RF Burns in the Ham Shack: Causes, Prevention and First Aid

Most radio amateurs worry about lightning and mains voltage, but a quieter hazard sits in the shack: the RF burn. It happens when a person touches a metal object that carries radio-frequency voltage while the transmitter is on. The sensation may be a warm tingle, a sharp pinprick or nothing at all, and the damage can be deeper than the skin suggests. It is rare, preventable and badly understood, so many hams only learn about it when someone is hurt. This guide explains how an RF burn happens, where it is most likely to occur, how to find and remove hot spots in your station, and what to do if it happens to you or to a visitor.

What an RF Burn Is

The topic is also covered by the general article on electrical injury.

A normal thermal burn comes from contact with a hot surface, and it starts at the skin. An RF burn is different: radio-frequency current flows through the body, and the tissue heats up from inside. Because the current prefers paths with low resistance, the heating is strongest where the current is concentrated, for instance at the fingertip that touches a wire or at the spot where a body part touches the metal. At HF frequencies the current does not flow only on the skin, as it does at much higher frequencies, but penetrates into the tissue, so the damage can be deeper than the small mark on the surface suggests. The medical background on burns is given in the article on burns.

The amount of heating depends on the power, the frequency, the contact area and the duration. A high-power transmitter on 160 or 80 metres into an antenna with a very high voltage at the end of the wire can deliver a painful burn in a second or two. At low power the effect may be limited to a tingle or a warm feeling, which is a warning sign that something in the station is wrong.

Where the Risk Is Highest

The places where an RF burn is most likely are the places where RF voltage is highest and where people touch. The end of a wire antenna, particularly an end-fed half-wave, has a very high voltage and a high impedance, so touching it while transmitting is dangerous. A tuner that sits in the open, with exposed terminals, is another case. The inside of an amplifier is a place where the voltages can reach thousands of volts, and the same holds for the output of a high-power tube stage. The antenna feed point of a short vertical, which is often close to the operating position, can also carry a large voltage if the tuning is poor.

The second class of risk arises from unwanted current on the outside of the coaxial cable, called common mode current. If the antenna system is unbalanced, or the ground is poor, part of the transmitter power returns by the outside of the shield and through the equipment, and it may be present on the metal case of the radio, on the microphone or on the key. A person who touches the chassis and another grounded object, such as a radiator or the water pipe, can have a current flowing through the body. This is the typical situation of “hot” equipment, in which the lips tingle on the microphone or a finger on the key feels warm. Our article on common mode chokes explains how this current arises and how it can be reduced.

Symptoms and Likely Causes

The following table links the symptoms you may notice with the probable causes and the first thing to try. Treat each of them as a sign of a problem to be fixed, not as something that must be tolerated.

Preventing RF Burns in the Shack

Start with the basic rule: never touch an antenna, a tuner terminal or a feed line connection while you transmit, and do not allow visitors, children or pets to do so. This sounds obvious, but many burns happen during a quick adjustment of a connector in the middle of a test. Switch off the transmitter, or at least reduce the power to zero, before you touch anything on the antenna side.

Next, reduce the common mode current. Place chokes on the coax at the radio and, if the antenna is unbalanced, at the feed point. A choke made from a number of ferrite cores on the coax works well, and the mix of the ferrite should suit the band, as described in our ferrite mix guide. Add smaller chokes to the microphone, key and USB cables if you have hot spots there. Make sure that the antenna is balanced as far as possible: a dipole with a good balun or a vertical with a proper set of radials will drive less current onto the outside of the coax. An end-fed antenna needs a counterpoise, otherwise the coax shield becomes the counterpoise, and the radio becomes hot. See the guide to the end-fed half-wave antenna for how it is done.

Third, take care of the ground. A proper RF ground and bonding of the equipment cases through short, thick conductors keeps all the metal parts of the station at the same RF potential, so that no voltage appears between them when you touch two of them. The details of a safe ground system are in our article on station grounding and lightning protection. A ground wire that is a quarter wavelength long on a given band acts as an open circuit at its end and is no ground at all on that band, which is a known cause of hot equipment on the higher HF bands.

Outdoor Antennas and the Public

Outdoors, the main rule is to keep the radiating parts out of reach. A wire antenna should hang high enough for people to walk below it without touching it, and the end of an end-fed wire should be insulated and kept far from places where children play. A vertical antenna, which has the highest voltage at the top and a high current at the base, should be surrounded by a fence or placed in a part of the garden that is not used. Metal fences, gutters, clotheslines and railings close to an antenna can pick up RF voltage by induction and cause a burn when touched, so keep the antenna away from them or ground them with a connection that does not make a loop. At portable events, such as a field day, mark the antenna area with tape and brief the visitors. Our article on RF exposure safety covers the related question of general exposure limits, which is a different issue from contact burns.

Finding a Hot Spot

To locate a hot spot, use a method that does not involve your body. A neon bulb or a small fluorescent tube held near a suspect part will glow when it is in a strong RF field, and an RF probe or a field meter gives a number. Always keep the power low during the search, and if you suspect a high voltage, use an insulated tool. Check each cable, case and metal part in turn, switching the power off before you touch. Note the band: hot spots depend on the frequency, so an equipment case that is cold on 20 metres may be hot on 80. When you find a place, apply the cure and repeat the test to see that the problem is gone.

Special Situations: Tuners, Amplifiers and Portable Stations

Some situations deserve extra care. An antenna tuner that is mounted outdoors at the feed point has high RF voltages on its terminals, and the cover should always be closed when transmitting. An internal tuner is safe as long as the radio is closed, but a manual tuner on the desk, with exposed rear terminals, can be touched by accident while you reach behind it. Cover the terminals or move the tuner so that the rear cannot be reached. Amplifiers are protected by interlocks, but the lid must never be removed while the unit is connected, because the voltages inside are also dangerous in the ordinary sense. Do not bypass an interlock.

At a portable site, the typical problem is a short vertical or a loaded whip set up near the operating tent. These antennas are simple, but their voltage at the base or the loading coil can be high, and a visitor who leans on the antenna may be burned. Place them away from the walkway, and set the radials on the ground where people do not walk. When you operate from a vehicle, the metal body acts as part of the antenna system, and touching a door handle while transmitting on a low band can give a mild shock. A quick test at low power will tell you whether this happens with your set-up. On a boat, the stays and rails are often part of the radiating system, and the same rule holds: no contact during transmission.

Finally, tell everyone who visits your station. A short instruction before a demonstration, such as do not touch the antenna or the cables and keep clear of the wire, avoids most incidents. It also helps to put a warning sign on the transmit antenna and on the tuner. Such small habits cost nothing and make the station safer for the family and for the neighbours.

What to Do If It Happens

If you feel pain or a burn, stop transmitting at once and move away from the source. Do not pull away violently from a conductor in a way that could injure you or knock over equipment; the first step is to switch off the transmitter. Look at the area. An RF burn may appear as a small white or dark spot, but the tissue under it can be damaged more deeply than it looks, and the damage may show only after a day or two. Cool the area with cool, not ice-cold, running water for several minutes and cover it with a clean, non-sticky dressing. See a doctor, and tell the doctor that it was a radio-frequency burn, because it is treated more carefully than a normal burn of the same size. Do not apply creams or oils before the examination.

After the incident, find the cause before you transmit again. Check the antenna, the tuner, the ground and the chokes, then correct the fault. If a visitor or a neighbour was involved, a short note of what happened will help you to keep the station safe in the future.

The Bottom Line

An RF burn is caused by radio-frequency current flowing through the body when you touch a metal part with RF voltage on it, and it can be deeper than it looks. The usual causes are a touch on an antenna or tuner during transmission, and common mode current that makes the radio case, the microphone or the key hot. Prevent it by keeping antennas out of reach, never touching connectors while transmitting, adding chokes, balancing the antenna and building a good ground and bonding system. If a burn happens, stop transmitting, cool the area, get medical advice and find the cause before the next transmission.

Frequently Asked Questions

What is an RF burn?

It is a burn caused by radio-frequency current flowing through the body, which heats the tissue from the inside. It happens when you touch a metal object carrying RF voltage during transmission.

Why does my microphone tingle when I transmit?

Probably because common mode current flows on the coax shield and the radio case. Add chokes on the cables and check the ground and the balance of the antenna.

Is an RF burn serious?

It can be. The surface mark may be small while deeper tissue is damaged, so any burn should be examined by a doctor.

Which antennas are most dangerous to touch?

End-fed wires, open tuner terminals and the tops of verticals, because the voltage there is high. Never touch any antenna while the transmitter is on.

Can low power cause an RF burn?

Seldom, but a warm or tingling feeling at low power is a warning. At 100 watts or more the risk increases clearly, especially on the lower HF bands.

How do I find the source of a hot chassis?

Check the ground and bonding, the balance of the antenna and the chokes on the coax. Test on each band, since the problem often depends on frequency.