Power Supply Sizing Calculator for a Ham Radio Station

Power supply sizing is arithmetic with one trap in it. The radio's transmit current is the number everyone looks up, and the number that actually causes faults is the voltage arriving at the radio under load, which depends as much on the cable between them as on the supply itself. Get both right and the station never surprises you.

Why 13.8 volts and not 12

Amateur equipment is designed around the voltage a running vehicle's electrical system actually produces, which is roughly 13.8 volts rather than the 12 volts a battery holds at rest. Every current figure quoted in a radio manual assumes that supply voltage, and every specification for transmit power assumes it too.

Radios typically tolerate about 13.8 volts plus or minus 15 percent, so roughly 11.7 to 15.9 volts. Below the bottom of that window they fold back power, produce distorted audio or reset entirely. Above the top they risk damage. That tolerance band is why sag matters: a radio that sees 11.5 volts on a transmit peak is outside its design window even though the supply is reading 13.8 with no load.

The current your station actually draws

Two things about that table. First, transmit current is peak rather than average, and a supply must handle the peak even though the average is far lower. Second, a 600 watt amplifier at 90 amps is a different class of problem entirely and normally comes with its own supply or runs from a dedicated one; do not try to run an amplifier and a transceiver from a single 30 amp supply.

Sizing rules that avoid every common fault

1. Add the radio's transmit current to the continuous draw of everything else. Only add a second radio's transmit current if the two can key simultaneously, which in a single-operator station they usually cannot.
2. Add 25 percent. A supply run continuously at its rating gets hot, ages faster and has no margin for the accessory you add next year. The difference in price between a 25 and a 30 amp supply is small.
3. Check whether the rating is continuous or intermittent. Cheap supplies advertise a peak figure and deliver considerably less continuously. A supply sold as "30 amp surge, 25 amp continuous" is a 25 amp supply.
4. Size the wire for voltage drop, not for heating. A gauge that will not melt can still drop enough voltage to fold back the radio.
5. Fuse both conductors at the supply and at the radio. The positive conductor always, and the negative too if there is any chance of an alternative ground path, which in a shack full of coax braid there always is.
6. Distribute through a fused block. A Powerwerx PD-5F fused distribution block$50.85 gives five individually fused outputs, so one accessory fault does not drop the whole station mid-contact.

Voltage drop, the fault that looks like a broken radio

A radio that resets on voice peaks, produces reports of distorted or clipped audio, or quietly delivers 60 watts when it should deliver 100, is nearly always suffering from voltage sag rather than a fault. The diagnosis takes one minute: put a voltmeter on the DC connector at the radio, key up into a dummy load at full power, and read it under load.

Aim to keep the total drop under about 0.5 volts at peak transmit. Note that the figures above are for the round trip, since current flows out along one conductor and back along the other, and using only the one-way length is the most common arithmetic error people make here. The full ampacity table by gauge is on the wire gauge reference.

Which supply to buy

For a 100 watt station, a 30 amp switching supply is the standard answer. A Powerwerx SS-30DV desktop power supply$164.17 arrives with Powerpole outputs already fitted, which saves an evening of crimping and means every DC connection in the station can use the same housing. A TekPower TP30SWII 30 A switching power supply$129.99 is a cheaper 30 amp switcher with analog meters and, more usefully, a noise-offset control that shifts the switching hash away from whatever frequency you are listening on.

For a station where receive noise is the priority, a linear supply such as the Astron RS-35M-AP linear power supply$551.00 is heavy, expensive and completely silent on the bands. That is the whole argument and it is a real one for a weak-signal or low-band operator. For everyone else a good switcher is the better buy, and the tradeoffs are laid out on linear versus switching power supply.

Standardise the connectors while you are at it. A pack of 30 A quick-disconnect power connectors, 40-piece$9.99 genderless 30 amp housings means a borrowed battery runs your radio and your radio runs from a borrowed supply without an adapter, which is the entire reason the amateur community converged on them. Full field on power supplies compared.

Switching supply noise, and what to do about it

A switching supply chops its input at tens to hundreds of kilohertz, and the harmonics of that switching frequency can land squarely in an amateur band as a strong birdie or a raised noise floor. It is not a fault; it is inherent to the topology, and quality of design determines how much escapes.

• Try the noise-offset control if the supply has one. Moving the switching frequency a few kilohertz often moves the birdie out of the segment you use.
• Choke the DC leads. Several turns of the DC cable through a FT-240-31 ferrite toroid core$11.67 at the supply end stops common-mode noise travelling down the wiring into the radio.
• Bond the supply chassis to the station ground bar with Tinned copper ground strap braid, 15 ft$22.99, using the same single-point ground as everything else rather than creating a second path.
• Move it away from the antenna feedline entry. A foot of separation between the supply and the coax entry panel is free and sometimes sufficient.
• Confirm it is the supply before rewiring anything. Run the station from a battery for five minutes. If the noise disappears, you have your culprit; if it does not, the problem is elsewhere and the noise hunting guide is the next stop.

Running from a battery instead

A deep-cycle battery kept on a float charger is the quietest power supply available and it keeps the station on the air when the mains fails, which is a genuine argument for anyone who got licensed for emergency preparedness. The tradeoffs are weight, the cost of a charger that suits the chemistry, and the fact that a battery must be fused at the battery itself rather than only at the radio.

For sizing, the same peak current figures apply but the question becomes how long it lasts rather than whether it can deliver, which is the battery runtime calculator. For safe handling, charging profiles and the cold weather rule that ruins lithium packs, read portable power and battery safety.