Автор Тема: Сверх-компактная антенна VLF и передатчик на пьезоэффекте  (Прочитано 318 раз)

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Оффлайн rw3adb

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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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Оффлайн rw3adb

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On Thursday, April 18, 2019 07:31:13 PM DK1IS wrote:

 

SNIP

 

> 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 ;-)

 

 

63.1 dBm - 32.1 dBm = 31.0 dB : the quartz adapted antenna radiated power is 1/1000 respect the T-Marconi, so the efficiency is 0.1%

 

Good DX ;-)

 

Claudio, IK2PII
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Оффлайн rw3adb

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Am 19.04.2019 um 17:28 schrieb Clemens Paul:

    РазвернутьРазвернуть

Hi Clemens and group,

here is the promised extension of my tests, for better comparability merged with the first results (my post on the reflector from 20190418):

Having no VLF quartz yet, I made some preliminary tests with a randomly available HC-6-U quartz, QRG 2434 kHz, wavelength 123 m.

    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
    alternatively antenna wire only resonated by two roller plus one fixed inductors in series, fed directly from 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 ;-)
    with inductive tuning at 2,434,000 Hz: RX level = - 60.1 dBm resulting in a gain of 3dB compared with quartz. -3 dB-bandwidth = 249 kHz resulting in a loaded Q = 9.8

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
    with indctive tuning at 2,234,000 Hz: RX level = - 57.3 dBm resulting in a gain of 1.7 dB compared with quarz. - 3 dB-Bandwidth = 390 kHz resulting in a loaded Q = 6.2

With all tests there was no additional resistive matching at the TX output. The tests at resonance showed the usual unbalance between lower and upper - 3 dB-points. With L-tuning the upper is further away from the resonance frequency than the lower. With quartz-tuning this effect is contrawise and of course absolutely much lesser accented. At a first glance L-tuning offers advantages due to its comparable huge bandwidth and the higher fieldstength at the RX site but this could shrink if you really go down to VLF frequencies, very short antennas and short distances. According to the author´s claims the quarz resonated system is usable for short range communication only and there the Q of L-tuned antennas will also grow to high values as well as the losses in the tuning circuit.

As soon as my ordered XTALs will arrive I´ll make further tests in the 4 kHz-range. If the results happen to differ very much from the preceding tests at 2.4 MHz I´ll report about it.

73 es Happy Easter,

Tom, DK1IS
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Оффлайн rw3adb

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A quartz piezoelectric crystal converts electrical energy to mechanical energy and vice versa.

It won't convert electrical energy into an electromagnetic wave by itself.

Magnetostrictive material such as Terfenol-D will convert mechanical vibrations into a fluctuating magnetic field.

This can generate an electromagnetic wave directly using the homopolar effect.

A piezoelectric transducer can be used to mechanically drive magnetostrictive material to produce an EM wave.

Efficiency is several orders of magnitude greater than a wire antenna enabling practical portable VLF transmitters.

73 de VA3VVV
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