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Antennas for dummies

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John Sorensen

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I built one of them FM transmitters, you know what I'm talking about, like FM Transmitter 2 or whatever. It works good, but what I'm wondering, is there an optimum length of antenna for it? I've seen stuff about 1/4 wave and 1/2 wave and so on, but I don't know if it applies to this.

j.
 
The purpose of an antenna is to radiate the transmitter power. Almost any form of antenna will radiate some power. To make the most of the transmitter power you do have you want to get the maximum power from the transmitter to the antenna. This requires that the transmitter, feedline and antenna be matched. Once maximum power is transferred the job of the antenna is to direct the power in a preffered direction, if there is a preferred direction.

Back to power transfer. A common impedance for a transmitter is 50 ohms. This means that if the transmitter was connected to a load (feedline and antenna) that behaves as if it were a 50 ohm load then maximum power will be transferred. Ideally the feedline should be the same - notice that some coaxial cables have a 50 ohm impedance. Next, the antenna needs to look like 50 ohms. Some antennas work best when they are resonant at the frequency of interest. A 1/4 wave antenna that is resonant with a proper ground plane will look like 50 ohms, or nearly so. A 1/2 wave dipole can look like 50 ohms at resonance. Realize that for both situation height above terrain, soil conductivity, drooping of radials, etc all play into the situation. Other arrangements of antennas can look like 50 ohm loads but that provide some directivity - and by directing the power these antennas are said to have some gain.

Clearly your antenna works. I've described a very complex subject with very few words. Improvements seem quite possible given what you have started with. While it might take a PhD in antenna science to come up with the best antenna for your situation you might experiment with simple variatons in what you have to see what improves or degrades the signal. Try adjusting the length, height, orientation, etc of what you have now. Try making a dipole, if space allows, and connecting it to the tranmitter with proper feedline. See if a matching network of some kind will help. Find this stuff by checking ARRL or RSGB sites.
 
A simple antenna can be made from 300 ohm TV lead-in wire. Cut a section that is 1/2 wavelength at the operating frequency and short both ends (connect the wires together). In the center of one side, make a cut and connect the lead-in. This makes a 300 ohm antenna which is called a folded dipole. The direction of transmission is at right angles to the plane of the dipole.
 
ok heres the the calculation your looking for.

say signal is 433mhz
433*10^6/3*10^8 = 1.4333.
1.4333./4 = .36
which would mean the the optimal antenna size is 36cm
 
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