Site Owner & Radio Enthusiast
What would you say if you had to guess whether AM or FM signals could travel farther?
Although you might assume FM signals are more suited for long distances due to their prevalence, it’s actually AM. Why can AM signals travel far?
AM signals operate on a lower frequency, increasing the size of their wavelengths. Larger wavelengths can permeate solid objects more efficiently, allowing them to cross longer distances. The traveling distance of AM signals grows even more at night.
This article will answer all your questions on AM signal travel, so don’t miss it!
Why Do AM Signals Travel Far?
As we established in the intro, AM radio waves can travel further than FM waves.
Now let’s explore the reasons why in more depth.
AM radio operates on a frequency range of 540 to 1700 kHz. FM radio uses frequencies between 88 and 108 MHz (at least in the United States; it can vary in other parts of the world).
As you can see, AM radio waves occur at a lower frequency between the two.
Low frequencies have bigger wavelengths, and high frequencies have shorter wavelengths.
That signal length is critical, allowing AM radio waves to pass through solid objects such as rocks, trees, hills, and buildings.
FM waves get stuck on these obstacles. The shorter wavelength prevents the radio wave from permeating through.
However, an FM radio wave’s ability to travel can be lengthened depending on where it’s broadcast from.
For example, the radio wave can travel further than usual in a wide open area because it’s unobstructed.
It will not outpace the distance an AM wave can travel.
How Far Can an AM Signal Reach?
Radio broadcasters and listeners must consider the distance AM and FM wavelengths can travel.
By day, an AM signal has an average broadcasting scope of 100 miles. By night, you can add on hundreds of miles more, says the FCC.
For comparison’s sake, an FM signal generally travels between 30 and 40 miles.
Do AM Signals Travel Farther by Day or By Night?
You’ll recall we mentioned a difference between an AM signal’s ability to travel at night versus by day. That’s no coincidence.
We just wrote a whole post dedicated to AM signal travel by night, so we’ll provide a brief recap here.
A radio wave’s ability to travel is affected by the ionosphere.
The ionosphere has molecules seeking bonds across several layers. The molecules will attach faster on some ionospheric levels than others.
During the day, the molecules remain unbonded more than at night. These unbonded molecules across the ionosphere operate the same as any obstacle.
An AM radio wave can pass through the obstacle, but it limits the extent of the signal.
This is why AM radio achieves an average broadcasting distance of 100 miles during the day.
The sun ionizes the molecules, so once it sets, they behave differently. The molecules will pair up at night, reducing their surface area and allowing AM waves to travel hundreds of miles.
The propensity for travel at that point is significant enough that the FCC requires some AM stations to reduce power or stop broadcasting at night if they don’t have safety measures.
Have you ever heard of such a measure required for FM radio? No!
You can usually tune into FM anytime, 24/7, because the signal distance doesn’t increase at night like an AM signal does.
Can You Extend the Distance of an AM Signal?
What if you want to extend AM signal range even more? You can try a few techniques, so let’s review them.
AM radio is even more prone to interference than FM, even if the former radio waves can pass through solid obstacles better.
As we discussed earlier, broadcasting in an open area is best.
The site should have few, if any, trees, hills, rocks, power lines, and buildings (well, besides the one you’re broadcasting from, if you’re indeed broadcasting indoors!).
Many everyday household objects can interfere with AM signals, dampening reception, as we wrote about here.
Be wary of anything electronic, from bug zappers to lights (even holiday lights), electric blankets, smartphone chargers, microwaves, computers and laptops, monitors, and televisions.
Consider powering off these devices when broadcasting or using AM radio.
If you’re listening to AM on a portable radio, you can also move the radio further from the source of these objects if you can’t unplug them for any reason.
Use at Night
AM radio waves travel much further at night.
Stations that you might have only barely been able to hear during the day because the interference was so severe can become clear and listenable once the sun sets.
However, we must stress that this only applies to some AM stations. Some will reduce their power load at night per FCC requirements.
When broadcasting at lower power, the distance the radio signals can travel might lessen.
Others have to stop broadcasting altogether!
Try an Antenna Booster
Positioning a portable radio antenna can improve the signal, but an antenna booster takes things one step further.
It amplifies the signal, enabling it to reach further distances.
You can always create an antenna booster or amplifier yourself or purchase one.
Here’s how an antenna booster works.
Most include an RF amplifier stage that can elevate the signal.
However, noise is equally amplified, so whether it’s a subtle hissing tone or a loud popping or humming, those sounds will come through loud and clear.
Make sure you know what’s causing interference around the AM radio signal, and do your best to minimize those sources of interference. This will spare everyone’s ears when they tune in.
Use a Transmitter
A radio transmitter is the last option to extend an AM signal’s length.
FM transmitters convert FM into AM signals, then transmit them over further distances, but you can also purchase an AM transmitter.
Transmitters can be high-level or low-level, the former of which uses high-level modulation.
That’s the standard choice when broadcasting on AM airwaves, especially if your transmitting power is mere kilowatts.
However, suppose you’re broadcasting but already have more transmitting power to your setup.
In that case, there’s no need to harness the power of high-level modulation. You can always use a low-power transmitter instead.
A radio transmitter includes a carrier oscillator, which produces a high-frequency carrier signal.
The carrier oscillator uses a sub-multiple frequency to keep the carrier frequency stable. This multiplies with a frequency multiplier to provide the required carrier frequency.
The transmitter also uses a buffer amplifier to isolate the frequency multiplier and carrier oscillator.
The amplifier sets the frequency multiplier’s input impedance and the carrier oscillator’s output impedance as the same.
Why do this? It prevents the multiplier from generating too large of a current.
There’s also a power amplifier. As the name suggests, this increases the carrier signal’s power.
AM signals can travel farther than FM signals due to the size of the wavelength.
The wavelengths are larger and thus have no issue passing through obstacles such as trees, buildings, and hills.
The traveling distance of AM signals amplifies even more at night.
Whereas the radio signal only travels about 100 miles during the day (which is still over 50 miles more than an FM radio wave’s average traveling distance), it becomes hundreds of miles more after dark due to sky wave propagation.
While you can use handy tools like a radio transmitter or antenna booster to lengthen an AM signal, remember that noise is often amplified with the signal.
Reducing noise as much as possible will enhance the clarity of the signal.