A power supply is the least glamorous part of a station, yet it decides whether your radio delivers its rated output, whether your receiver stays quiet, and whether everything stays cool. Buying one by guesswork is how many operators end up with a supply that sags on transmit or hisses into the receiver. Sizing one is simple arithmetic plus a few good habits. This guide shows you how, using numbers you can check against your own radio's manual.
Why 13.8 Volts, and Why Size Matters
Most transceivers and mobile radios are designed around a nominal 13.8 volts DC. That is roughly what a vehicle charging system delivers, and it suits a 12 V battery. Manuals allow some tolerance around that figure, so check yours for the exact range.
The supply must hold that voltage steady while the radio pulls a large, sudden current each time you key up. If the supply is too small, the voltage sags. The result can be reduced output power, distorted audio, or a radio that shuts down mid-transmission.
Start With the Radio's Transmit Current
Open the manual to the specifications page and find the current drain at maximum transmit power. As a rough guide, a typical 100 W HF transceiver draws somewhere around 20 to 25 A at full output, while receive current is usually only a few amps or less. Radios in the 200 W class need roughly double the transmit current. Those figures are approximate, so your manual is the authority.
The transmit figure is the one that matters, because it is the peak your supply has to handle. Never size a supply from the receive current.
Add Everything Else on the Same Supply
List each device that will draw from the supply at the same time as the radio transmits. Common candidates include:
- An external automatic antenna tuner, which usually draws modest current but more while tuning
- A cooling fan or a small 12 V accessory
- Lighting, a wattmeter or a switch box powered from the DC line
- A second radio, only if it could realistically transmit while the first one does
Many USB interfaces draw from the computer rather than the supply, so they usually do not count. Check each device's label or manual for its current rating.
Think About Mode and Duty Cycle
Peak current is not the same as average current. On SSB, speech only demands full power on voice peaks, so the average draw is well below the maximum. CW keys the transmitter on and off.
FM, RTTY and many digital modes such as FT8 hold the transmitter on for many seconds at a stretch, so the radio and its supply work close to full load for much longer. This is why operators commonly turn power well down on digital modes, often to around half of the radio's rating or lower. Check the manual for the radio's recommended duty cycle.
Whatever the mode, transmit only within your licence privileges and power limits. These differ between the USA and Canada, and you need the right licence for the band and mode. If you are still studying, the licence exam practice covers both countries. A look at the live HF propagation page before you call CQ can also help you avoid running more power than you need.
Add Headroom
Take your total transmit current and multiply it by about 1.2, which gives roughly 20 percent headroom. Then round up to the next available supply size. A supply that runs at 70 to 80 percent of its continuous rating stays cooler, tends to have less ripple and should last longer. If you might move to a higher-power radio later, size for that now. A good supply outlives several radios.
A Worked Example
These numbers are illustrative only. Substitute the figures from your own manuals.
- Radio at 100 W output: about 22 A at full power, per the manual
- Tuner and fan: about 1 A combined
- Total transmit load: about 23 A
- With 20 percent headroom: 23 x 1.2 = 27.6 A
- Next common size up: a supply rated 30 A continuous
If that same station would only ever run 50 W on digital modes, the real load is far lower. You would still buy for the radio's full rating, because you will not always operate the same way.
Continuous Rating vs Peak Rating
Read supply specifications carefully. Many list a higher peak or surge figure alongside a lower continuous one. Size against the continuous rating, since that is what the supply can deliver hour after hour without overheating. Also check whether the rating applies at 13.8 V and at your mains voltage. Treat vague labelling as a reason to look elsewhere.
Linear or Switching: Think About Noise
Linear supplies are heavy and less efficient, but they are typically very quiet electrically. Switching supplies are light, cool and efficient, but a poorly filtered one can radiate broadband hash that raises your HF noise floor or produces steady tones across the band. Many well-made switching supplies are quiet, especially models designed with amateur radio use in mind.
You can test a supply yourself. Tune the receiver to a quiet spot on a band you use, note the noise level, and compare it with the radio running from a battery if you have one. Try moving the supply and its cables away from the receiving antenna's feedline. Snap-on ferrite chokes on the DC and mains leads often help, and keeping the mains cable and coax apart is a good habit.
Wiring, Fusing and Voltage Drop
At 20 A, even a small amount of resistance costs you voltage. A total resistance of 0.05 ohm, for instance, drops a full volt at 20 A. Thin, long or loose connections are the usual culprits, so:
- Keep DC leads short and use the cable supplied with the radio, or heavier
- Do not extend the radio's cable with lighter wire
- Keep the fuses in the radio's power lead, as the manufacturer intended
- Use a fused DC distribution block if several devices share the supply
- Check that the red lead goes to positive and the black to negative before powering up
If your radio shows supply voltage, watch it while transmitting. A big drop between receive and transmit points to a supply or wiring problem.
Safety matters too. Never open a mains-powered supply, because it can hold dangerous voltage even when unplugged. Give it room for ventilation, and keep it dry.
Common Mistakes
- Sizing from receive current or from the radio's output power in watts instead of its transmit current in amps
- Forgetting accessories that run at the same time
- Running a supply at 100 percent of its continuous rating with no headroom
- Trusting a peak rating as if it were continuous
- Using thin, long DC cables, or extending the radio's own cable
- Skipping fuses, or replacing a fuse with a higher-rated one
- Placing a switching supply right beside the receive antenna or feedline
A Pre-Purchase Checklist
- I have the radio's maximum transmit current from its manual
- I have added the accessories that run at the same time
- I have applied about 20 percent headroom
- The supply's continuous rating meets that number at 13.8 V
- I have checked its noise reputation, or can test it against my receiver
- I have the right cable gauge and fusing ready
If you plan backup power, our guides on LiFePO4 vs AGM batteries and a 72-hour emergency power plan pick up where this article leaves off.
Wrap-Up
Choosing a power supply comes down to four things: the radio's transmit current, the accessories that share the supply, sensible headroom and a quiet design. If the station will ever run unattended, for example through Remote Rig, a properly fused, comfortably sized supply matters even more. Once it is running, note your power settings in the free online logbook so you can see how the station performs over time.
How HamRadioList can help
Once your supply is sorted, keep track of the power you run and how it performs with the free HamRadioList logbook. If you plan to operate an unattended radio, look at Remote Rig and Station Bridge, where a reliable, well-sized supply matters even more.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Remote Rig - operate your own radio from anywhere on Android, iPhone, Windows or Linux, with live push-to-talk audio and CAT control.
- Live HF propagation - see current band conditions before you call CQ.
- License exam practice - practice questions for the US and Canadian amateur exams.
- Station Bridge - the software that connects your radio, audio interface and computer to Remote Rig.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.