Active circuit HF VHF UHF Antenna , 3 to 3000 MHz, In its simplest form, an active antenna uses a small whip antenna that feeds incoming RF to a preamplifier, whose output is then connected to the antenna input of a receiver. Unless specifically designed otherwise, all active antennas are intended for receive-only operation, and thus should not be used with transceivers; transmitting into an active antenna will probably destroy its active components. A well designed broadband active antenna consider field strength of the desired signal (measured in microvolts per meter of antenna length), atmospheric and other noise, diameter of the antenna, radiation resistance, and antenna reactance at various frequencies, plus the efficiency and noise figure of the amplifier circuit itself.
Active circuit HF VHF UHF Antenna, The fact that there are two independent preamplifiers in the AA-7 makes faults easeir to diagnose than with many other devices. If a problem occurs, only at one setting of S1, concentrate on that part of the circuit. If the problem is common to both settings, the components and the connections common to both preamps should be checked. Maker sure the jumper wires are in place.
Parts List and other components:
- · Semiconductors:
- · Q1 = MFE201, SK3991, or NTE454. N-Channel, dual-gate MOSFET (see text)
- · Q2 = 2SC2498, 2SC2570, 2N5179, SK9139, or NTE10. NPN VHF/UHF silicon transistor (see text)
- · Note: If you use the NTE107 as a replacement, make sure to insert it correctly
- · into the pcb. The orientation is different than as shown on the parts layout
- · diagram. (e-c-b seen frontview for NTE107). See this Data Sheet
- · Resistors:
- · All Resistors are 5%, 1/4-watt
- · R1 = 1 Mega Ohm
- · R2 = 220K
- · R3,R6 = 100K
- · R4 = 100 ohm
- · R5 = 10K potentiometer, (pc mount)
- · Capacitors:
- · C1,C2,C5,C6 = 0.01μF, ceramic disc
- · C3 = 100pF ceramic disc
- · C4 = 4.7 to 10μF, 16WVDC, radial lead electrolytic
- · Additional Parts & Materials:
- · B1 = 9-volt alkaline battery
- · S1,S2 = DPDT PC mount pushbutton switch
- · J1,J2 = PC mount RCA jack
- · ANT1 = Telescoping whip antenna (screw mount)
- · MISC = PCB materials, enclosure, enclosure, battery holder and connector,
- · wire, solder, etc.
- · KIT = NO kit is available at this time. I may supply kits on demand.
There are other characteristics or phenomena associated with preamplifiers and active antennas that does not mean that your circuit is malfunctioning. For example, if you have strong AC hum in the HF setting, the antenna is too close to an AC cord or powerline. HF signals may be clearer at the VHF/UHF setting than ast the HF setting. Why? Although either preamp may be used for HF, the signal strength will be greater with the VHF/UHF preamp. However, the HF signal-to-noise ration is better with the dual-gate-MOSFET-based preamp. Try both and use the best for your particular receiver conditions.
Some portable receivers not enclosed in metal cases may break into oscillation when connected to any RF preamplifier. Try reducing the AA- 7's gain and make sure that good grounds are provided with the interconnecting coax cables. A preamplifier will intensify any problems due to poor receiver design: overloading, images, or any other problems with selectivity and image rejection.
