Hi group,
a little bit OT, but the public interest seems to rise ;-)! At April 15, Roman, RW3ADB, pointed out here a similar article leading to a more detailled link:
https://www.nature.com/articles/s41467-019-09680-2.
The paper describes a kind of matching a low source impedance to a very high load impedance by using a piezoelectric resonator. Thus it is possible to feed an extremely short antenna with a very high voltage in resonance without using lossy L-C-circuits, resulting in a relatively "high" antenna current. Of course such a system has a very small bandwidth and is frequency-wise very sensitive to stray capacitances at the high-impedance side. The authors turn this to an advantage with regard to a fast capacitve re-tuning when using it in DAM systems (Direct Antenna Modulation). A second problem is that the crystal has to stand voltages in the 100 kV range which is impossible for commercially available quartz resonators. Therefore the authors use a special lithium niobat crystal at about 35 kHz with 94 mm length which is mounted in a chamber filled with hexafluoroethane gas - not so easy for a normal ham ...
Anyway, the idea is fascinating. Having no VLF crystals up to now, I made some preliminary tests with a randomly available HC-6-U quartz, QRG 2434 kHz, wavelength 123 m. And yes, it works!
TX: Sythesizer Schomandl MG100M, 300 Hz ... 100 MHz, smallest step 0.1 Hz, Ri = 50 Ohms, P = 10 dBm, output tied to first leg of quartz, alternatively direct to the antenna wire
RX: Perseus SDR with an active rod antenna on my garage, fed by a solar system, no external wire connections, air-line distance 100 m to my shack
Antena wire: 2 m or 4 m, laid out in the shack, tied to second leg of quartz, alternatively direct to the TX output
Results with 2 m antenna wire:
with quartz resonator: fres = 2,433,908.8 Hz, RX level at resonance = - 63,1 dBm, - 3dB-bandwidth = 48.3 Hz resulting in Q = 50391
without quartz resonator: RX level = - 85,4 dBm resulting in a gain of 22.3 dB with quartz
comparision: my 13 m T-Marconi with top load 4 x 33 m, tuned to 2434 kHz and fed by the synthesizer with 10 dBm results in a RX level of - 32.1 dBm ;-)
Results with 4 m antenna wire:
with Quartz resonator: fres = 2,433,773.6 Hz, RX level at resonance = - 59 dBm. - 3 dB-bandwidth = 49.4 Hz resulting in Q = 49376
without quartz resonator: RX level = - 75.4 dBm resulting in a gain of 16.4 dB with quartz
So it seems the best effects can really be achieved with very short antenna wires. The long range performance is in no way comparable with more fullsized antennas but for special applicatios the features could be interesting. For instance realising some capacitive feedback from the vicinity of the antenna rod could result in a small low power beacon with integrated matching - it even could be used as a presence detector with direkt frequency read out ...
- But yes, Rik, I`ll certainly will keep my Marconi - making QSOs on VLF costs enough efforts even when using a big antenna. But I´ve alredy orderd some VLF crystals on the bay ...
73 es Happy Easter!
Tom, DK1IS
