Cables

RF cables are, for frequencies higher than HF, almost exclusively coaxial cables (or "coax" for short, derived from the words "of common axis"). Coax cables have a core wire, surrounded by a non-conductive material (which is called dielectric or insulation), and then surrounded by an encompassing shielding which is often made of braided wires. The dielectric keeps the core and the shielding apart. Finally, the coax is protected by an outer shielding which will generally be a PVC material.
The inner conductor carries the RF signal and the outer shield is there to keep the RF signal from radiating to the atmosphere and to stop outside signals from interfering with the signal carried by the core. Another interesting fact is that the electrical signal always travels along the outer layer of the central conductor: the larger the central conductor, the better signal will flow. This is called the “skin effect”.

Even though the coaxial construction is good at containing the signal on the core wire, there is some resistance to the electrical flow: as the signal travels down the core, it will fade away. This fading is known as attenuation, and is measured in dB/m. The rate of attenuation is a function of the signal frequency and the physical construction of the cable itself., and a table of these values can be found in the next chapter. Obviously, we need to minimize the cable attenuation as much as possible, keeping the cable very short and using high quality cables.

Quarter wave omnidirectional antenna for 2.4 GHz

This is a simple and cheap omnidirectional antenna good for surveying signal strength using wlan tools. It is built using an N-type female chassis mount connector and five short lengths of copper or brass wire. The driven element is the wire soldered into the central conductor of the connector, with a length of one quarter wavelength. The four radials are also one quarter wavelength long, and are soldered into each corner hole of the connector. Each groundplane is cut to length and then bent over at 30 degrees from the horizontal to attempt to match the impedance to 50 Ohms. The gain of this antenna will be between 3 dBi and 4 dBi depending on final tuning.


Parts list:
- one N-type chassis mount female connector with four-holes flange
- 20 cm of copper or brass wire of 2 mm of diameter

Tools required:
- ruler and goniometer
- pliers
- a file for metal
- a small rat-tail file for metal
- one powerful soldering iron of about 80 -100 W
- one soldering iron
- solder
- vice
- hammer

Construction:

1. With the pliers, cut the wire in 5 pieces of 4 cm each.

2. File the flange of the connector near the holes in order to remove the plated surface. Use the rat-tail file to do the same on the internal part of the holes. This is done to prepare the surface of the connector for tinning.
3. Power on the high power soldering iron and let it heat for a couple of minutes. Apply the soldering iron to the connector until it gets hot (really hot! Be very careful!), but avoid melting the dielectric. Then tin the area around and inside the holes by applying solder until it flows. Avoid filling the holes with solder. Tinning is required to facilitate the process of soldering the wires to the connector.
4. Smooth with the file one side of each of the wires. Tin the wires for around 1 cm at the smoothed end, using the high power soldering iron.
5. Bend at 90 degrees 0.5 cm of the tinned side of the wires with the pliers. Do it for four of the wires, leaving one straight. Help yourself with the vice and the hammer.
6. Place firmly the connector in the vice, avoiding damaging the screw. Place the tinned bend side of one wire in a hole of the flange. Keeping it with the pliers, position the wire horizontally and along the direction of the diagonal. Apply the high power soldering iron shortly to the connector and with a small amount of solder, solder the wire to the connector. Avoid melting the dielectric of the connector. You may find useful getting somebody else keeping the wires in place with the pliers while you are soldering.
   
7. With the low power soldering iron, tin the central pin of the connector. Keeping the straight wire vertical with the pliers, solder its tinned side in the hole of the central pin.

8. Trim the exceeding part of the wires under the flange.
9. With the pliers, bent over the four radials at 30 degrees from the horizontal plane. This is done to match the impedance to 50 Ohms. To facilitate this operation, you may draw the 30 degrees angle on paper, and compare the antenna with it as shown.

10. Trim the radial at a length of 3,05 cm measured from the corner of the flange. Smooth the end of the wires with the file.

11. Trim the central wire at 3.05 cm measured from the flange surface. If you have a Spectrum Analyzer with Tracking Generator and a Directional Coupler, you can leave the central wire 0.5 cm longer and check the curve of the reflected power of the antenna. Trimming the wire at steps of 0.1 cm or less, you can tune the antenna to have the minimum reflected power at a frequency of 2.44 GHz. The pictures below show the display of the Spectrum Analyzer at the beginning and at the end of the tuning procedure. A difference of a few millimeters changes the frequency of resonance of the antenna of some hundred MHz. You are done!

Radio Laboratory Handbook of the ICTP “School On Digital Radio Communications for Research and Training in Developing Countries”

BiQuad antenna

The BiQuad antenna is simple to build and offers good directivity and gain for Point-to-Point communications. It consists of a two squares of the same size of 1⁄4 wavelength as a radiating element and of a metallic plate or grid as reflector. This antenna has a beamwidth of about 70 degrees and a gain in the order of 10-12 dBi. It can be used as stand-alone antenna or as feeder for a Parabolic Dish. The polarization is such that looking at the antenna from the front, if the squares are placed side by side the polarization is vertical.

Horn antenna

The horn antenna derives its name from the characteristic flared appearance. The flared portion can be square, rectangular, cylindrical or conical. The direction of maximum radiation corresponds with the axis of the horn. It is easily fed with a waveguide, but can be fed with a coaxial cable and a proper transition. Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the center of the dish reflector.

The use of a horn, rather than a dipole antenna or any other type of antenna, at the focal point of the dish minimizes loss of energy around the edges of the dish reflector. At 2.4 GHz, a simple horn antenna made with a tin can has a gain in the order of 10 - 15 dBi.

Yagi antenna

A basic Yagi consists of a certain number of straight elements, each measuring approximately half wavelength. The driven or active element of a Yagi is the equivalent of a center-fed, half-wave dipole antenna. Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called reflectors and directors, or passive elements altogether. A reflector is placed behind the driven element and is slightly longer than half wavelength; a director is placed in front of the driven element and is slightly shorter than half wavelength. A typical Yagi has one reflector and one or more directors. The antenna propagates electromagnetic field energy in the direction running from the driven element toward the directors, and is most sensitive to incoming electromagnetic field energy in this same direction. The more directors a Yagi has, the greater the gain. As more directors are added to a Yagi, however, it becomes longer. Following is the photo of a Yagi antenna with 6 directors and one reflector.

Yagi antennas are used primarily for Point-to-Point links, have a gain from 10 to 20 dBi and a horizontal beamwidth of 10 to 20 degrees.

1/4 Wavelength Ground Plane

The 1⁄4 Wavelength Ground Plane antenna is very simple in its construction and is useful for communications when size, cost and ease of construction are important. This antenna is designed to transmit a vertically polarized signal. It consists of a 1⁄4 wave element as half-dipole and three or four 1⁄4 wavelength ground elements bent 30 to 45 degrees down. This set of elements, called radials, is known as a ground plane.


This is a simple and effective antenna that can capture a signal equally from all directions. To increase the gain, however, the signal can be flattened out to take away focus from directly above and below, and providing more focus on the horizon. The vertical beamwidth represents the degree of flatness in the focus. This is useful in a Point-to-Multipoint situation, if all the other antennas are also at the same height. The gain of this antenna is in the order of 2 - 4 dBi.