Antenna switch with PIN diodes

A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. At high frequencies, the diode behaves as a resistor. The high-frequency resistance is inversely proportional to the DC bias current through the diode. Therefore, if suitably biased, the PIN diode acts as a variable resistor. It has low reverse capacitance, that will attenuate RF signal, unless the diode is forward biased. These properties make it suitable for RF switches.

The device presented here can be used to switch two antennas. It can prove useful in a number of situations. You can have a VHF and a UHF antenna, or you can have two similar antennas that are pointed in opposite directions for different signals. This switch does not require an extra cable and does not cause significant signal losses. A low DC voltage is sent over the coaxial cable to forward bias one diode. If the polarity of this voltage is reversed, then the other diode will be biased.

Transmission line baluns for VHF and UHF

A balun is a device that converts a balanced signal to an unbalanced signal. Depending on construction, a balun may transform impedance too and may be used for connecting signal lines of different impedances. In radio applications, baluns are used most of the time for adapting a transmission line to the antenna. This post will show you some easy to build baluns, designed for receiving antennas working in VHF and UHF radio and television bands. However, these baluns can be also used for transmitting antennas. They are easy to build because you only use cables similar to the antenna feeder cable.

Most used antenna class for all radio communications is the dipole. Since it's called a dipole, it has two connection points. Therefore it is suited for balanced signals. The antenna can be connected to a receiver using a twin lead cable. But in this case the receiver must have balanced input and similar impedance with the antenna. But the most used type of cable is the coaxial type, which is unbalanced and has a different impedance too. Let's see what are the basic types of connections between the antenna and the feeder cable.

Make a wideband antenna matching transformer

Most do-it-yourself TV antenna projects I could find use a readymade impedance matching transformer between antenna and cable. But what should you do if you can't find one at your local supplier? Let's attempt to make one with nothing more than some copper wire and a ferrite core. You could use some hard to find double hole pig-nose RF ferrite cores, but this transformer performed fairly good with ordinary toroidal cores.

This matching transformer can be used for VHF and UHF reception (from 50 to about 800 MHz). The input is 300 ohms balanced and the output is 75 ohms unbalanced, ready for coaxial cable connection. I found out that a device similar to what is presented here is used in commercially available antenna preamplifiers with 300 ohms input.