Dick said:
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 .
This is most interesting. Some of us have active antennas from the Decca
transmitter sites used for the monitor receivers. G3GRO has used one of
these active antennas with a whip antenna, which when raised well above
other electromagnetic obstructions (as suggested by G3JKV) gives results at
least as good as with a resonant transmitting antenna. My own Decca active
antenna, located on the side of the house only 7m high does not perform that
well.
I have received an active antenna, for testing in the EU environment, from
Frank Gentges, K0BRA, with e-mail advice to mount the antenna as high as
possible.
However, the Decca active antennas did not use whip antennas, instead they
used copper drums about .5m in diameter and .5m long, mounted only 1.5m
above the ground (three of them located only 2m apart) . The Decca active
antennas appear to have a couple of opamps with a resistor on a link, which
can be changed to alter the gain (I do not have a circuit).
Could these be a Prof Nordholt's design? Does this design need to be
mounted high above electromagnetic obstructions to perform well as with the
conventional active antenna with a whip?
Regards,
Peter, G3LDO
<[email protected]>
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