Content-transfer-encoding: 8bit

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