Keyword: Using TV pull the antenna as FM dipole antenna
The importance of antenna emission element length You will find the following introduction to emphasize the length of the antenna of 1/2 or 1/4 wavelength, why? Because the length of the antenna is wrong, the impedance and the design value difference are poor, resulting in a decrease in emission efficiency. For example, the correctly produced half-wave coupling impedance is 75 ohm generic resistance, and if the length is halved, the impedance becomes 15-400 j ohms, which is 15 ohm resistors, 400 ohm capacitive impedance. The resistive part of the antenna impedance is part of the effective part, while the sensing portion only causes the reflected wave, and the energy feedback back the transmitter. This phenomenon generally describes the standing wave ratio (SWR), the stronger the station, the better, 1.0 represents the reflection wave is zero. If no feeder cable, the antenna's sensing can be poured with an additional adjustable inductor or a capacitor, and the matching of the antenna is tuned. For example, a shortening dipole antenna is shortened, and a 400 ohmic capacitive sensing pin of a 400 ohmic impedance is used to turn the antenna to 15 ohm pure resistance. However, this also requires the output amplifier of the transmitter less than 15 ohms, which has certain difficulties in the circuit design, and the frequency of the antenna has also become very narrow. The wavelength of the radio wave in the air is 300 / fm, where F is an emission frequency (MHz), such as a wavelength corresponding to 100 MHz.
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The most common antenna of the dipole antenna is the center-driven half-wave dipole antenna, as shown in the figure. The production method is to welder a crude copper wire (or copper tube, aluminum tube, or shielding of coaxial cable) to the BNC socket. The thickness of the thick copper wire is approximately 1/4 wavelength, ie 75 / fm, where F is a transmit frequency (MHz). If you are not sure you use the frequency, use 70cm. Such components require two sets, and the B figure shows how the two sets are connected together. The crude copper wire on the above part should only be connected to the center of the BNC socket, and the crude copper wire below the part should be connected to the BNC housing. The original impedance of the dipole antenna is 75 ohms. Since two semi-semi-semi-semi-semi-symmetrical (ie, balance), there is a "1: 1 balance-unbalance transformer (Balun) when the coaxial cable (unbalanced) is connected. The simplest production method of this converter is to route the cable about 10-20 cm in diameter. The antenna should be placed vertically, at least 50 cm air should be left between the cable and the antenna.
Using a TV stalk antenna as an FM dipole antenna, a typical television stalk antenna can be used as an FM half-wave dipole antenna. This antenna generally comes with Balun (such as a small box of the left), but you have to test the power you want to emage (the coil and the magnetic core are not hot in advance). Another solution in Balun is to use the coaxial cable as a feeder, and give it a 4-5 circles of approximately 10 cm in diameter (see the above description). The antenna should be placed vertically, each pulling stalks to 75 / f, the two stalks should be a straight line (the same as the figure in Figure A), and should not be V-shaped. Cables and the antenna stalk should not be parallel, but at least 50 cm should be left.
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