Active Antenna for AM/FM/SW


Active Antenna for AM/FM/SW:
This simple little circuit can be used for AM, FM, and Shortwave(SW). On the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna. It is further more designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.

MPF102/NTE451 - N-Chan JFET

Parts List:
R1 = 1M
C1 = 470pF
C2 = 470pF
L1 = 20uH to 470uH (see text)
Q1 = MPF102, 2N4416, or NTE451

A KIT is available and may contain any value uH for L1, between 20uH and 470uH, to get you started. Change this value to suit your needs.
Parts only, no pcb: [Parts KIT]

L1 can be selected for the application. A 470uH coil works on lower frequencies and lie in AM, for shortwave try a 20uH coil. The KIT is supplied with a value whatever is available up to 500uH. The color code for L1 is generally yellow-violet-brown for a 470uH type but still this can vary by the type of inductor.
This unit can be powered by a 9 volt alkaline battery. If a power supply is used, bypass the power supply with a 0.04uF capacitor to prevent noise pickup. The antenna used on this circuit is a standard 18-inch telescoping type, but a thick piece of copper, bus-bar, or piano wire will also work fine.

The heart of this circuit is Q1, a JFET-N-Channel, UHF/VHF amplifier in a TO-92 case. Although many different types of FET's may be used--such as the MPF102, or the 2N4416--bear in mind that the overall high-frequency response is set by the characteristics of the FET amplifier. The direct replacement for the MPF102 is the NTE451. Second runner up is the 2N4416.

Output is taken from jack J1 and run to the antenna-input of your receiver.

Although this little circuit can easily be mounted on a piece of vero-board, I have supplied the printed circuit board and layout diagram if you wish to make your own.

Active Antenna PCB      Active Antenna Lay-out

ERROR FIX: Printed Circuit Board had a fault, both pads of R1 were incorrectly connected to the gate. The pcb and layout above are corrected.
Thanks Bill, AD7EX.


Source: "Popular Electronics" magazine, July 1989 issue. Posted with permission.
Copyright © Gernsback Publications, Inc. 1989. (Gernsback Publishing is sadly no longer in business since January 2000).


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Copyright © 2003 - Tony van Roon