can disturb the antenna function. The jointing of the connector and tube is sealed watertight with silicone mass. In the very lowest point of the tube there have to be drilled in a small hole for condensed water outlet. 

The open end of the can needs a cap. The plastic material needed here should pass by microwave oven test .

Antenna mounting to the antenna mast conduit can be made of some kind of band round the cylinder to prevent the can flatten or dent.

Improved model

If the bottom of the can is not smooth, an extra bottom can be added inside the can. It can be made from steel tin or aluminium material which is cut through the inside diameter of the can.
 
More effective version

The waveguide antenna can be equipped with a funnel which increases the sensitivity of the antenna by collecting hf signal from larger area. This adding increases the gain of the antenna by twice or 3 dB.

 

The picture shows how the funnel is cut from smooth tinned steel. Dot lines show margins that need to joint. This antenna is made of air conditioning conduit with dia D = 100 mm where the bottom added tin steel. The antenna dimensions :  D = R1 = 100 mm,  D2 = R2 = 170 mm, Lg/4 = 44 mm, Lo/4 = 31 mm, 3/4 Lg = 132 mm. 

It is possible to increase the outer dia D2 even more. The idea of the funnel is from the satellite receiver horn found in ARRL antenna book.

 

The open end of the funnel is closed with a microwave proof plastic cap. Tighting of the joint of the N connector and condenced water hole are similar as in the basic model.

Theory of the Waveguide Antenna

The different wavelengths in the waveguide tube. They are marked as Lo, Lc and Lg.

Lo is the wavelength of  the hf signal in open air or  Lo/mm = 300 / (f/GHz).
Lc is the wavelength of the low cut frequency which depends on tube dia only  Lc = 1,706 x D

Lg is standing wavelength inside the tube, it is function of both Lo and Lc

A waveguide which is closed on the other end acts similar as a short circuited coaxial cable. The coming hf signal reflects from ending point and there forms so called standing wave when incoming and reflecting signals in different places are either weakening or amplifiering each others:

If there is a measuring probe which is moving in axial direction inside the tube, there can be found some minimum and maximum points in certain intervals. At the closed end the signal is zero and so will be in halfwave intervals. The first maximum point is quarter wavelength from the closed end. This will be the best place to outlet signal to coaxial line. Notice that maximum area is quite flat. So the place of the outlet must not be very accurate.

The important thing is that the standing wavelength Lg is not the same as wavelength Lo, counted from hf signal. Large tubes are near as open air where Lg and Lo are almost the same but when tube diameter becomes smaller the Lg increases effective until it becomes a point when Lg infinite. It corresponds the diameter when hf signal doesn't come to the tube at all. So the waveguide tube acts as a high pass filter which limit wavelength Lc = 1.706 x D. Lo can be calculated from nominal frequency: Lo/mm = 300/(f/GHz). Inverse values of Lo, Lc and Lg forms a right angled triangle where becomes the equation of Pythagoras:                             (1/Lo)2 = (1/Lc)2 + (1/Lg) 2

which can be solved

Lg = 1 / SQR((1/Lo)2 - (1/Lc)2)

In the antenna the N connector is situated in maximum point or length of Lg/4 from the closed end. Total length of the tube is selected so that the next maximum place hits on the open end of the tube or 3/4xLg from the closed end. The latter is only supposed by my own and found be not the worst decision.


Editor