As a ham radio enthusiast, you’ve really enjoyed accessing AM radio. After enough experimentation, you’d like to expand your horizons to FM, but all you have are AM antennas. Will these suffice for FM?
You can’t use an AM antenna for FM. The two have separate modulation modes and exist on different frequencies. AM antennas increase the amplitude of a signal, something an FM signal doesn’t need. Using it would create a muddled, low-quality sound.
This guide will examine when you need an AM vs. FM antenna and recommend FM antennas to expand your ham radio hobby. Make sure you don’t miss it!
Can You Use an AM Antenna for FM? Why Not?
While technically, you could use an AM antenna for FM, it’s inadvisable.
Why is that?
AM and FM are too different. Let’s explore.
FM, which stands for frequency modulation, increases the speed of the frequency, not the amplitude.
FM operates on a high-frequency range of 88 to 108 MHz.
By comparison, AM is on a frequency band of 535 to 1,605 kHz.
The increase between AM frequencies is 10 kHz, whereas, with FM frequencies, it’s 200 kHz.
In addition to the other points we’ve touched on, the bandwidth differences between AM and FM signals are stark.
An FM station gets 150 kHz of bandwidth, which is 15 times greater than the bandwidth an AM station receives.
This is why the quality of FM radio outpaces AM significantly.
Another noteworthy difference between AM and FM is the broadcasting range.
AM has a huge broadcasting range due to its low frequencies, and it can broadcast even further at night.
This is the primary reason radio enthusiasts wish to use AM antennas for FM radio, hoping to expand the broadcasting distance of an FM station.
Wavelength size influences how well a radio wave can travel through solid objects.
The sizable wavelengths of AM frequencies enable the frequencies to pass through these objects, so trees, buildings, and hills won’t affect AM waves to the same degree as FM waves.
AM radio is prone to interference, much more so than FM. This is despite the radio wave’s ability to travel further.
What Happens If You Use an AM Antenna for FM?
Considering how different AM and FM radios are, what if you used an AM antenna for FM?
Well, it wouldn’t work the way you think it would. AM antennas are made for AM radio.
For instance, let’s look at an AM loop antenna. It features a copper coil or spiral surrounding a non-conductive, X-shaped frame built from wood. The loop shape could be square, diamond, or another geometric shape.
The copper windings go around the frame and may form a corkscrew (in the case of a spiral loop antenna).
An edge-wound loop antenna’s copper cable traces along the edge of the frame. The copper winding attaches to a farad capacitor.
AM loop antennas can be small or large, with smaller loops having a smaller surface area than larger ones.
You can even make a loop antenna at home if you have the right equipment (and are up for a challenge!).
However, whether using a loop antenna or another type for AM, it doesn’t have enough components to work for FM.
Even if it worked, you would have a messy sound and a lower frequency range than intended.
What Kind of Antennas Do You Need for FM?
FM antennas are designed for FM radio, so you won’t see the loop style. Instead, here is an overview of the types of FM antennas for indoor and outdoor use.
A half-wave vertical FM antenna features a vertical radiator near the antenna base.
The feedline and antenna require a matching device if you’re operating a coax with this antenna.
For example, a coli series suffices for matching.
The gain of a half-wave vertical antenna is relatively high, especially for indoor antennas, and is two times greater than a bidirectional dipole.
Speaking of, let’s now look at bidirectional bipoles.
Recommended for indoor urban environments where you want top-notch FM reception, this type of antenna has a single dipole, such as a ribbon dipole, or less crude designs.
Its gain is zero decibels when used for FM broadcast band antennas.
A multi-dimensional or multi-element array is an outdoor FM antenna, and yet another name is a unidirectional antenna.
It has a higher gain than the antennas we’ve looked at, which allows it to grab the station signals from far away.
Since a multi-element array has directional data, you should expect to move the antenna through an antenna rotator system when changing stations.
A multi-element array should go higher up in a rural or urban setting to increase reception.
Also known as a turnstile antenna, a crossed dipole is an outdoor antenna with dual dipole antennas.
They’re installed at approximately right angles and then receive phase quadrature feedings.
The primary benefit of a crossed dipole is that you don’t have to adjust the antenna.
However, the -3 decibel gain is relatively low, especially for outdoor FM antennas.
The last type of outdoor FM antenna is the unidirectional dipole.
Bigger than a crossed dipole, a unidirectional dipole has greater gains by focusing the signal unidirectionally (hence the name).
A unidirectional dipole works in a range of roughly 20 miles.
An AM antenna will not work for FM.
Instead, for best results, use an AM antenna for AM radio and an FM antenna for FM radio.
This will ensure the most authentic sound fidelity and the greatest traveling range that wavelength can achieve.