Hallo,
https://incompliancemag.com/creating-a-novel-compact-antenna-that-can-go-where-radios-cant/
Friends are there any news? Home made
new antenna??
'73 from Richard - IK7FMO
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
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