Instructables

How do you determine the length of an antenna and the size of a fractal antenna?

In an instructable, the formula for calculating antenna length was given as "length in meters =300 / frequency in MHz".  The example given was for a Bluetooth antenna with a frequency of 2450MHz.  The antenna length was calculated as 300/2450 = 0.1225m = 12cm.  The author then divided it by 4 to make a "quarter wave" antenna length of 30mm.  Why divide it by 4 and what is the benefit of a "quarter wave" antenna.  What is the effect of the conductor leading to the antenna?  If the 30mm antenna is soldered to a 4mm lead, does that result in a 34mm antenna?  For a fractal antenna, should the 30mm length of wire be folded into a fractal shape or should the total fractal antenna be 30mm in width?  I have read somewhere that one of the benefits of a fractal antenna is that they are able to receive signals of different wave lengths.

Re-design4 years ago
A quarter wave antenna is sometimes used because it's shorter than a full wave antenna.

A quarter wave works better than a random wave antenna because of harmonics, I think that's the right word.  You get less SWR (google it) using a quarter wave than you would with a 3/16 or a 5/16 wave antenna.

THe feed line does not enter into the calculation if it is shielded wire or a balance line.  If you just feed the antenna with a hunk of wire then all of the wire not becomes an antenna and all of your design and math goes out the window, but less of your signal goes out of your antenna.

An antenna is able to receive a signal of any frequency. But frequencies that are multibles of dividibles of the antenna length are recieved much more stronger than those that are at odds with the math.
netboss3 years ago
Re-design is correct on everything except his feedline reasoning. Using shielded or balanced wire has nothing to do with de-coupling your radiator although it is important in matching your induction/resistance to your receiver/transmitter. SWR represents RF that is reflected and not collected or transmitted, therefore wasted radio waves. If your antenna is not resonant at the tuned frequency you lose much of the signal, possibly all of it. Antenna's are resonant at 1/4, 1/2, 5/8, full-wave, multiples of full-wave and several other multiples.

You could create a simple dipole for your FM radio by taking a piece of two-conductor speaker wire, compute your size by taking the middle of the FM band, 98mhz would be close. 300/98 = 3.06 meters. Split the two wires apart at one end until each side is 1.5 meters long. Tape it up at the junction where you reached 1.5 meters. Spread the two separated sides at a 90 degree angle (in a T shape). The 90 degree turn is what separates(de-couples) the radiator from the feedline. Spread your wires and use thumbtacs to keep it in place and you have a very effective FM antenna. Better than the vertical 1/4 wave on your car.
Quick question. If you want to make a catch all antenna that would pick up everything from say short wave to UHF. Could you use a short length antenna that is able to be divided from the wave length of many frequencies?