Radio Waves & Wavelength

 

The most important aspect to understand when attempting to intercept MH/HF (shortwave) radio communications is how radio waves actually work. Learning how and why you can receive a transmission from the other side of the world on HF while you cannot receive the next town using VHF is the main difference between HF and V/UHF communications, and both government and commercial users exploit that fact every day.

Radio Waves

For this tutorial I assume that you are are of what a frequency and a wavelength are, but will recap:

1. A frequency is a count of how many times an electrical AC signal (radio wave) cycles from 0V to + max, to - max, then to 0V again per second. 1 Hz is 1 cycle per second (or the base unit of frequency).

2. A wavelength is simply the physical (and electromagnetic) length of each particular electrical cycle.

3. Radio waves travel at the speed of light (c) - Einstein showed us nothing can go faster than c (without time travel)

Frequency / Wavelength Relationship

There is a mathematical relationship between the two which is very important when it comes to radio communications.

λ=ѵ/ƒ

Where:
(λ) = Wavelength (m), 
(ƒ) = Frequency (Hz),
(ѵ) = Velocity - in this case the speed of light - c (in Km/s)

Knowing that c is constant (300,000 Km/S)  - this shows Wavelength (λ) reduces as Frequency (ƒ) goes higher.

E.G.1.  Find wavelength for 300 MHz signal

Wavelength (λ) = Speed Light (C)/Frequency (ƒ)

λ = 300,000,000 / 300,000,000
=1 metre

Showing the wavelength at frequency 300 MHz, is 1 metre.

 

E.G. 2. Find wavelength for frequency 477 MHz signal

λ = 300,000,000 / 477,000,000

= 63 cm

 

E.G.3  Find wavelength for 3.5 MHz.

λ = 300,000,000 / 3,500,000

λ = 85 metres

So this shows clearly that the higher the frequency is, the smaller the wavelength is.
Conversely the lower the frequency is, the larger the wavelength is.

Since Shortwave Listening takes place between 3 - 30 MHz we must consider that the wavelengths are huge, compared to V/UHF scanner bands (2m - 30cm).

So, as we show later, in the antennas section, short wave listening can require antennas of up to 85m to be resonant at that band.. In this example your antenna must be large enough to contain the massive 85m long signals.

 

Most radio networks frequencies are named by their wavelength or band - E.g. 160m, 70m, 10m, 2m, 70cm, etc.

Now you can calculate approximate wavelengths and associate them with frequencies. If you want to receive a frequency of a particular wavelength you must consider that your antenna should be resonant to it's wavelength.