|What is a phase-locked loop circuit?|
|A. An electronic servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator|
|B. An electronic circuit also known as a monostable multivibrator|
|C. An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator|
|D. An electronic circuit consisting of a precision push-pull amplifier with a differential input|
Frequency synthesizers that use phase-locked loops are also popular. A phase-locked loop circuit is an
electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled
oscillator, and a stable reference oscillator.
Let's look at another example. The effective radiated power relative to a dipole of a repeater station with 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain is 317 watts. (E9A16). In this case, the total gain of the system is 10 dB – 4 dB – 3.2 dB – 0.8 dB, or 2.0 dB. 2.0 dB corresponds to a power ratio of approximately 1.585, and the effective radiated power equals 200 W × 1.585 = 317 W. In this system, high feedline and duplexer losses are almost completely negating the benefit of using such a high gain antenna.
Finally, the effective radiated power of a repeater station with 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain is 252 watts.
(E9A17) In this example, the total gain of the system is 7 dB – 2 dB – 2.8 dB – 1.2 dB, or 1.0 dB. 1.0
dB corresponds to a power ratio of approximately 1.26, and the effective radiated power equals 200 W × 1.26 = 252 W.
Feedpoint impedance, antenna efficiency, frequency range, beamwidth
Other antenna parameters are also important, of course. One of the most basic antenna parameters is the feedpoint impedance. Why would one need to know the feed point impedance of an antenna? To match impedances in order to minimize standing wave ratio on the transmission line. (E9A03) The reason that it’s important to minimize the standing wave ratio, or SWR, is that if you’re using coaxial cables, minimizing the SWR will also help you minimize losses. If you minimize losses, you’ll radiate more signal.
Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. (E9A04) For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 Ω, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it.
Another antenna parameter that’s frequently discussed is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. (E9A14) Radiation resistance plus ohmic resistance is included in the total resistance of an antenna system. (E9A05)
If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. You calculate antenna efficiency with the formula (radiation resistance / total resistance) x 100 percent. (E9A09)
Vertical antennas are sometimes criticized as being inefficient antennas. Soil conductivity is one factor that determines ground losses for a ground-mounted vertical antenna operating in the 3-30 MHz range. (E9A11) If soil conductivity is poor, ohmic resistance will be high, and the antenna's efficiency will be low. One way to improve the efficiency of a ground-mounted quarter-wave vertical antenna is to install a good radial system. (E9A10)
The frequency range over which an antenna satisfies a performance requirement is called antenna bandwidth. (E9A08) Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth.
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