THz waves

[Stefano]

This topic will be about generating, detecting and imaging with THz waves.

 

Regarding generating EM waves in the radio spectrum, this are the Italian laws:

 

-LPD (Low Power Device): operating at 433 MHz, maximum power 10 mW, they can be used freely; now obsolete, they should be replaced by SRD (Short Range Device);

-PMR 446: operating at 446 MHz, maximum power 500 mW, they require a declaration of use and the payment of 12 €/year;

-CB (Citizen’s band): same as above, 27 MHz and maximum power 4.5 W;

-To be an amateur radio operator (“ham”) you also need a license, that you obtain by passing a written exam.

 

Hopefully, THz frequencies are outside those ranges, and so they can be generated without authorizations. They can only travel a few meters in air and can not go through walls.

 

But anyway I am more interested in detecting them, not generating them.

 

I was inspired by this series of videos;

 

The easiest thing is making the antenna: very thin, very long (to have high gain), and it shouldn’t be too difficult (even if the diameter would need to be about 0.3 mm). At THz frequencies everything becomes messy, and a couple of pF of parasitic capacitance is enough to ruin everything. It isn’t easy.

[enricosavazzi]

[...]THz frequencies are outside those ranges, and so they can be generated without authorizations. They can only travel a few meters in air and can not go through walls.[...]

I think the Italian law used to be that everything else outside the permitted bands is forbidden. I don't know whether this has changed the last 40 years. At the time I left Italy they had just relaxed the laws about FM transmitters, and private FM radio stations, often just personal stations with a range of just a couple of Km, were popping up everywhere. Those were also the early days of private TV channels in Italy. No one would have believed that within a few years media moguls would become the richest people in Italy by making money mostly from private TV stations.

 

THz radiation should be safe to generate and use (law-wise, although not necessarily health-wise) because the authorities have no way to detect them beyond a range of a few m. The government should also not be particularly interested in hunting transgressors because these wavelengths are not used in communication systems. It might quickly change if someone is caught using portable THz transmitters to blind the THz inspection machines in airports, though.

 

Laws already exist, for example, to prevent tampering with the frequencies used in mobile networks in order to create phone-free areas. I think possession of this type of device may be a crime in itself, even though the device is not being used. So the safest thing to do with THz transmitters is building or purchasing the key parts, like antennas and transducers, as separate components, and always keeping the transmitters indoors on private property once assembled.

[Stefano]

To detect them I was thinking about an axial mode helical antenna, very long to have very high gain (in my case I am interested in a small beam angle, which is small for both transmission and detection). The antenna should be even smaller than what I wrote above: it should have a diameter of about 0.1 mm and a pitch of about 0.07 mm. Then, how can I detect the signal? Even building a LC circuit tuned to 1 THz is very hard. The inductor and capacitor need to be so small (in value) that your circuit becomes a C shaped piece of metal, with the capacitor being the gap and the inductor being the incomplete single winding.

 

Then... how about transistors? They shouldn’t work at frequencies even near 1 THz. Can I get a very weak signal, maybe by trying to reduce its frequency or by nesting transistors to increase the gain (like I suggested here https://www.ultravioletphotography.com/content/index.php/topic/3634-some-ideas/page__st__80)?

[GaryR]

Most secure radio frequencies are trunked and encrypted these days, so it's unlikely that you'd hear anything, even if you dialed in the correct frequency. It's more likely that you may be in trouble with local laws, if you possessed a radio that could decode restricted frequencies. In Canada, there aren't any restrictions on scanners, but USA still blocks the old cellphone band (824-849 / 869-894 MHz) on all their radio scanners. US citizens can legally own an unblocked radio, but are breaking the law if they monitor those frequencies...the point is, how would anyone know, if you're not transmitting.

 

I don't know of any SDR (Software Defined Radio) device that can operate in the THz range, but I suppose it's just a matter time before someone invents one. The widely available HackRF One SDR receives from 1 MHz to 6 GHz.

[Stefano]

I want to see natural sources (mainly thermal) of THz radiation. I don't want to decode anything.

[Stefano]

I have to avoid using the antenna in normal mode, so I have to filter the signal coming from it.

[Stefano]

Can I try using nested transistors to detect the signal? Then filter it with a very small value capacitor in series or a very small value inductor in parallel? Actually winding the antenna with a 0.1 mm diameter isn't that easy... I can use a super high AWG enamelled copper wire, wound around something. If I use a metal wire as the core I will have to remove it

[Stefano]

Is there someone here who has good electronics knowledge?

[Andy Perrin]

Is there someone here who has good electronics knowledge?

No (or to be more precise, if there are, I haven't seen evidence of it), that's why I suggested you needed a more EE-focused forum. Awhile back a member here referred me to this one:

https://www.eevblog....forum/index.php

 

I have good basic electronics knowledge (simple op amp circuits, logic, etc.) but you need a true expert in RF and microwaves for this.

[ulf]

Already at 1GHz electronics has become rather sensitive to the physical shapes of conducting parts and their surrounding electromagnetic fields.

You go from common conductor design to strip-lines, micro-strips and waveguides.

The dielectric constant of materials become more and more important and designs must be done with special RF-optimised materials.

For PCB designs, already at lower GHz you omit the thin normally green insulated laquer on some areas with sensitive microstrip-designs. https://en.wikipedia...wiki/Microstrip

 

Just as Andy say THz technology is a field for very specialised knowledge and techniques.

For someone without that this is an area of advanced technological "Magic", to be handles by specialist gurus.

You must have very much luck to reach any success fiddling with this at home.

 

An analog to this in UV might be to compare our struggling imaging and possibly reach to 300nm with our converted cameras, with trying EUV: https://en.wikipedia...let_lithography

[Stefano]

If you have 80K+ to spend...

 

https://www.google.com/url?sa=t&source=web&cd=2&ved=2ahUKEwiczJ3v5orpAhWm_CoKHTEhAVQQFjABegQIAxAC&url=https%3A%2F%2Fwww.terahertzstore.com%2Fproducts%2Fterahertz-cameras%2Fterahertz-camera-for-thz-imaging-cea-leti-tzcam.htm&usg=AOvVaw2SWkAAegJabWErACIHmtmL

 

The lens is sold separately and costs more than $10,000.

 

https://www.google.com/url?sa=t&source=web&cd=12&ved=2ahUKEwizgZuk6IrpAhVHl4sKHb3rCF8QFjALegQIAxAC&url=https%3A%2F%2Fwww.terahertzstore.com%2Fstore%2Fproducts%2Fterahertz-cameras%2Fterahertz-camera-lens-attachment-for-tzcam.htm&usg=AOvVaw0wgI_Ogjay8-dpkOS85c-S

 

Surely a cool toy to play with. Just a bit expensive.

[ulf]

The uncooled microbolometer technology used is similar to the one used in thermal cameras.

 

Bolometer-technology isn't new.

Read about Langley's bolometer here:https://en.wikipedia.org/wiki/Bolometer

[Stefano]

So I guess I can modify a thermal camera to see THz waves? Ignoring diffraction-related issues. Microbolometers should be sensitive to a wide variety of EM waves, since they rely on heat (and, BTW, infrared radiation and heat are NOT the same thing, although closely related). And I would need to change the lens.

[dabateman]

If you have 80K+ to spend...

 

https://www.google.com/url?sa=t&source=web&cd=2&ved=2ahUKEwiczJ3v5orpAhWm_CoKHTEhAVQQFjABegQIAxAC&url=https%3A%2F%2Fwww.terahertzstore.com%2Fproducts%2Fterahertz-cameras%2Fterahertz-camera-for-thz-imaging-cea-leti-tzcam.htm&usg=AOvVaw2SWkAAegJabWErACIHmtmL

 

The lens is sold separately and costs more than $10,000.

 

https://www.google.com/url?sa=t&source=web&cd=12&ved=2ahUKEwizgZuk6IrpAhVHl4sKHb3rCF8QFjALegQIAxAC&url=https%3A%2F%2Fwww.terahertzstore.com%2Fstore%2Fproducts%2Fterahertz-cameras%2Fterahertz-camera-lens-attachment-for-tzcam.htm&usg=AOvVaw0wgI_Ogjay8-dpkOS85c-S

 

Surely a cool toy to play with. Just a bit expensive.

 

See if you can find one on ebay for $100.

 

Also looks if you buy in bulk there might be a discount.

Actually not joking about searching ebay. Some stuff is really cheap now. I might get a fully working spectrometer soon.

 

[Stefano]

Sometimes you can find really valuable things on ebay, if you are careful and know where to look.

 

This guy found a true UV laser diode, a 375 nm direct (non DPSS) laser diode, in a scrap part he bought for $80 on ebay. If you want a similar diode (maybe it is the exact same one) from Thorlabs it would cost more than $4000.

 

[Andy Perrin]

I got my TriWave on eBay that way. Deals are out there but sometimes you have to wait a looong time. I hunted for it for ages, since seeing Nick’s post on the board here.

 

I would say you have a better chance of buying a THz cam than building one.

[Stefano]

I would have a 1 THz LC oscillator with a 0.3766 pF capacitor and a 0.0672 pH inductor. I can make the capacitor using 2 wires, 1 mm in diameter, 5 cm long and 2 cm apart, but that inductor is simply impossible to make. Using a helical antenna I would probably use it in normal mode for much lower frequencies rather than axial mode for 1 THz. I can filter out lower frequencies by using a very low value capacitor in series, maybe.

[Stefano]

I never understood how you can use a diode to receive radio waves if the voltage on the antenna is usually much lower than the forward voltage of the diode. Is it possible because you connect the antenna to a LC circuit which resonates and the voltage becomes higher?

[Andy Perrin]

I never understood how you can use a diode to receive radio waves if the voltage on the antenna is usually much lower than the forward voltage of the diode. Is it possible because you connect the antenna to a LC circuit which resonates and the voltage becomes higher?

It is actually an issue for them -- that's why people use germanium or Schottky diodes which have a lower forward voltage of 0.2-0.3V. So you need a very strong AM signal for one of those radios to work. LC circuit doesn't amplify the signal, it can only decrease the signal outside the chosen band.

 

(LC circuits can definitely increase voltage, but total power still has to be conserved. I think I need to be careful with the word "signal" here.)

[Stefano]

Is it different for FM?

[Andy Perrin]

I'm not sure enough about how FM receivers work to say, really? Definitely I don't see a reason why an FM receiver would even need a diode since it's not trying to detect the envelope of the signal, which is the purpose of the diode.

[Stefano]

You have to amplify the signal, and if you use semiconductors in the amplifier you always have a voltage drop (I think).

[Andy Perrin]

Well if you amplify the signal then there's not an issue. I thought the question was about passive receivers (no power input except the radio wave).

[Stefano]

The question is about both. How can you hear your radio if the voltage on the antenna is too low? If it is higher than the voltage drop of common diodes, there should be no problem. But since radios normally have to amplify the signal in order to drive a speaker, and the voltage across the antenna is in the order of microvolts (from what I understood) how is this possible? If the voltage is very low, you probably can't even detect it since it is AC.

[Stefano]

I have to understand this in order to even hope to detect terahertz waves. If I have, say, 0.0001 V AC across the antenna, I need a way to detect it. A diode can't do that. A transformer to boost the voltage would be completely useless at those frequencies, even if it is an air-core one. It seems an impossible task.