[Photo] Using TV pull stalk antenna as FM dipole antenna

October 15, 2022

The importance of antenna element length

You will find that the following introduction emphasizes that the length of the antenna is 1/2 or 1/4 wavelength, why? When the length of the antenna is not correct, the impedance and the design value are much different, resulting in a reduction in transmission efficiency. For example, the impedance of a properly fabricated half-wave dipole antenna is 75 ohms purely resistive. If its length is halved, its impedance becomes 15-400j ohms, that is, 15 ohms resistance and 400 ohms capacitance reactance. The resistive part of the antenna impedance is the part that is effective for transmission, while the inductive part only causes reflected waves to feed energy back to the transmitter. This phenomenon is generally described by the standing wave ratio (SWR). The closer the standing wave ratio is to 1.0, the better. 1.0 means that the reflected wave is zero.

If no feeder cable is used, the inductive reactance of the antenna can be removed with an additional adjustable inductance or capacitance pin. This is called matching tuning of the antenna. For example, the dipole antenna shortened by half in the above example can use a 400 ohm inductance series inductance pin to remove the 400 ohm capacitance of the antenna, making the antenna 15 ohm purely resistive. But at the same time, the impedance of the output amplifier of the transmitter is required to be less than 15 ohms, which has certain difficulties in circuit design, and the bandwidth of the antenna also becomes very narrow.

The wavelength of radio waves in air is 300 / f meters, where f is the emission frequency (megahertz), for example, the corresponding wavelength of 100 megahertz is 3 meters.

Dipole antenna
The most commonly used antenna is a center-driven half-wave dipole antenna, as shown in Figure A on the right. The production method is: welding a thick copper wire (or copper tube, aluminum tube, or the shielding layer of the coaxial cable) to the BNC socket. The length of the thick copper wire is about 1/4 wavelength, or 75 / f meters, where f is the transmission frequency (megahertz). If you are not sure about the frequency you use, use 70cm. Two sets of such parts are required. Figure B shows how to connect the two sets of parts together. The thick copper wire in the upper part should only be connected to the center of the BNC socket, while the thick copper wire in the lower part should be connected to the BNC housing. The original impedance of the dipole antenna is 75 ohms. Since the two halves of the dipole are symmetrical (that is, balanced), a "1: 1 balun" is required when connecting with a coaxial cable (unbalanced). The easiest way to make this converter is to wind the cable around 4-5 turns with a diameter of about 10-20cm. The antenna should be placed vertically, leaving at least 50cm free space between the cable and the antenna.

Using TV pull stalk antenna as FM dipole antenna
	The general double-angle TV pull antenna can be used as an FM half-wave dipole antenna. This kind of antenna usually comes with a balun composed of a high-frequency transformer (such as the small box on the left), but it is necessary to test in advance whether this transformer can withstand the power you want to transmit (the coil and the magnetic core do not heat). Another solution for Balun is to directly use the coaxial cable as the feeder, and use 4-5 circles with a diameter of about 10cm (see the introduction above). The antenna should be placed vertically, each stalk should be 75 / f meters long, and the two stalks should be in a straight line (the same as in figure A above), rather than being V-shaped. The cable and antenna stalks should not be close in parallel, but at least 50cm should be left free.

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