Antenna switch with PIN diodes

How to switch between two antennas with a simple and reliable device built with PIN diodes. Designed for VHF and UHF bands.

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.

The circuit is very simple and consists of an outdoor section that will be placed on the mast, near the antennas and an indoor power supply unit.

Antenna switch schematic

The PIN diodes are biased with a 5V supply. Current is limited with the 220 ohm resistor. You can use any power supply you want, even with different output voltage. You will replace the 220 ohm resistor with a different one which maintains a biasing current of 15-20 mA through the diodes. The voltage drop on a pin diode is no more than 1V, so substract 1V from the supply voltage and divide the result with 15mA. This is how you calculate the series current limiting resistor value (which is 220 ohms in the actual schematic).

An important condition must be met in order to bias the diodes. The circuit must be closed at DC! Therefore this circuit will work with folded dipole antennas and the type of balun presented here. You cannot use the classic 4:1 transmission line balun or the Q3Q 1:1 transmission line balun (described here). But you can build a wideband 4:1 balun, which is a DC closed circuit. This one will work with any kind of dipole (not only folded type).

The balun shown in the schematic can be built on a toroid core with an outer diameter of 12 - 15 mm (for example the T50 core) by winding 7 turns of a twisted pair of wire. I used some wire from an ethernet cable. Then one end of the winding connects to the 300 ohms balanced terminals of the dipole and the other goes to a 75 ohms input.

Wideband VHF/UHF balun

The PIN diodes can be BA244, SMP1307 or any other type that can handle VHF and UHF frequencies. The cheapest device in which you will find PIN diodes is a DiSEqC switch for satellite installations. However I don't know if those diodes will work on VHF frequencies. Try to find their datasheet. Below you can see the PIN diodes in a DiSEqC switch (SMD marking is A8, I couldn't find a datasheet). And the DiSEqC switch can have another use in this project. You can build the switch in its case.

PIN diodes in a DiSEqC switch

The choke coil you see on the schematic is build by winding 4 turns of 0.3 mm diameter wire on a ferrite core of 3-5 mm diameter. The 470pF DC blocking capacitor on the coaxial wire should be shielded in a metal case that connects coax ground to antenna connector ground.

There are two ways in which you can build this device. You could make a switch with 300 ohms balanced input ports, where the antennas connect using twin lead cable or you can leave the balun out at the antenna terminals. Here are both designs.

PCB example for antenna switch

The PCB is not drawn to scale. You can build this board on the fly by scratching the few tracks. The switch can even be built without any PCB. The left version contains a screw terminal block where twin lead cable from antenna connects to. The balun is included in the box. The right version is just a box with three F connectors and the diodes connected between them as shown. You don't need a PCB for this one.

The simplicity of this circuit comes with some drawbacks. It's difficult to add an amplifier to your receiving installation and you're limited to only two antennas. Multiple input ports can be added, each with its own diode that needs to be biased by more complex circuit. A microcontroller could be used to select and hold bias on a diode based on pulses received over the coaxial cable.

References

• Elektor Electronics. Circuit 143 in 303 Circuite Electronice (Elektor 303 Circuits, Romanian edition). Teora, 1998, ISBN 973-601-857-1.
• Wikipedia contributors. "PIN diode." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 14 Apr. 2018. Web. 13 May. 2018.