FM transmitter antenna. (2025 Update)

Jul 20, 2006 Last reply: 8 months ago 4 Replies

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Boki 20 years ago

Dear Experts,

My device, include a FM transmitter. ( 85MHz ~ 108MHz )

In my calculation:

The antenna length ( meter ) = ( 300 / frequency (MHz) ) * 0.25 * 0.96

so I get about 0.72m, is that right ?

The problem is that my PCB is about 5x5 cm2, many buttons around, and need to include Bluetooth anttena ( on board anttena ).

I wish the FM TX can be done by on board anttena also, but it seems not possible, someone suggest me to add external anttena into housing and then connect to PCB...

Please advice, thank you very much!

Best regards, Boki.

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Joe G (Home) 20 years ago

72cm is the full wavelength....

a classic 1/4 wave antenna with ground is very popular.

I think you need to do some home work on antenna theory - termination, impedance matching and propagation in free space.

try jfgi.us

Regards JG

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Meindert Sprang 20 years ago

No, it's 1/4...

Meindert

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mc 20 years ago

No, 72 cm is a quarter wave (1/4 wave) for the FM broadcast band. The full wavelength is about 3 m.

More generally: the way you get a small antenna is to use a loading coil (inductance in series with the antenna). If the antenna has to be tiny, it will have to be inefficient, but some are more inefficient than others.

The Radio Society of Great Britain's VHF/UHF Handbook is full of useful, practical ideas. There is a lot of ham radio work on 140-150 MHz using antennas 2/3 the size of what you will want.

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EE123 20 years ago

.72m (72 cm) is correct for a monopole. With a monoploe, you need a groundplane to provide the image current. This will work for a single ended drive scheme. Otherwise, you need to double the length to use a dipole, which is driven by a balanced driver (balun).

Good luck, Dave

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