Omnidirectional Antenna

The Omnidirectional antenna is probably the most common antenna available. Just about every Wi-Fi device you can buy comes with an omni antenna. This is because the omni is so easy to set up, and generally works in consumer environments without much planning . There are few different types of omni antennas. Omni signals spread out sideways , but not vertically.

Top view of coverage an Omni Antenna

An omni antenna sends and receives signals equally in front, behind , to the left , or to the right of the antenna. However, when you go above or below the antenna, signal strength drops off significantly. The trade-off you make when choosing a high-gain antenna is this focusing, or thinning, of the above and below energy. The low-gain omni works better vertically than a high-gain omni, but it won't extend as far horizontally. The omnidirectional antenna sends and receives signals in all directions equally. This is generalization , but it's mostly accurate.

Side view of an omnidirectional antenna signal

Even though an omni antenna does not work very well above and below, it is not considered a directional antenna. Wi-Fi antennas are generally rated in two-dimensional space that assumes it is mounted parallel to the Earth's surface. Knowing how the beam is shaped, and that an antenna is not truly omnidirectional will help you choose the right antenna for your Wi-Fi.

(source : Wi-Fi Toys Extreme Tech)

Building A Classic Paper Clip Antenna

This article will show you how to put together the ultimate homebrew antenna -- a working Yagi antenna for 2.4 GHz Wi-Fi out of litle more than paperclips stuck together. This model is commonly called the Frisko antenna , after the French Frisko brand of ice cream cups whose wooden spoons were used in the first prototypes. 

The current designs of most external Wi-Fi cards put the antenna in a lawed position, with the antenna very close to the computer. This means that the pattern of emissions is often blocked by computer itself. Not only that, the small packaging of wireless cards prevents an optimal design for the internal antenna to pick up wireless network devices more than a couple of 100 feet away.

This is one of the reasons that attaching even a small external antenna can greatly improve signal strength, especially if it is oriented properly.

Dipole Antenna you will build in this article is folded dipole . The Dipole antenna is just the the simplest antenna. The dipole is a half wave antenna that consists of two opposing radiating elements. It's made up of two quarter-wavelength poles that are not connected to each other and fed in the middle by the transmission line. A standard dipole is open on each end, but it can also be folded over on itself.

The Antenna design shows a simple dipole made from steel paperclips . Each arms of the dipole is 31 mm in length, or 1/4 of a wavelength for Wi-i channel 6. The center conductor is soldered to right arm, while the shield is soldered to the left arm. It doesn't matter to which side you solder. 

The dipole antenna is unique in that it can be mounted vertically or horizontally. When standing vertically, the dipole antenna is omnidirectional. When horizontal, this antenna will radiate outward in two directions off the sides (and slightly upwards), like turning a donut on its edge.

(source ; Wi-Fi Toy Extreme Tech)

Adding A Driver / Amplifier for Increased Output Power

Addition o an RF power stage that you can use to extend your communications range. This part of the transmitter is arranged for class-A linear service. 

Although it is not necessary to use a linear amplifier for CW or FM amplification, there are some advantages : (1) a linear amplifier produces a lower level of harmonic currents; (2) it is easier to drive when a low-power stage is used to excite it. (3) the keyed waveform of the overall transmitter is less clicky than when using a class C amplifier after the keyed stage.

The Output power from Q2 is approximately 1 watt. This level of power will enable you to work DX when band conditions are good.

(source : W1FB Design Notebook)

150 mW CW Transmitter Circuit

A Schematic diagram of A 150 mW CW transmitter, Fixed value capacitors are disc ceramic , 50 V or greater , Resistors are 1/4 or 1/2 watt carbon film or carbon composition, C1 and C2 are feedback capacitors , C3 is a 10-100 pF ceramic or mica compression trimmer. 

A two circuit phone jack is used for J1 and an RCA phono connector or SO-239 coax connector may be used for J2, L1 is a 2.3 uH toroidal inductor. Use 24 turns of no. 26 enamel, wire on an Amidon T37-6 toroid core or other brand. L2 has three turns of no. 26 enamel, wire wound over the +12 V end of the L1 winding. 

Q1 is a 2N4400 or 2N4401 or equivalent transistor . A 2N2222A may be substituted, but will deliver less output power . Y1 is a fundamental crystal, 30 pF load capacitance.

(source : W1FB Design Notebook)

Broadband Antenna with reflector

In Yagi antennas, reflector antennas and in-phase excited antennas are suitable for amateur production.

a) Broadband dipole with angular reflector and broadband reflector

Due to the large feedback factor, this antenna is (Fig. 160) Suitable for places with a medium and strong signal, disturbed by reflections from rearward direction, ie especially to urban districts with height segmentation development. The radiation diagram in Fig. 160c applies to the frequency 730 MHz, at which this antenna has the highest gain. The return factor is in the whole scope IV. and V. television band almost uniform. The electrical parameters are shown in Fig. 161. Fig. 162 is a view of the actual embodiment antennas with angled reflector.

b) Excited full-wave floor system for channels Nos. 21 to 81

This antenna (Fig. 163) is very suitable for amateur construction, because is not as demanding on accuracy as Yagi antennas and always gives good results. The reflector wall can be either made of mesh with a mesh size 20 X 20 mm (Fig. 163a), or can be made according to Fig. 163b

In both cases, however, all the wires crossing each other are necessary solder with tin or tin the whole reflector wall in fire (galvanic tinning is not enough). The electrical values ​​are almost equal in the whole band

specific (see also Fig. 171). The antenna is also suitable for places with inhomogeneous ones field, is significantly less sensitive to the quality of the incident field than antennas Yagi type.

The gain of an excited whole-wave floor system can be maintained while maintaining the basic

increase by about 2 dB by adding a series of directors to each emitter (Fig. 164). Director carriers are threaded at both ends, enabling easy installation even in an already finished antenna.

A further increase in gain is achieved by arranging four antennas in the system (Fig. 165). The spacing of the antennas in the axis of the system is chosen so that the distance between individual emitters was constant. The reflectors of these antennas therefore overlap, it is advisable to shorten them accordingly. It is advisable to merge the antennas in series with a parallel connection .

(source : translated from Anteny by Tomas Cesky)