FMA9901A FM ​​broadcast dedicated antenna

Due to the rich content, good sound quality, wide frequency response, and the upper limit of the audio frequency response of FM radio with good quality can reach 15000Hz, especially with the emergence of FM stereo broadcasting, it is more popular with the public, so many audio enthusiasts regard FM radio as a new sound source. Although many users are using some top FM tuners, when the signal-to-noise ratio of the received radio station is close to or lower than the receiving sensitivity of the radio, the output audio signal cannot achieve high fidelity effect, or even cannot be received. Usually, the outdoor field strength is more than 15 decibels stronger than the indoor field strength, which makes it "innately advantageous" to set up outdoor antennas compared to indoor antennas. In addition, large-volume high-gain multi-unit antennas can be set up outdoors. Therefore, when building an FM-HiFi system, not only should you choose an ideal FM receiver, but the antenna is also an important part that cannot be ignored.

The FMA9901A receiving antenna introduced in this article is an outdoor antenna specially used for various FM tuners. Since its launch in 1999, it has been well received by many domestic FM radio listening enthusiasts. The antenna consists of five parts: a dedicated FM radio antenna stand, a high-gain antenna amplifier, a steering motor, a 16-meter coaxial cable, and a power box.

The FMA9901A FM ​​broadcast antenna uses a folded half-wave dipole type three-unit antenna frame, with a receiving center frequency of 100MHz. The dipole uses an 8mm hollow aluminum tube after electroplating. Figure 1 shows the actual size of the antenna frame for reference.

The amplifier is composed of FM filter circuit, low noise amplifier transistor, rectifier circuit and steering control circuit. Figure 2 is the schematic diagram of the antenna amplifier. After the signal is received by the antenna, it is transformed from 300Ω to 75Ω by impedance converter B1. The input loop L1, L2, C1, and C2, C3, L3 respectively form T-type low-pass and high-pass filters, so that the passband of the entire amplifier is limited to 80MHz-110MHz, so as to filter out VHF signals other than FM and improve the anti-interference ability of the amplifier. Since the signal received by the antenna varies in strength, in order to prevent the output level of the amplifier from exceeding the maximum output level and causing distortion, and because the amplifier cannot effectively amplify weak signals with extremely low signal-to-noise ratio, only a low-noise NEC3356 (same performance as 3358) microwave tube is used for signal amplification, and the signal is output through a high-pass filter composed of C5, C6, and L4. L6 and C13 form a high- and low-frequency separation circuit. A 12V DC (AC) voltage is added to the antenna amplifier (motor) through L6. Due to the action of C13, the 12V DC (AC) voltage will not enter the FM receiver, and the FM signal will enter the FM receiver through C13 coupling. Since L6 has the function of blocking high frequencies, the FM signal will not enter the power supply.

The above antenna frame and amplifier only constitute a high-gain unidirectional reception. In order to achieve all-round high-gain reception, the antenna is also equipped with a steering device. The motor uses a 12V AC permanent magnet synchronous motor with an external speed of 4 rpm and a torque greater than 4 kg. The failure rate within 1 year is less than 2%. Since it uses low voltage and low current, the power used by the motor, the amplifier and the FM signal are all transmitted on a 14-meter 75Ω coaxial line, making the operation very safe and convenient. Just press K, the turn indicator D3 lights up, and the 12V AC power supplies the motor through C12. The motor M starts working and will not interfere with the broadcast when rotating.