Hi Guys , I read about a circuit to build a low-power FM transmitter(powered by 2 AA size batteries). For the antenna a 20-30 cm. long wire was suggested.
Could you tell me what kind of wire should be used and how i am supposed to orient it to recieve the RF signals on a portable FM radio held a few metres away(on its line-of-sight)
Thank you.
Posts: 35 | Location: New Delhi,India | Registered: 07-16-02
I imagine that 18-gauge wire should work just fine. Here are some links to antennas that others have constructed for FM broadcast use ! You might also consider salvaging the existing telescoping antenna from an old junk FM radio, at low power it should do just fine ! If you decide to take it further, radio shack has cheap FM-antenna kits and impedance matchers.
A transmitter connected to a common pole-shaped antenna will send out a signal in the shape of an expanding doughnut (imagine the doughnut with the pole in the center of the hole). The wave is 2-dimensional and will expand equally in all directions except up and down.
If the pole is perpendicular to the ground, the wave will expand parallel to the ground. If you lay the antenna parallel to the ground, the wave will expand up into the sky and down into the earth.
The receiving antenna must be "polarized" the same way; if the transmitter is perpendicular to the ground, the receiving antenna must be, also, otherwise you lose considerable power in the signal.
Type of wire:
As long as the wire is not very long and the antenna is attached to the transmitter (no line in between), you can use most any guage of wire with a copper element.
The receiving antenna can be of any length or guage as long as the receiver can be tuned, but the less distance the received signal has to travel along the wire the better.
(Aircraft use nothing more than a wire in the fusalage to receive HF signals; all they have to do is isolate from ground.)
Posts: 3632 | Location: Washington, US | Registered: 06-03-02
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