Some AM stations don't just turn down after sunset — they collapse to a tiny fraction of their daytime power. A station running thousands of watts by day might be authorized for 28 watts, or 2 watts, or a single watt at night. Operators call it “flea power.” Here's why it happens, and how a transmitter actually produces a power so small.
Noncommercial hobby reference. Corrections welcome — chris@n6jet.com.
By day, an AM signal travels by groundwave — it hugs the earth and serves a local area. After sunset the ionosphere changes, and AM signals begin to reflect back to earth as skywave, carrying hundreds or even thousands of miles. That's wonderful for distant listening and a problem for the band: suddenly a small-town station can splash all over a frequency that belongs to someone else.
On a clear channel, one or two dominant Class A stations (50,000 watts) have their nighttime skywave service federally protected. Any lesser station sharing that frequency has to stay out of the way after dark. It can do that one of two ways: aim a directional array that puts a null toward the protected station, or go non-directional and simply cut power until its signal is too weak to reach. Non-directionally, that “too weak to reach” floor can work out to just a handful of watts. A station whose night power is under 250 watts (and field below 107.5 mV/m at 1 km) is licensed Class D — full power by day, flea power by night.
The same station, day versus night. The night figure is the most power it can radiate without interfering with the clear-channel station it hides under.
| Call | kHz | City, State | Day | Night |
|---|---|---|---|---|
| KYET | 1170 | Golden Valley, AZ | 6,000 W | 1 W |
| KSCO | 1080 | Santa Cruz, CA | 10,000 W | 28 W |
| WFED (former 1050 facility) | 1050 | Silver Spring, MD | 3,500 W | 44 W |
Night powers like these run from a single watt up into the tens or low hundreds. On 1080, KSCO has to protect Class A KRLD in Dallas; non-directionally, the most it can radiate and still protect Dallas is 28 watts. KYET, a daytime 6 kW station, drops all the way to one watt. The instrument that tells the story is the antenna-base ammeter: a station reading 5 amps by day may read under 200 milliamps at night.
You don't ask the transmitter to generate one watt — nothing in a transmitter rack makes a clean, fully-modulated one watt on its own. Instead you do it in two steps:
1. Set the transmitter to its lowest stable power preset. Modern solid-state rigs have a floor below which they can't regulate or modulate cleanly. A Nautel J1000, for example — a 1,000-watt AM transmitter — can run cleanly all the way down to 10 watts, but no lower.
2. Pad the rest of the way down. Put a resistive attenuator (a pad) in the line between the transmitter and the antenna tuner. To go from 10 watts to 1 watt — a factor of ten — you use a 10 dB pad, which passes one watt to the tower and dumps the other nine into its load resistors.
The pad is a passive, linear network, so it passes the audio modulation through untouched. Two design rules and you're done:
Starting from a low transmitter preset (10 W) instead of full power means the pad only has to dissipate a few watts, so it's a small, cool, simple network rather than a big load bank.
Before purpose-built low-power transmitters, engineers got their flea power by running a bigger transmitter at its lowest setting and throwing most of the output away — into a dummy load, or famously into a bank of incandescent light bulbs, switched on one at a time as more power needed dumping. It worked, but a bulb's resistance changes as it heats, so the impedance match (and the power level) would drift. Engineers prefer fixed resistors for exactly that reason. Either way the principle is identical to the modern pad: make a stable, well-modulated signal, then waste almost all of it and radiate a sliver.
It's tempting to think you could just turn the transmitter down to one watt and skip the pad. You can't — below its floor the power-control loop loses regulation, the modulation stops tracking, and the signal can't hold its spectral mask. There's a real FCC case that makes the point. Station WGRP (940 kHz) was licensed for 2.2 watts at night and tried to make it with a J1000. Nautel told the Commission the transmitter is only certified down to 10 watts and can't produce 2 watts without audio distortion. Rather than pad down to 2.2 watts, the station let the transmitter run where it physically landed — about 27 watts — and programmed it to switch to that unauthorized level automatically. The FCC found the violation willful. The lesson is exactly the rule above: if your licensed power is below the transmitter's floor, you reach it with a pad, not by abusing the transmitter.
In 2026 the watt is often almost ceremonial. Most of these low-power night AM stations now simulcast on an FM translator, so after dark the audience is really listening on FM and the AM's one or two watts mainly serve to keep the license alive. The flea-power signal still goes out every night — a transmitter quietly producing ten watts into a resistor that gets a little warm, with a single clean watt trickling out to the tower — but the people are on FM. It's a fitting end for one of the stranger corners of broadcast engineering: a station that is a local lion at noon and a nightlight by midnight.
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