Piezo-electric antenna? I think bollocks, how about you? - Page 1
Forum section: electronics / rf-microwave

author: vk6zgo
author_rank: Super Contributor
timestamp: April 17, 2019, 05:26:39 am
content: I found this on a local ham forum. (WAHAMs).The poster apparently had more luck than I did with the following site:https://newatlas.com/pocket-sized-vlf-antenna/59287/(This newatlas site looks a bit "suss" with another article on a "snow powered nano generator")He found pictures & all sorts of stuff which I couldn't find, but there was this link:https://www.sciencedaily.com/releases/2019/04/190412085213.htm?utm_medium=cpc&utm_campaign=ScienceDaily_TMD_1&utm_source=TMDAfter my initial reply, indicating disbelief, he posted this link, which is apparently the "mother lode":https://www.nature.com/articles/s41467-019-09680-2It all looks very "high tech" but I'm very far from convinced.I learnt at a young age, that in this life you get nothing for nothing & "bugger all" for sixpence!

author: Berni
author_rank: Super Contributor
timestamp: April 17, 2019, 07:21:40 am
content: I'm not really an RF engineer but it looks like a perfectly good idea to me.The basic problem is that radiating any significant amount of RF at 30KHz with a 10cm long antenna requires the antenna to be driven with a ridiculously high voltage. The usual way of making this high voltage is with a transformer or impedance matching network and then feeding it into a piece of wire.What they do instead is excite a piezoelectric rod with the input signal, bringing the rod to resonate at the transmit frequency. Because the rod is long and has a very high Q makes the piezoelectric effect inside generate a massive 125kV AC voltage on the end. Basically this is a transformer-less version of a tesla coil that happens to be smaller for a given voltage.The clever bit is that they add capacitance around the base of the rod to shift its frequency around a bit in order to do FSK modulation. This is the same idea as the 5 to 30pF capacitors we commonly put on crystal oscillators, the capacitance shifts the frequency of the oscillator slightly.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 17, 2019, 05:01:03 pm
content: Without reading it too carefully it seems legit. The idea of using a piezo-electric element as an impedance matching network is certainly reasonable. Piezo materials are nice in this type application because the speed of sounds is much less than the speed of light. I didn't follow the high speed modulation bit (aka "violating the Bode-Fano limit), but what Berni says sounds reasonable. There are approximately infinite ways to "exceed" the Bode Fano limit (i.e., violate its assumptions), so it isn't really surprising.

author: mikerj
author_rank: Super Contributor
timestamp: April 17, 2019, 05:51:05 pm
content: Quote from: vk6zgo on April 17, 2019, 05:26:39 amThat article also looks fine. It's a triboelectric generator using snow movement to producing tiny amounts of power for e.g. remote sensing applications.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2019, 01:50:32 am
content: Heh, sure, if short range transmitters are what you need!Doesn't work for receive. At least not without a PLL or lock-in or whatever to tune it dependent on the received signal. Those invite problems in a similar* way to regenerative sets ("just with more steps").*Actually, not really similar at all, but it's interesting to think about why that is (and isn't).Even then, whether you're sensing by amplitude or phase or both, the receiver (be it PLL, lock-in or other) can still only respond as fast as the Q, so it truly is an asymmetrical situation.Or to put it another way: they can modulate it on transmit, but it must be done open loop, i.e., there's no possible hope of reading the radiated voltage/phase and stabilizing that inside a feedback loop, because that still changes at a rate determined by electrical Q. Indeed, the amplitude will change proportionally (inversely? I forget) with modulation, because you're effectively varying the electrical length of the antenna, which changes the energy distribution and therefore the amplitude (but not the total energy, which remains ~constant from cycle to cycle, because Q >> 1).Ironically it's the inverse of the AM radio situation, where a small receiving antenna is possible but the transmitter must be large. This still seems to imply poor efficiency and range, though.Tim

author: Marco
author_rank: Super Contributor
timestamp: April 18, 2019, 05:27:01 am
content: For receive there's less necessity, ferrite provides big gains.

author: radiolistener
author_rank: Super Contributor
timestamp: April 19, 2019, 06:57:39 pm
content: I'm very skeptic about this antenna, there are three issues:1) It doesn't matter if antenna is small, because it still has energy loss on any body placed near the antenna. For 30 kHz, even small antenna needs to be placed at least 1600 meters away from the ground. Ideally it should be placed into 5 km vacuum area.2) Small antenna needs very high voltage and current. It leads to energy loss due to heat.3) There is required tuner, which will helps to keep the part of energy which is not radiated for one cycle in the antenna system.Such piezo antenna in theory can solve 3. But regarding to issues 1 and 2, they cannot be solved, because they are present for any kind of antenna.

author: Marco
author_rank: Super Contributor
timestamp: April 19, 2019, 07:28:57 pm
content: Quote from: radiolistener on April 19, 2019, 06:57:39 pmIt's easier to have a low ohmic connection to a capacitor plate on a piezo crystal than having a low ohmic km long wire.

author: radiolistener
author_rank: Super Contributor
timestamp: April 19, 2019, 08:08:38 pm
content: Quote from: Marco on April 19, 2019, 07:28:57 pmThe energy in the long km wire produces radiation. The energy in the capacitor is just heating wires and capacitor plates and adding dielectric loss. And these energy losses in the capacitor and inductor are multiplied by Q, which is 300000 for this antenna...

author: Marco
author_rank: Super Contributor
timestamp: April 19, 2019, 09:54:09 pm
content: Exactly the opposite ... in an electrically short antenna high Q is a must. Energy has to be able to slosh around with low losses (ie. high Q) because so very little of the stored energy gets radiated over a given time period.The fact it has such high Q is why it's efficient, but it makes modulation hard.

author: radiolistener
author_rank: Super Contributor
timestamp: April 19, 2019, 10:29:13 pm
content: Quote from: Marco on April 19, 2019, 09:54:09 pmNo. There is a good example - magnetic loop antenna.It also small and has low efficiency just because it has high Q and as result extreme high voltage and amperage. It leads to high heat losses, especially at soldering points. If you will be able to eliminate this energy loss due to heating, you can get very efficient antenna, but it is too hard even with using thick copper pipe.Magnetic loop Q is about 1000. With Q=300000 heat energy loss will be much more high.High Q for a small antenna is required because it is small and radiates a small portion of energy from transmitter at one cycle. There is needs high Q in order to keep energy circulating inside antenna system and avoid reflection back to the transmitter. It allows to pump RF energy into antenna and store it until it will be radiated. This is how works antenna tuner. Higher energy = higher voltage and higher amperage. Higher voltage and higher amperage = higher heat loss and higher radiation loss. The problem here is that all components which is used to keep that energy inside antenna system has energy loss due to heating and dielectric losses at extreme voltage and amperage.

author: Marco
author_rank: Super Contributor
timestamp: April 19, 2019, 10:48:15 pm
content: You could damp the Q of a loop antenna by just using higher resistance wire, quite obviously wouldn't make it more efficient.

author: radiolistener
author_rank: Super Contributor
timestamp: April 19, 2019, 10:53:50 pm
content: Quote from: Marco on April 19, 2019, 10:48:15 pmYes higher resistance = higher heat loss and lower efficiency. And as result it leads to lower Q.But lower Q allows to get wider bandwidth.Sometimes it's better to get wider bandwidth to the detriment of efficiency.Very sharp resonance is also not good because you're get narrow bandwidth and needs to tune antenna more frequently.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2019, 10:58:42 pm
content: Quote from: radiolistener on April 19, 2019, 10:29:13 pmWhich Q?There is the Q of the components used, and the system Q.A system Q of 1000, with component Q of 300,000, will radiate very efficiently indeed.If you meant the other way around (component Q of 1000, with an attempted system Q of 300,000), then of course, system Q cannot be higher than component Q, and this would be extremely inefficient indeed!Cooling requirements aside, a superconducting loop would do very nicely for example. The present case is simply the electromechanical analog: using a high-Q mechanical resonator, and electric coupling, instead of magnetic coupling.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: April 19, 2019, 11:06:15 pm
content: Quote from: T3sl4co1l on April 19, 2019, 10:58:42 pmBut I don't believe that transform mechanical energy to electrical energy back and forth will be more efficient than electrical oscillations in the antenna system (antenna + tuner).

author: Berni
author_rank: Super Contributor
timestamp: April 19, 2019, 11:44:06 pm
content: High Q means lower losses are required in the system because this means the system has to be able to resonate really really well.Piezo ceramics can indeed resonate with very low loss. I have a large quartz crystal tuned for 9KHz that uses a bar of quartz as big as a pinky finger. You can connect the thing up to a signal generator to get it swinging at , disconnect it and leave it on the bench for over a minute and then connect it to a scope probe, and you still can see a 9KHz tone on it.There are no long wires that need to transfer energy around (like a loop antenna) where resistance could sap away the power. The energy is staying inside the ceramic, just changing form between electrostatic and mechanical forms. Sure the ceramic will still get hot when driven really hard, but the losses are not that terrible to make it unpractical. For example ferite rod antennas work great at low frequency too, but there losses are too high to be practical for transmitting.Still that being said this novel piezo antenna will likely never be as efficient as a good massive dipole antenna for the same frequency. But it is most likely a massive improvement in performance compared to any other VLW antenna small enough for a person to carry in one hand. Supposedly performing well enough to be usable in a mobile application. The attraction behind a resonant loop antenna is its small size, not its efficiency, same goes for this even smaller piezo antenna.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2019, 01:18:14 am
content: Loss is loss, it doesn't matter where in the system it is!And, if the radiation resistance is the dominant contributor to Q, then that's all you need for efficiency.Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: April 20, 2019, 03:30:37 am
content: Quote from: radiolistener on April 19, 2019, 11:06:15 pmBut that is why we use piezoelectric (quartz) crystal oscillators! A crystal has an equivalent LC circuit with much, much lower ESR than you could get in a comparable real inductor.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 20, 2019, 03:54:37 am
content: Quote from: Berni on April 19, 2019, 11:44:06 pmIsn't that a pretty good indication that it isn't radiating any of that energy?Quote

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 20, 2019, 06:31:51 am
content: A high Q means low bandwidth because the peak of the frequency response is higher. This is also why you want a very high Q resonator for a low-phase noise oscillator.If your overall antenna has a high Q, it means little energy is lost when put in (loss being any form through which energy is leaving the resonator - it might not necessarily mean loss in terms of heat). A highly efficient antenna (where efficiency is meant as 'the ability for the antenna to radiate') thus has a low Q - energy you put in leaves the resonator because it is radiated out.I suspect in this case they wish to make the antenna a low dielectric/mechanical/conductive loss device, and as a result lose as little energy internally to those factors. If you then shape the material such that it resonates, it can be a highly efficient resonator? And because of the piezo material it can be small with respect to the free-space wavelength (similar to dipoles in a high-permittivity substrate such as silicon, where they are much smaller than their free-space counterparts)

author: Berni
author_rank: Super Contributor
timestamp: April 20, 2019, 08:00:16 am
content: Quote from: vk6zgo on April 20, 2019, 03:54:37 amYes the tiny amount of radiation is the very problem why making a small 30KHz antenna is so hard. The VSWR of such an antenna is horrible since like 99.99% of the input power is reflected back out.The very high Q resonance is what is used to overcome this problem. Normally the reflected power travels back out along the feed coax into the RF power amplifier where its burned as heat. But a highly resonant circuit has the ability to recycle the reflected power and use it to add extra amplitude to the next wave of oscillation. Because almost all the power in such an antenna is being reflected this causes the resonant oscillation to build higher and higher making the circuit oscillate at a few kV even tho the input is only a few volts. At that point the amplitude is so large that even if a small fraction of a % is radiated that is still a large amount of power. This is what stops the resonance from building up any higher once the radiated/loss power is the same as the input power. As long as you keep your losses low the radiated power is now a large percentage of the input power (Like 1 to 50% ?) and the VSWR is now closer to 1 since the resonance is trying to suck power from the input in order to keep going.This same trick is used with wireless power transfer such as inductive Qi phone chargers. The coils are too far apart to make a nice efficient transformer, so resonance is used to easily raise the amplitude of the TX side until energy starts flowing out. At the same time resonance is used on the RX side to suck as much energy as possible from the near by coil despite a crappy coupling factor.

author: radiolistener
author_rank: Super Contributor
timestamp: April 20, 2019, 10:43:16 am
content: Quote from: Berni on April 20, 2019, 08:00:16 amI'm almost agree with your explanation, but you're forgot that large amplitude leads not only to higher radiation, but also tohigher heat loss and higher dielectric loss.Yes, high Q of antenna system is needed to keep energy inside antenna system and prevent reflection back to the transmitter. But at the same time high Q of antenna system doesn't means higher antenna efficiency.There is example. You can remove antenna at all and make high Q tuner which will keep all summed up energy and it will dissipate it into heat with almost zero EMF radiation efficiencyWhen we talking about antenna efficiency, it depends on the resistance ratio:η = Rrad / (Rrad + Rloss)whereRrad - radiation resistanceRloss - loss resistanceRloss = Rs + Rd + RnwhereRs - resistance due to skin effectRd - resistance due to dielectric lossRn - resistance due to heat loss on bodies placed into Near Field regionIn other words, antenna efficiency is a ratio between EMF radiation loss and heat loss.Antenna system with high Q uses more oscillation cycles in order to radiate EMF and heat. It means that energy will be loss multiple times (Q times to simplify). It leads to energy loss Q times for EMF radiation, but also it leads to energy loss Q times for heat.And don't forgot that very small antenna doesn't means mobility. Very small antenna (relatively to wavelength) still requires to keep away from any body for at leastR = lambda / (2 * pi)distance (the outer boundary of reactive near field region for very short radiator). For 30 kHz it will be 1590 meters. Otherwise you will get high loss on bodies placed into reactive near field region.With high power required to radiate VLF with a small antenna (due to low efficiency) and high Q factor it will be dangerous to be near such antenna due to extreme high magnetic and electric field strength.

author: radiolistener
author_rank: Super Contributor
timestamp: April 20, 2019, 11:15:52 am
content: Quote from: ejeffrey on April 20, 2019, 03:30:37 amHm, may be it's true, then this antenna is just attempt to replace impedance match circuit (antenna tuner) with a better efficiency crystal. But there are still other issues exists. The crystal should be as large as possible in order to get better efficiency. This may be problematic for crystal. And such antenna still should be placed far away from the ground in order to eliminate heat loss on the bodies falls into reactive near field region. It may be useful for spacecraft or weather balloon, because they are far away from the ground.Also, so high Q factor leads to low speed communication, it will be about 0.05 baud or something like that for 30 kHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2019, 01:08:38 pm
content: Quote from: radiolistener on April 20, 2019, 10:43:16 amRight. Suppose we have a capacitive plate, or wire, or top load (to use the Tesla equivalent) that has an impedance of several megohms. Suppose that it's mostly radiative loss (i.e., say 5MΩ radiation loss in parallel with 20MΩ equivalent parallel loss).But it's such an extremely high impedance, that we have no hope of tuning it as an antenna, with an LC tuner. Right?Right.Suppose then, we magically have a resonator, with a Q of 300,000, with a tap having an impedance of 5MΩ. Its equivalent parallel loss at that tap is therefore 5MΩ * 300,000 = fuck all resistance. Suppose it also has a tap down around, maybe not 50Ω, that would be a huge ratio, but say it's kΩ where we can still match to it reasonably with an LC tuner. Or maybe 50Ω is reasonable after all, it's a huge Q -- I don't know. Just mechanical tweaks really.Finally, suppose we wire these components together.Am I to understand that, just because the resonator hassome kind of loss whatsoever,that now, all of a sudden, it will be the dominant loss in the system, even though its equivalent loss is three hundred thousand times weaker than the load we've attached to it?I don't know about that.Quote from: radiolistener on April 20, 2019, 11:15:52 amThere we go.Yes, the crystal only needs to be a resonant transformer. It will inevitably have a high Q, which is possible thanks to its good mechanical properties and symmetrical mounting.Ground effects can be ameliorated somewhat by using a dipole (nothing stopping one from using a symmetrical resonator as a combination balun and transformer!), but it will still depend upon nearby equivalent dielectric losses. In effect, the radiation resistance as seen by the antenna might be dominant (giving high efficiency into the fields, near and far), but the radiation resistance, as integrated over the near field volume, might still include a lot of losses within that volume, so that the far field radiation still ends up a small fraction of it.On the upside, the impedance is very much higher than the ground impedance, which is not the case for a magnetic loop antenna, where the ground impedance is a sizable fraction. That is to say, the mag loop might have an impedance of fractional ohms, which is merely 1/1000th of the ~100s ohms ground impedance; but this antenna will have megohms, which is 10,000s (or more) times the ground impedance, so the efficiency can be that fraction higher.QuoteYes, so now we can come back to the original paper: by parametrically varying the resonator, transmission rates for a particular modulation (namely, FM, with some inevitable AM as well) can be much higher than the Bode-Fano limit of a LTI antenna.Reception rates won't be improved, or at least, you could vary the parameter in lockstep with what you're receiving and pick it up correctly, but, how would you know what you're receiving unless you've already received it? D'oh. So, no help there. Such symmetry violations are the price of exceeding the Bode-Fano limit.The proof of the pudding is ultimately in the eating, of course: if, say, the military isn't using this technology, it must not be worthwhile. They wouldn't need it for airborne transmitters, because they have line of sight to any band they wish (ground, aviation, satellite..). It would be useful for subs, but only when they surface (the impedance of ocean is even better, ~10s ohms, by the way), otherwise the E-field is shorted out by whatever's around it of course. And again, they can do that with a much more effective wideband antenna, if they're going to risk exposing anything above the surface at all.If it truly is more efficient as a transmitter, and can be scaled up to considerable power levels, it may prove cheaper to operate than existing methods. That may take time to implement though, so maybe the military isn't so hasty to adopt it.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2019, 01:10:48 pm
content: Something cute I don't think anyone twigged on yet...Quote...They hooked it up to a fuckin' Arduino and pair of shields!Tim

author: Berni
author_rank: Super Contributor
timestamp: April 20, 2019, 01:16:12 pm
content: Again i was not trying to say resonant antenna designs are efficient. They are not.The main benefit of a resonant antenna is that it can be physically much smaller while still having the capability to radiate out a significant percentage of the input power. Yes a good deal of the input power ends up as heat inside the antenna, but a dipole antenna of the same size would reflect almost all of the power back and burn it as heat inside the RF amplifier instead, making all together even less efficient when the whole system is considered. When you need an antenna to be physically smaller than 1/100th the wavelength then resonant designs are the only ones that can radiate a good percentage of the input power.If the antenna doesn't need to be small then yes resonant is a bad way to do it.All antennas can be dangerous to stand next to if a ton of RF power is pushed into them. Be it at 30Khz or 5GHz. But yeah 30Khz does mean you have to keep your power a lot lower.