Choosing a Ham Radio Power Supply: Linear vs Switching
The Box Nobody Thinks About Until It Fails
Ask a group of hams what they spent the most time researching for their shack and antennas, transceivers and amplifiers will dominate the answers. The power supply is the afterthought: something that turns mains into direct current, sits under the desk and is bought on price. Then a transmitter sags on voice peaks, a digital mode overheats the supply, or a noisy switching unit paints birdies across the band the operator actually wanted to hear, and suddenly the neglected box is the most interesting component in the station.
This guide explains how to choose a supply for a typical HF transceiver, what really separates linear from switching designs, and how the wiring between supply and radio matters as much as the supply itself. It sticks to principles rather than model recommendations, because product lines change quickly and your radio’s manual is the authority on what it needs.
What Your Radio Actually Asks For
Most amateur transceivers are designed to run from a nominal 13.8 volts direct current, which approximates the voltage of a charged lead-acid battery under float charge. Your manual lists the accepted voltage range and the maximum current draw on transmit. Two details in that specification deserve attention.
Voltage Tolerance and Where It Is Measured
The manual describes the voltage the radio needs at its own power terminals, not at the supply output. Every connector, fuse and metre of cable between the two drops some voltage under load, so a supply that reads perfectly on its own meter can deliver noticeably less to the radio at the moment you key up. That is why the sensible way to test a setup is to measure voltage at the radio while transmitting into a dummy load, not to trust the supply’s front panel.
Transmit Current and Duty Cycle
The current figure in the manual is the peak requirement at full rated output. What matters for the supply is how long that peak lasts. Single sideband voice is intermittent: the average power is far below the peak. Digital modes, RTTY, FM and long CW dashes are close to continuous, and a supply that is comfortable on SSB can run hot in a long FT8 session or a contest run on RTTY. If you operate digital modes for hours, choose more headroom than a voice-only operator would. Our guide to a WSJT-X and FT8 setup that works the first time explains why those modes keep the transmitter keyed for long stretches, and that is exactly the load profile a supply must survive.
Linear Supplies: Heavy, Quiet and Inefficient
A traditional linear supply uses a mains transformer to step the voltage down, rectifies and filters the result, then uses a pass element and regulator to hold the output steady. The design is simple and has been trusted for decades. Its strengths are cleanliness and robustness: there is no high-frequency switching, so there is essentially nothing for a nearby receiver to hear, and a well-built unit tolerates abuse well.
The costs are weight, size and efficiency. The transformer is heavy, the regulator dissipates the difference between input and output voltage as heat, and a linear supply that delivers high current is a warm object that needs ventilation. For a fixed home station on a shelf, that is often acceptable. For a portable operator carrying gear up a hill, it is a bad deal.
Switching Supplies: Light, Efficient and Sometimes Noisy
A switching supply chops the rectified mains at a high frequency, uses a small transformer to step it down and regulates the output by controlling how the switching happens. The result is a unit that is a fraction of the weight of a linear supply, runs cooler and wastes less energy. Most modern amateur supplies are switching designs, and the technology has matured enough that many are well behaved.
The catch is that the switching process is itself a source of radio-frequency energy. That energy can travel down the mains cable, out the DC output leads and through the air. On a poorly filtered unit, it appears in a receiver as a comb of carriers at regular spacing or as a raised noise floor on particular bands. Whether it becomes a problem depends on the unit’s filtering, on cable routing and on how close your receive antenna is to the supply. Some switching supplies marketed for amateur use are designed specifically to keep emissions low; others, such as cheap generic units sold for other purposes, may not care about the HF spectrum at all.
How to Test a Switching Supply for Noise
The test is free. Tune the receiver to a quiet spot on each band you use with the antenna connected, switch the supply off and note the noise level, then switch it on and compare. Repeat with the radio powered from a battery if you have one. If the noise rises when the supply is on, you have a source worth chasing. Ferrite chokes on the DC and mains leads can help, and our article on common-mode chokes and stopping RFI at the source covers how to choose and place them. For a broader hunt through the house, see finding and killing RF noise in a modern home. The ARRL also maintains a guide to radio frequency interference that is worth bookmarking.
Comparing the Two Designs
| Factor | Linear supply | Switching supply |
|---|---|---|
| Weight and size | Heavy and bulky because of the mains transformer | Light and compact |
| Efficiency | Lower, with more waste heat | Higher, runs cooler |
| Receiver noise | Essentially none from switching | Varies widely; can produce birdies or a raised noise floor |
| Portability | Poor | Good |
| Robustness to abuse | Generally very tolerant | Depends on build quality and protection circuits |
| Typical price at a given current rating | Often higher for the same current | Often lower |
| Best suited to | Fixed shack with a sensitive receiver | Portable use, mobile shack, crowded desks |
The honest summary is that neither design is universally better. A quiet, well-filtered switching supply is a perfectly good choice; a cheap unmanaged one can spoil an otherwise good station. When you buy, look for evidence of amateur radio use and reviews that mention receive noise, not just specifications.
Sizing the Supply: How Much Current Headroom
Start with the transmit current in the radio’s manual. Then add the accessories that share the supply: an antenna tuner with its own power feed, a computer interface, a cooling fan, a second radio or a small amplifier driver. Choose a supply whose continuous rating comfortably exceeds the total. Headroom serves several purposes. A supply run well below its limit runs cooler and lasts longer. Voltage regulation holds better away from the current limit. And if you upgrade to a radio that draws more current, you will not have to buy again.
Be careful with the difference between continuous and peak ratings. Some datasheets quote a surge figure that the unit can supply only briefly. What you want is the number the supply can deliver indefinitely at a specified ambient temperature. If the specification does not distinguish, assume the lower figure.
Regulation, Ripple and Protection
Good regulation means the output voltage stays steady as the load changes, which keeps your transmitter’s output clean on voice peaks. Low ripple keeps hum off your transmitted audio and out of your receiver. Overvoltage protection, often called a crowbar, protects a radio from the consequences of a failed regulator. Not every budget supply includes it, and it is worth paying for because the alternative is an expensive repair. Current limiting and thermal shutdown are also valuable, provided they act gracefully rather than by causing sudden power cycling in the middle of a contact.
Wiring: Where Most Voltage Is Actually Lost
A good supply with poor wiring is a bad supply. Follow the manufacturer’s instructions for the DC cable supplied with the radio and keep it as short as practical. If you need to extend it, use heavy enough conductor for the current and length involved, and avoid thin extension leads, corroded terminals and barrel connectors that were never meant for high current. A short heavy cable is one of the cheapest reliability upgrades in the shack.
Fusing
Fuses protect the wire, not just the radio. Place a fuse in the positive lead close to the source of power, sized according to the radio manufacturer’s guidance and the wire’s capability. A fuse near the supply ensures that a short circuit anywhere along the cable is interrupted before the wire overheats. Carry spares of the correct type and rating, and never replace a blown fuse with a larger one to stop it blowing.
Connectors and Distribution
Many operators standardise on a modular two-pole connector often called Powerpole, because it is polarised, genderless, easy to crimp and widely used in emergency communications. Whatever you choose, use one convention everywhere so that a radio, a battery and a supply can be swapped without thinking, and be strict about polarity. A power distribution block with individual fusing turns a rat’s nest of leads into a tidy, safer arrangement and lets you add accessories without stacking wires on a single terminal.
Grounding and Safety
The supply’s mains safety earth is not optional decoration. Treat it as part of the station’s safety system, and read the ARRL’s page on electrical safety before doing any work inside a mains-powered unit. The wider question of bonding, RF grounds and surge protection is covered in our guide to station grounding and lightning protection.
Mobile, Portable and Backup Considerations
If your supply also serves as the charger for a backup battery, check that it is designed for that job. Some units can charge a battery directly, others cannot manage the inrush current or lack protection against back-feed when mains fails. A dedicated charge controller or an isolating arrangement is usually the safe approach. For field work, a supply is only part of the story; our article on battery and power options for portable operating covers the alternative of running straight from batteries and the trade-offs of each chemistry.
A Practical Buying Checklist
- Read the radio manual for voltage range, maximum transmit current and any note on supply type.
- Add accessory current, then choose a continuous rating that leaves comfortable headroom.
- Prefer designs with overvoltage protection, current limiting and good regulation.
- For a switching supply, look for amateur radio user reports on receive noise and be ready to add ferrites.
- Plan the wiring: short, heavy DC leads, a fuse at the source, consistent connectors.
- Test at the radio: measure voltage under transmit into a dummy load and compare noise with the supply on and off.
Frequently Asked Questions
What voltage does a ham radio power supply need to deliver?
Most HF transceivers are designed around a nominal 13.8 volts DC, but the exact accepted range and current requirement are in your radio’s manual. Always check that figure rather than assuming.
Are switching power supplies bad for ham radio?
Not inherently. Well-filtered switching supplies are widely used. Poorly filtered ones can radiate noise that shows up as birdies or a raised noise floor, so test for noise and add ferrite chokes if needed.
How much current headroom should I choose?
Add the radio’s transmit current to any accessories sharing the supply and choose a unit whose continuous rating comfortably exceeds that total. Digital-mode operators, whose transmissions are close to continuous, should allow more margin than SSB-only operators.
Where should I put the fuse?
In the positive lead as close to the power source as practical, sized according to the radio manufacturer’s guidance and the wire’s capability, so that a short anywhere along the cable is interrupted.
Can I run my radio directly from a battery instead?
Yes, provided the battery voltage stays within the radio’s accepted range under load and the wiring is fused. Many portable operators do exactly this.
Why does my radio show lower voltage when I transmit?
Cable, connector and fuse resistance cause voltage drop under load. Measure voltage at the radio during transmit and shorten or thicken the DC run if it sags too much.
The Bottom Line
A power supply should be chosen for the load it will actually carry, the noise it will or will not add to your receiver and the wiring that connects it to the radio. Linear supplies are heavy and quiet; switching supplies are light and efficient but vary in cleanliness. Size for continuous current with headroom, fuse at the source, keep the DC run short and heavy, and verify with a voltage measurement at the radio during transmit. Do those things and the humble box under the desk will disappear from your thoughts, which is exactly what a good power supply should do. More shack-building guides live on the LY4A blog.