One-Chip Active Whips
             Mark Connelly - WA1ION - 6 MAY 1991

     Buffer amplifier integrated circuits are commonly used in
the test equipment industry for active probes used to observe
the waveform on a circuit point in a "non-intrusive" manner.
Such amplifiers typically have an input impedance of greater
than 100K and an output impedance of 50 ohms or lower (5 ohms
is typical).  Amplifier gain is in terms of current; the voltage
at the low-impedance output is approximately the same as at
the high-impedance input.  Active probes, by providing the
impedance transformation, have little effect on the circuit
point being observed while providing an output suitable to
be switched through coaxial switch matrices to 50-ohm
measurement instrument inputs.  Performance to at least 30 MHz
is typical.  The semiconductor makers of the world offer many
of these important buffer amplifier IC "building blocks".
National Semiconductors LH0033 & LH0063, Analog Devices AD9630,
Burr-Brown 3553AM, and VTC VA033 are examples of these.

     Such chips are well suited to the function of an active
whip antenna amplifier.  The whip rod acts as a high-
impedance voltage source (with minimal current-delivering
ability) that must be converted to a 50-ohm impedance for
input to a receiver.  Active whips can fill an important niche
in the DXer's antenna "arsenal" - especially for DXers with
little available land for wire antennas and for those not
inclined to use loops.  Some of the buffer amplifier devices
can deliver in excess of 10 volts peak-to-peak (Vp-p) of
relatively undistorted output; therefore, strong-signal
handling is generally not a problem.

     The devices are usually operated with two equal-amplitude,
opposite-polarity power supplies.  Doing this allows operation
down to DC.  If RF-only operation is desired, a single supply
and capacitive input/output coupling may be used.  

     The intention of this article is to stimulate DXers to
try this alternative design approach to active whips.  Many
good articles have been written about constructing active
whips with discrete transistors (usually FET input to FET or
bipolar output).  The whips I've built from "canned buffers"
have performed as well, or better than, most of the conventional
two-transistor models.