Hi Jim,
congrats on your great LF work! I was really impressed by your
electro-mechanical VLF receiver concept, and now I'm very interested in your
antenna measurements. I would be pleased to receive a copy of the data.
The usual statement seems to be that in the near field of the electric
antenna,
the H field is proportionally smaller and the E field larger than in the
far field -
ie E/H is greater than 120pi ohms.
No and yes. In the reactive near-field of a small antenna, the magnitudes of
both field components rise more steeply towards the source. For an electrical
dipole,
E ~ 1 / r^3 (= electrostatic dipole) and
H ~ 1 / r^2 (corresponding to the Biot-Savart rule for a short length of
current),
with a common crossover to the 1/r far-field occuring at lambda/(2pi) .
Formulas and graphs showing the field transition for Marconis and ground
loops at 9 kHz can be found on Geri's www.qru.de/radiation.html.
Regarding the "missing dB's" in the near field, I have also experienced that
the effective height of a marconi antenna was typically only 50..75% of the
physical height of its top load, even though the capacitance of the top
exceeds that of the vertical part. As you found, the difference is more
pronounced with low antennas, agreeing with the assumption of E-field
shielding by trees and houses.
To calibrate the radiation efficiency of my antenna, I have used relative
measurements of received signal powers rather than transmitted field strength:
First, I measure the induced voltage of a remote transmitter (DBF39) in a
single-turn nonresonant loop of known area (32 cm diameter), and calculate
the local magnetic fieldstrength. As H is hardly affected by near-field
capacitive objects, E = H * 377ohm gives the undisturbed electric field DBF
would have here without obstructions. The power density E * H times the
aperture area of an ideal monopole 3/4 * (lambda / (4 pi))^2 is the available
RX power one would expect from a lossless antenna. Then, measuring the
actually received power as a fraction of that gives the antenna efficiency.
Due to reciprocity, this is also the efficiency for transmitting (including
nearfield losses), as long as there are no nonlinearities like ferrite-core
hysteresis or corona losses. In my case, I found eta = 0.08%, i.e 100mW EMRP
(or 183mW EDRP) from 125W TX power.
73 and best wishes
Markus, DF6NM
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