How to calculate the AM Antenna Length | applied electronics engineering

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How to calculate the AM Antenna Length

By Applied Electronics - Tuesday, February 28, 2017 No Comments
AM radio are the first and simplest radio. Today they are still used and build by HAM radio amateur fellows. One of the simplest kind is using a single transistor which was shared in How to make a simple one transistor radio at home? blog post. One of the main component there is the antenna. Here we talk about how to calculate the AM antenna length for that circuit. The circuit diagram is re-shown here for convenience.

How to calculate the AM Antenna Length


The formula for calculating the AM antenna length is as follows,

L = 300/ f

where f is the desired AM frequency in MHz and L is the length of the antenna in meters.

This formula is due to the famous relation,

speed of light = wavelength * frequency

Now in the case of AM radio, the frequency lies in between 530KHz to 1600KHz. Let the AM wave frequency be 800KHz then the AM antenna length is calculated as follows,

L = 300/0.8
or,
L =375 meters

This is full length of the antenna

But you can also tune to harmonics of the AM wave. For example you can tune to AM signal harmonics by designing a halfwave length antenna. The length of the antenna then becomes,

Lhw = L/2
or,
Lhw = 187.5 meters

And you can also design AM antenna with quater wavelength as follows,

Lqw = L/4
or,
Lqw = 375/4
or,
93.75 meters

The advantage of using half wavelength or quater wavelength antenna is that the length of the wire used to construct the antenna is reduced.

See also How AM modulation circuit works.

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