PIN Diode in Ham Radio: Fast RF Switching Explained

Inside many modern transceivers there is a component that clicks no relay and wears out no contacts, yet connects the antenna to the transmitter one moment and to the receiver the next. It is often a PIN diode, a semiconductor that behaves, at radio frequencies, like a resistor whose value you control with a small direct current. The PIN diode is quiet, fast and long-lived, which is why it has taken over the antenna changeover and the band switching in a large part of the amateur equipment built in the last forty years. This guide explains how it works, what its important numbers are, how to bias it correctly and where homebuilders can use it, including a well known trick with a very ordinary power diode.

What Makes a PIN Diode Different

An ordinary junction diode has a P region and an N region that touch each other. A PIN diode has a thick layer of almost pure, or intrinsic, semiconductor between them, and this extra layer gives the device its name. When a forward current flows, charge carriers are injected into the intrinsic layer and stay there for a relatively long time, called the carrier lifetime. At a radio frequency, the period of the signal is much shorter than that lifetime, so the carriers do not have time to leave and return with every cycle. The diode therefore stops acting as a rectifier and becomes a linear resistance, whose value depends on the average stored charge, that is, on the DC bias current.

That is the whole principle. With a forward bias of a few milliamperes to a few tens of milliamperes the resistance of the diode is a fraction of an ohm to a few ohms, a nearly perfect closed switch. With the bias removed or reversed, the diode is nearly an open circuit, and what remains is a small capacitance of a picofarad or less across the intrinsic layer. In between, the resistance can be set to any value from tens of ohms to several kilohms by changing the current, which makes the PIN diode a voltage- or current-controlled attenuator as well as a switch.

The Numbers to Look For

A data sheet for a PIN diode lists a handful of values, and each one has a practical meaning. The forward resistance at a given current tells you the insertion loss of the switch in the on state. The reverse capacitance, together with the frequency, tells you the isolation in the off state, since a capacitor of one picofarad has a reactance of about 160 ohms at 1 GHz but only about 8 kilohms at 20 MHz. The reverse breakdown voltage, together with the thickness of the intrinsic layer, tells you how much RF voltage the diode can stand when it is off. The carrier lifetime gives the lowest frequency at which the device still behaves as a resistance rather than a rectifier. A final value is the switching speed, which is typically between a fraction of a microsecond and a few microseconds.

The effect of the carrier lifetime on distortion deserves attention. A diode with a long lifetime remains a good linear resistor down to very low frequencies, so it produces little intermodulation. A diode with a short lifetime is faster but starts to rectify at the low end, which creates distortion. For the HF bands, with an ordinary transmit signal of 100 watts, designers want a diode that is slow enough to stay linear on 160 metres and fast enough for the keying speed in CW.

Series and Shunt Switches

There are two basic ways to use a PIN diode as a switch. In a series switch the diode is placed in the signal path. Forward bias closes the switch and the signal passes with very little loss, while no bias opens it and the signal is blocked by the small capacitance. In a shunt switch the diode connects the signal line to ground. With no bias the signal passes unhindered, and with forward bias the line is short-circuited and the signal is blocked. A shunt switch gives better isolation at high frequencies because the short circuit does not depend on a tiny capacitance, but it needs a quarter-wave section or a matching network to present a high impedance back to the source.

The classic antenna changeover combines both. A series diode connects the antenna to the transmitter during transmit, and a shunt diode a quarter wavelength away grounds the receiver input at the same moment, so that the receiver is protected from the transmitter power. On HF the quarter-wave line is replaced by a lumped equivalent with two capacitors and an inductor, sometimes called a pi network. The result has an isolation of 40 to 60 decibels, and the receiver sees only the tiny leakage that remains.

How to Bias a PIN Diode

The bias current must be added to the RF path without disturbing it. Two components do this job. A DC blocking capacitor keeps the bias from leaking into the rest of the circuit, and an RF choke or a resistor feeds the bias to the diode without letting the radio frequency escape into the control line. The choke must have a high impedance on the lowest band the switch has to handle, and a bypass capacitor at the cold end sends any residual RF to ground. A mistake in this part is one of the common causes of an RF leak into the keying and control wiring, and the cure is the same as in other places, with ferrite beads on the wires, as the guide to the ferrite mix explains.

The DC part is simple Ohm’s law. If the control line supplies 12 volts and the diode drops about 0.7 volts at 10 mA, a series resistor of roughly 1.1 kilohms sets the current. A larger current gives a lower resistance and a lower loss, but too much current wastes power and heats the diode. A good design uses the lowest current that gives the required insertion loss and distortion, often 10 to 20 mA in a receiver path and more for the transmit path at high power. In a battery-powered portable rig this consumption is a consideration, and it is one of the places where a relay can be better, since it uses power only while it changes state.

Where PIN Diodes Appear in a Radio

The transmit-receive changeover is the best known use, especially in a transceiver that offers full break-in. When you key a CW signal at 40 words per minute and want to hear the other station between the dits, a mechanical relay cannot follow, but a PIN diode switch can. The article on full break-in CW and QSK describes the operating side of the same feature. A second use is band switching, where a diode connects the correct filter or coil for the selected band, controlled by the microprocessor without a mechanical band switch with its long wires. A third use is the antenna selection inside the radio, for example between two antenna sockets.

A fourth use is variable attenuation. A pair of PIN diodes in a pi or bridged-T arrangement gives a 50 ohm attenuator whose loss changes with a control current. Such attenuators are used for AGC in the front end, ahead of the first amplifier, so that a strong signal is reduced before it can overload the receiver. The behaviour of the automatic gain control is described in the article on AGC. In an HF amplifier, PIN diodes also switch the low-pass filters for the bands, which are explained in the article on the low-pass filter.

The 1N4007 Trick

A PIN diode is a specialised part, but a very common rectifier, the 1N4007, has a thick, lightly doped junction and a long carrier lifetime of several microseconds. Builders discovered that at HF the 1N4007 behaves very much like a PIN diode, with low resistance at a few milliamperes and low capacitance with reverse bias. For this reason it appears in many QRP transmit-receive switches and in homebuilt antenna relays, where its price of a few cents is attractive. It works well from the lower HF bands and up to perhaps 30 MHz, although on 160 metres its lifetime is just sufficient, and it is not as fast or as good at VHF as a real PIN diode designed for the purpose.

If you use rectifier diodes this way, accept that the parameters are not guaranteed and may vary between batches and manufacturers. Test the loss and the isolation with a vector network analyzer or a NanoVNA, and measure the intermodulation if the switch is going to sit in a receiver path. A real PIN diode costs only a little more and has data you can design with, so it is the better choice for anything above a hobby project.

Power Handling and Safety

When the switch is off, the RF voltage across the diode can be high. At 100 watts into 50 ohms the peak voltage is about 140 volts, and with a mismatch it can be a good deal higher, so the reverse breakdown rating must exceed it with a margin. A common remedy is to use two diodes in series or to apply a reverse bias, which widens the depletion region and both lowers the capacitance and raises the voltage that the diode can take. With a forward-biased diode the dissipation is the square of the current times the resistance, which is small, but a too-small bias at high power can let the diode rectify, and the resulting heat can destroy it.

In a station with an amplifier, protect the receiver path against a failure. If the bias supply fails in a way that leaves the transmit path open and the receive path connected, the full power can reach the receiver input, and the usual design includes a fail-safe so that the receiver is shorted unless the control line positively selects receive. Check the design of any amplifier you buy or build for this point. In general, treat the changeover as part of the safety chain, and consult the guide to HF linear amplifiers before adding one to your station.

Relay or PIN Diode?

Neither is better in every case. A good relay has a very low loss, high isolation, handles high power and works at any frequency from DC to VHF, but it is slow, makes noise and has a limited life. A PIN diode is silent, very fast and lasts for decades, but has more loss at high power, needs a bias current and must be designed with some care. Many transceivers use PIN diodes for the receiver path and a relay for the high-power transmit path, which combines the speed where it is needed and the low loss where the power is high. Whatever you choose, the principle of the PIN diode is a good one to know, because it explains how a small current can steer a large radio signal.

The Bottom Line

A PIN diode is a diode with an intrinsic layer that behaves at RF as a resistance controlled by the DC bias current, from a fraction of an ohm when forward biased to a small capacitance when off. It is used in series and shunt switches for the transmit-receive changeover, band switching, antenna selection and variable attenuators for AGC. Bias it through a blocking capacitor and an RF choke, choose the current as low as the loss and distortion allow, protect the off-state breakdown with a margin, and consider a fail-safe for the receiver. A 1N4007 can replace a PIN diode in simple HF projects, but a purpose-made part is better when performance matters.

Frequently Asked Questions

What is a PIN diode used for in a ham radio?

Mostly for fast, silent switching: the transmit-receive changeover, band-filter switching, antenna selection and, with a variable bias, as an attenuator for AGC.

Why does a PIN diode not rectify the RF signal?

Carriers injected into the intrinsic layer stay there longer than one RF cycle, so the diode acts as a linear resistance whose value depends on the DC bias current.

How much bias current does a PIN diode switch need?

Typically 10 to 20 milliamperes for a low-loss receive path and more for transmit paths at higher power. The correct value is in the data sheet for the specific diode.

Can I use a 1N4007 as a PIN diode?

Yes, on the HF bands, where it is a common choice in QRP switches because of its long carrier lifetime. Its parameters are not guaranteed, and it is poorer at VHF.

Why does a PIN diode switch need a choke and capacitors?

The choke feeds the DC bias without letting RF into the control line, and the blocking capacitor keeps the bias out of the rest of the signal path.

Is a relay or a PIN diode better for QSK?

A PIN diode, because it switches in microseconds without noise. A mechanical relay cannot follow fast CW keying between the dits.