Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Re: DIY FM antenna

Posted by Glenn2 (A) on March 18, 2003 at 15:53:52

In Reply to: DIY FM antenna posted by EKG on March 17, 2003 at 05:47:01:

All you need is a simple vertical dipole.

Simply attach two pieces of wire or metal to the end of some 75-ohm coaxial cable. One to the centre core and one to the screen. Extend them in opposite directions. Done.

Each 'arm' needs to be 95% of a quarter of the wavelength of the middle of the FM band (98MHz).

So, wavelength = speed of electromagnetic radiation / frequency
=299,792,458 / 98,000,000 = 300/98 = 3.06 metres

Divide by 4 to get 1/4 wavelength (the antenna must be 1/2 wavelength in total) and chop off 5% because the electrical length and the physical length of the antenna are different.

(3.06 / 4) * 0.95 = 0.726 metres = 72.6cm (28.6 inches)

So, a piece of coax with a 28-inch piece of wire attached to the core and another to the screen and you're done.

It's usually better to have the one connected to the centre going up and the other down but try both ways. I find wire better as you can fiddle with it more and hang it across windows where it will work better, and it's easier to attach (i.e., solder). If you mount it horizontally, it will be bi-directional and may work better if you hang it over the window facing the direction of the main transmitting aerial.

If there's a particular station you want to optimise it for, just change the 98 to whatever you need.

Glenn

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