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Re: LF: <Tech> More antenna results

To: [email protected]
Subject: Re: LF: <Tech> More antenna results
From: [email protected]
Date: Mon, 8 Oct 2001 20:16:34 EDT
Cc: [email protected]
Reply-to: [email protected]
Sender: <[email protected]>
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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