Instruction manual

SM100 • 5
electromagnetic waveform is also produced. This waveform is not susceptible
to the ravages of electrostatic bursts so it is more desirable to receive.
Well, this is exactly the problem we encountered here at Ramsey. While trying
to “keep an ear on” the football sports broadcasts 90 miles away, power line
noise and low power conditions caused us to string a long wire antenna in
hopes of receiving a stronger signal. This worked somewhat, but didn’t really
satisfy our “rigid” requirements for noise free reception. So we experimented
with an active antenna design, and viola, our signal magnet was created!
SM100 CIRCUIT DESCRIPTION
Our signal magnet antenna consists of two parts, the antenna section
(SM100A), and the control box (SM100C). Have a look at the schematic
diagram and follow along.
We were presented with an interesting problem when initially designing the
signal magnet. We needed to run DC power, a variable control voltage, and
the RF signal onto a single wire. The power and RF seemed simple enough,
with a DC blocking capacitor at the antenna and the controller, but how do we
also add the variable voltage? The solution, a zener diode! This can “regulate”
the DC voltage at the antenna to the necessary supply voltage to run the RF
preamplifier. The only drawback with this idea is that the DC supply must start
at 12 VDC and tune up to 27 VDC.
The control assembly consists of the voltage sources necessary to run the unit.
Diodes D1 and D2 along with capacitors C1 and C4 comprise a voltage
doubler that provides the unregulated 30 Volt DC supply. Zener diode D3
provides the regulation for the high (27V) DC voltage supply. The “tune” knob
varies the voltage from 12 VDC to 27 VDC while transistor Q1 “buffers” the
output voltage to avoid any changing on the output, as this would interfere with
the tuning of the unit. Jacks J1, J2, and J4 route the RF input to your radio.
The antenna assembly consists of a similar voltage regulator circuit using
diode D7 to supply the 12 VDC for the RF amplifier. Transistors Q4 and Q5 are
the low noise amplifier section of the circuit. Varactor diode D6 is the tunable
component in the circuit whose internal capacitance changes proportionally to
the reverse bias voltage applied to it.
The final bit of magic is the “Faraday” shielded ferrite stick antenna. The shield
effectively eliminates any electrostatic interference present. This is the input to
the entire circuit and will be mounted in the weatherproof PVC pipe. We’ll
discuss this in more detail later when we’re assembling this section of the
circuit.