Antena question

Apr 14, 2008 3 Replies

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Claude 18 years ago

Greetings dudes.

I am building a 3 volt FM transmitter, just strong enough to scare the bejesus out of the wife when she is listening to her crappy music :o)

The project suggests using a half wave antenna instead of a single wire. I understand the use of a 50 Ohm coax , it is the wire they hook up in a loop between the signal and the ground ( respectively the centre of the coax and the shield) every site that I have checked states that this loop should be

150/Mhz. Here comes the stupid question, what are the units? Do they mean
150 mm per Mhz? I will be attempting transmissions between 99.0 and 100 Mhz providing I can properly tune the Cap. does this mean my antenna would have a 15 meter antenna in a loop shape? This sounds wickedly big?

Mathematically challenged old geezer. Montreal Canada

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Tim Wescott 18 years ago

Radio wavelengths are about 300 meter/MHz; 150m/MHz is right for a half-wave.

Google on 'dipole antenna'. It's easy (you can make it from lamp cord or speaker wire), it works well, and it's not critical to tune. If it didn't blow up your transmitter final amplifier, you could probably use a coat hanger, so a dipole should be more than enough.

Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html

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Bob Eld 18 years ago

loop

and

Mhz

have

The units are Meters. The wavelength of 100 MHz is 3 Meters. Therefore a half wave length is 1.5 meters or 59 inches.

W.L. = c/f where WL = wavelength in meters. C = speed of light, 3 X 10^8 meters/sec. and, f = frequency in Hertz.

The width of the loop should be 1.5 meters, not the length of the wire. The loop is usually squashed flat and is called a folded dipole. BTW, the impedance of such a loop is 300 ohms not 50 ohms so you'll need a balun or other impedance matching device.

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Claude 18 years ago

Thanks a lot. Wave lenght, this is all starting to make sense ( I stress the word starting :o) RF seems to be akin to black magic. So far every little wire in my projects involving oscillators have caused havoc by causing spurious transmissions ( feed backs that shouldn't be there etc. Seems like the atmosphere in my shop is laughing at my capacitors :o)

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