Monday, 14 February 2011

Active antenna 1 to 20dB, 1-30 MHz range.



Rodney A. Kreuter Tony van Roon

"When fate or nasty neighbors prevent you from stringing a long-wire receiving antenna, you'll find that this pocket-size antenna will give the same, or even better, reception. This "Active Antenna" is cheap to build" and has a range of 1 to 30Mhz at between 14 and 20dB gain."

For conventional all-frequency short-wave reception, the general rule is "the longer the antennal the stronger the received signal." Unfortunately, between nasty neighbors, restrictive housing rules, and real-estate plots not much larger than a postage stamp, short-wave antennae often turn out to be a few feet of wire thrown out of the window--rather than the 130 feet of long-wire antennal we would really like to string between two 50-foot towers.Fortunately, there is a convenient alternative to the long-wire antenna, and that's an active antenna; which basically consists of a very short antenna and a high-gain amplifier. My own unit has been in operation successfully for almost a decade. It works satisfactory.

The concept of a active antenna is fairly simple. Since the antenna is physically small, it doesn't intercept as much energy as a larger antenna, so we simply use a built-in RF amplifier to make up for the apparent signal "loss." Also, the amplifier provides impedance matching, because most receivers are designed to work with a 50-ohm antenna.

Active antennas can be built for any frequency range, but they are more commonly used from VLF (10KHz or so) to about 30MHz. The reason for that is because full-size antennas for those frequencies are often much too long for the available space. At higher frequencies, it is quite easy to design a relatively small high-gain antenna.The active antenna shown below (Fig. 1), provides 14-20dB gain at the popular short-wave and radio-amateur frequencies of 1-30MHz. As you would expect, the lower the frequency the greater the gain. A gain of 20dB is typical from 1-18 MHz, decreasing to 14dB at 30MHz.
Circuit Design:
Because antennas that are much shorter than 1/4 wavelength present a very small and highly reactive impedance that is dependent on the received frequency, no attempt was made to match the antenna's impedance--it would prove too difficult and frustrating to match impedances over a decade of frequency coverage. Instead, the input stage (Q1) is an JFET source-follower, whose high-impedance input successfully bridges the antenna's characteristics at any frequency. Although many different types of JFET's may be used--such as the MPF102, NTE451, or the 2N4416--bear in mind that the overall high-frequency response is set by the characteristics of the JFET amplifier.

Transistor Q2 is used as an emitter-follower to provide a high-impedance load for Q1, but more importantly, it provides a low drive impedance for common-emitter amplifier Q3, which provides all of the amplifier's voltage gain. The most important parameter of Q3 is fT, the high-frequency cut-off, which should be in the range of 200-400 MHz. A 2N3904, or a 2N2222 works well for Q3.
The most important of Q3's circuit parameters is the voltage drop across R8: The greater the drop, the greater the gain. However, the passband decreases as Q3's gain is increase.

Transistor Q4 transform Q3's relatively moderate output impedance into a low impedance, thereby providing sufficient drive for a receiver's 50-ohm antenna-input impedance.

Active Antenna Schematic Diagram (C)

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