The term band is thrown around constantly in ham radio discussion and it is included in  many questions in US license exam pools.  So what exactly is a radio band?

Parts of the radio frequency spectrum allocated for a common purpose are called a band.  Besides amateur radio, the reader may be familiar with USA commercial broadcast ranges known as AM band and the FM band, or another personal communication chunk of frequencies called the citizens band (CB).

With amateur radio, bands are generally associated with a number (2m or 40m, for example).  This number is approximately equal to the wavelength of that span of frequencies We say approximately because a specific wavelength is only valid at one exact frequency and that may be outside the actual range of the band.

Shown below is a chart of the current US amateur bands (dated 2017): These 18 bands are are named by their approximate wavelength.  For those who care about such details, the named wavelength (λ) may not actually be inside the designated frequency range.  A table of these is given below: Some do, but why don’t all the meter bands line up with wavelengths inside the band?

Lots of discussion on this issue can be found on the internet and two particularly good links are given in the first two references below.  It’s a real combination of history, simplicity, misuse, and existing broadcast band names.

0.7m is the same as 700mm or 70cm; the 70cm name is more commonly used Continue reading

# Wavelength

We recently discussed radio frequency (RF) signals and radio waves.  Now let’s review the related concept of wavelength because it is often used in ham radio.

At any frequency it takes a certain amount of time for a wave to complete one cycle.  A cycle is any repeating feature of the waveform.  Radio waves have sinusoidal form. Because the wave moves over time, it travels a certain distance in any given period. Wavelength is the distance a wave travels in one complete cycle.  We measure this in meters. Viewed in 3D animation, it’s not only cool to look at, but may help us understand it a little better. The red and blue sine waves are the electric and magnetic fields oscillating at right angles to each other at the radio frequency.  The constant wavelength (λ) follows E field peaks between the X and Z axes. The radio wave is moving along the Y axis (lower right).

Radio waves are typically oscillating millions of times per second (MHz).  They are traveling near the speed of light (300 million meters per second). The time it takes for a radio wave to complete one cycle equals the speed of light (approximately) divided by the radio frequency: Simplifying the math shows us that to calculate wavelength, we simply divide 300 by the frequency in MHz.  The millions (Megas) cancel each other out.  The resultant wavelength is in meters. For the center of one popular HF band the wavelength would be:  300/14.2=21m  See how it works?

The wavelength at the center of our most common VHF radio band would be:  300/146=2.05m

Logically, higher frequencies complete one cycle in less time than lower frequencies. This means that the wavelength of higher frequencies is shorter than that of lower frequencies.  Frequency and wavelength are inversely proportional. Wavelength is simply an inverted way of thinking about radio frequency; they are mathematically related.

It helps to visualize the two overlaid on a RF spectrum chart: You can see how the yellow wavelength values above the blue frequencies increase in opposite directions.  Note also how the values line up in 1/10/100s and 3/30/300s per the speed of light relationship.

Wavelength becomes practical when dealing with antennas where element lengths need to be some fraction of a particular RF wavelength.

Wavelength is also the most common descriptor of radio frequency bands.  We will follow up with this in a future topic.

Wavelength is not terribly mystifying but it isn’t very obvious either.  Hopefully this gives you a better grasp of this important subject.

A fairly technical yet easily understood video relating frequency, wavelength and the speed of light is worth watching here.

# Understanding Antennas-A Simplified Perspective

A PowerPoint slideshow, Understanding Antennas / A Simplified Perspective for Ham Radio Operators is downloadable here:  Understanding Antennas-A Simplified Perspective

This presentation provides a working understanding of amateur radio antennas without being overly technical or dry.

The target audience is newer hams with limited knowledge of antennas.  It is presented at the Technician license level. You will see Continue reading