----- Original Message -----
Sent: Sunday, February 18, 2001 9:26
PM
Subject: LF: Fw: Active antenna
To All from PA0SE
It has been mentioned several times that the input stage of an
active receiving antenna is easily damaged by lightning in the vicinity due to
the very high input impedance of the amplfier.
Several years ago Dr. (now professor) E.H. Nordholt of the
Technical University Delft in The Netherlands developed an alternative design
for an active antenna that does not suffer from this problem.
Instead of making the input impedance very high it has
been made very low instead by means of negative feedback. As a result the
E-field probe, a piece of copper tube of 50cm long and 3cm diameter, now feeds
a current into the FET at the input. Because of the probe's
capacitive impedance the current increases with frequency at
6dB/octave.
This is compensated by the negative feedback .
The active antenna has a frequency response that is flat
from 5kHz to over 30MHz. It stays linear in field strengths up to 10V/m.
Great attention has also been given to proper noise behaviour
("noisemanship"). The input is protected by diodes against high voltages.
Because signal voltages at the input are very small due to the low input
impedance little signal current flows into the diodes and they do not impair
the favourable characteristic of the active antenna.
In a test the antenna was not damaged by sparks jumping
to the probe from a car ignition coil.
I have sent a similar text, together with a circuit diagram,
to the reflector before but it may have been too long for distribution.
But particulars can be found in:
E.H. Nordholt, D. van Willigen: "A new approach to active antenna
design", IEEE Transactions on Ant. and Prop., vol. AP-28, no. 6, November
1980.
73, Dick, PA0SE
