Off the shelf loop antenna for frequencies < 1kHz

[anfractuosity]

Hi,

I'm just wondering if anyone could recommend
a loop antenna for frequencies < 1kHz, that I could buy
off the shelf?  (I wouldn't be too confident to make an antenna
myself)

I would be planning on attaching it to a sound card.

Thanks!

[coppercone2]

the spec on commercial loop antenna usually begins at 1Khz, even for the big old ones.

These kinds exist
https://electro-metrics.com/product/antenna-loop-em-6874-20-hz-30-mhz/

So yeah making it yourself is hard if you want to make it as nice as that, you are talking pretty thick brass thats bent with a slot on top.

If you want unshielded its as easy as buying a 2 meter long flat bar aluminum from home depot and bending it around a barrel or something, and terminating it to a plastic electrical box with some bolts and solderable star washers.. that is a 2 hour project if you have a vise, and a torch helps to get the end bends. If you don't mind a square shape, you can do it by hand on the floor. I don't think you would have problem screwing it down a block of wood either if you paint it

https://www.amazon.de/-/en/7314-Solder-terminal-Soldering-Screwed/dp/B01N468DT2

[anfractuosity]

Thanks a lot for the link to a particular commercial antenna!

For some reason I thought the loop had a coil of enamelled wire over it, but it sounds
like that's not the case?

I will definitely look more into DIY versions too.  I'm a bit unsure
as to what determines the lowest frequencies you can receive?

I just found https://www.eeweb.com/reception-of-radio-waves-below-22-khz/ where they mention
using a pre-amp prior to the soundcard, but I assume I could start by connecting the antenna to hot/cold
of an XLR plug.

[coppercone2]

if you have multiple loops that is a different antenna then what I was thinking of. I was thinking of a active loop with 1 turn

[radiolistener]

For what reason you're need it? If you're needs calibration, then you can use something like link above. But if you're needs high sensitivity it doesn't works for that purpose, in such case you're needs resonant antenna with high Q LC circuit. Usually it will be a large dimension inductor with a high Voltage variable capacitor.

you can also use a long traveling wave antenna. Such antenna often is used for ELF and VLF band communications with submarines.

Just remember that larger dimension antenna is more effective, because it cover larger part of electromagnetic wave in the space.

Shortened antennas needs to be resonant in order to keep efficiency. At ELF band resonance have to be very sharp and antenna requires precise tuning on exact frequency. It won't allow you to see wide bandwidth on the waterfall.

[coppercone2]

I am not even sure how what those little loops are used for in the lower frequency range. I have seen them sitting around before ,but its like desktop ornaments. Its as good as a miniature saint louis arch

Because they have like sensors that are used for low frequency range for components inside of equipment (wands)... so what use is a bench top 20Hz antenna? I am not sure.

[anfractuosity]

I'd like to attempt to do something similar to this paper - 'Experimental Generation of ELF Radio Signals
Using a Rotating Magnet' where they generate low frequency RF by spinning a permanent magnet.

I've got a similar magnet to what they used now, but am a big stuck as to the receiving aspect, which
requires the loop antenna.

They say -

"The portable ELF receiver, shown in Fig. 2, consists of three orthogonal 1 Ω−1 mH square-loop antennas with a
matched, low-noise amplifier for each channel"

[mag_therm]

1 Ω−1 mH square-loop  ??

Possible capacitor values for a resonant loop:

Capacitors non polar:
Surplus 5 uF oil filled motor capacitors wiil maybe have low loss.
Put 5 in parallel = 25 uF

At 800 Hz Xc = -j 7.9 Ohm

So the loop needs j 7.9 Ohm = >  1.6 milliHenry

Using a solenoid of 1 * 1 metre diameter ( or square):  A = pi* D^2/4 metre -squared
1 metre long: len = 1
A = 3.14 * 1 *1 /4 = 0.785 metre_squared
L = Uo * N^2  * A/len
1.6 E-3 = 4* pi*1e-7* N^2 * 0.785 / 1

gives > N = 40 turns.

Use wire thick enough to give your bandwidth. Check my calc and adjust to the needs.

[Wallace Gasiewicz]

An efficient magnetic Loop antenna would have to be huge for 1 KHz. Huge both in length and in the size of the radiator (pipe).
Surface area has to be huge for any efficiency at 1 KHz. You did not say if you are trying to receive or transmit.
A Mag Loop s just a resonant LC circuit. If it is small like the ones in a radio, it does not emit much radiation.
Sure, you can receive a signal from a resonant LC circuit even inside a radio with a probe, but how close do you want to be?
There are various calculators on line  that quantify these factors for you. There are also groups on groups.io that specialize in these antennas
These antennas typically have very low resistance, much less than the one ohm that is listed in the commercial loop referred to.
I think this thing is used for testing at close distances and probably is powered by quite a few watts in practice. I do not think it is anywhere near efficient at all.
Again, it is an LC circuit. The loop is the inductor and the separation space at the top is the capacitor portion of the LC circuit.
Mag Loop Developers use silver solder, brazing and welding to lower resistance. Even regular solder joints are not acceptable.
The voltages developed in the LC circuit typically are in the thousands, even with only a Ham transceiver input.

The caps used are rated in the tens of thousand volts. Vacuum Variable caps are one type used at HF Freq. Your voltages will be less since it is powered from an audio card.
When you start winding the antenna to make a resonant coil. the efficiency or radiation resistance goes down. It becomes terrible. even a two turn loop is hard to develop.

[anfractuosity]

Thanks for all the replies, sorry to clarify I'd like to just receive with the loop antenna
and I'd be transmitting low frequencies to it via a spinning permanent magnet, like in the paper.

"Use wire thick enough to give your bandwidth" -- is the thickness of the metal of the loop, the main parameter
that determines the bandwidth then?

"The caps used are rated in the tens of thousand volts. Vacuum Variable caps are one type used at HF Freq. Your voltages will be less since it is powered from an audio card." - I wasn't sure what was meant here,
do you mean the LC circuit would end up generating very high voltages? Which would need to be stepped down before being fed into the soundcard?

Many thanks

[TimFox]

If you need several uF with high Q (low loss) for 1 kHz, metalized polypropylene is the modern capacitor choice.

[Wallace Gasiewicz]

The surface area of the antenna is what transmits so both length and diameter is important. They do not use wire but 2-3 inch tubing . Sometime even bigger plastic tubes wrapped with foil.
For Rx not much voltage. But for Tx:
Off the cuff, 100 W from a 50 ohm Ham transmitter into a 10 foot dia loop will generate 10K Volts after you match the loop with your transmitter with the appropriate match like a gamma or another small loop. Matching is not easy, either.
Huge amounts of power are transferred back and forth  from the "L" to the C" of the Tank circuit.
People have burned holes in expensive 15,000 volt Vacuum Variable Caps.
I built a Mag Loop and it worked just fine for 40 Meters to way over 10 Meters
I do not know how much power your transmitter is going to make. Maybe a few watts?
For Rx any really good cap will do but for Tx you need high voltage caps. They make caps from tubing (Trombone caps for example) and metal plates and use variable caps with big plate distances. Butterfly caps also. You probably need a variable cap for tuning, maybe an old variable cap from an old transmitter would work for low power. These antennas have to be tuned exactly to the resonant freq. The "Q" is very high and tuning is very sharp, as a result of the high Q.
Tuning the loop is important for Tx and also RX. It will do a good job of eliminating nearby and far away frequencies. Again, it is nothing but a fancy very large LC Circuit. A LC circuit is a tuner. In this case a very high "Q" tuner.
Join the Magloop group and read some of the info:
https://groups.io/g/MagLoop/messages
Or this group:
https://groups.io/g/HelicallyLoadedMagLoop/messages
I really do not think that a small loop is a good idea for VLF or ELF. I think it would be way too inefficient.

[eliocor]

http://www.vlf.it/
Nothing more, nothing less.... 
Just some examples:
http://www.vlf.it/feletti2/idealloop.html
http://www.vlf.it/cr/differential_ant.htm
http://www.vlf.it/poggi3/dualpolarization.html
http://www.vlf.it/romero3/ics101.html
http://www.vlf.it/torsten/_B3CKS-ANTENNA.htm
http://www.vlf.it/minimal2/minimal2.html
http://www.vlf.it/minimal/minimal.htm
http://www.vlf.it/bikeloop/bykeloop.htm
http://www.vlf.it/inductor/inductor.htm
http://www.vlf.it/easyloop/_easyloop.htm

[coppercone2]

ok that is a receiver. Those are basically inductors. These very low frequency antennas for receiving are basically electromagnets, usually wound on a specific high mu core, which you can buy, but its a little expensive.

https://www.stormwise.com/page26.htm

That might be one way to do it.

or if you want air core (not like a typical portable AM radio, which is a rod), you can do something like this
https://physicsopenlab.org/2020/05/03/loop-antenna-for-very-low-frequency/


*****
you can buy this kind of loop antenna here, for a price.
https://www.lfengineering.com/products.cfm

[radiolistener]

is the thickness of the metal of the loop, the main parameter that determines the bandwidth then?

No, the antenna bandwidth is determined by antenna Q-factor:

BW = 1/Q

where BW is normalized bandwidth (relative to the resonant frequency)

The thickness of wire affects Q-factor (and as result also bandwidth), but thickness is not the main parameter which determine it.

If you want to find the main parameter which affects antenna bandwidth, for resonant type of antenna it will be a physical size of antenna. Small resonant antenna has narrow bandwidth, because it requires high Q-factor. Large resonant antenna has wide bandwidth, because it has small Q-factor. But this rule doesn't works for a non resonant antenna, such as a traveling wave antenna, because it has a large dimension, but low Q-factor, so it has wide bandwidth.

But note, there is a back side of wide bandwidth. Wide bandwidth antenna has a bad sensitivity and low efficiency. If you want to receive very weak signal, your antenna needs to have very narrow bandwidth with a sharp resonance.

[coppercone2]

for the mechanical generator I am not sure I would go with high Q. That is useful when there is something like a crystal working. Don't know what RPM the system will be good at, 1KHz mechanical sounds an awful lot like "lets try here". and "oh look its still not tearing itself apart on a dremel".

What I want to know about those magnetic generators is basically if you have just a magnet, just a electret, or a magnet on top of a electret mounted to the rotor.

I think you want broad band + fft to see what its doing across the spectrum, harmonic contents, etc.. for initial design. How do you know if the shaft is maybe doing something silly and its being a troll? When its like a stable oscillator then you can go into developing a high fidelity receiver. If you had a tested working transmitter with a spec, then you could go right into that.

I expect its going to be ugly. And you are close by, you need that Q when you start taking far away measurements.

[Wallace Gasiewicz]

One of the limiting factors of mag loops is the issue that the Radiation Resistance for a small radiator is Small. Therefore the actual resistance to the freq used must also be very low to get any efficiency. Therefore the Q is very high and band width is low. If you increase the resistance the antenna does not work very well as a radiator.

[RoV]

Anfractuosity, complexity depends on what you are going to obtain.
If you just want to play with a rotating magnet in extremely near field (I mean up to a few meters of distance), you can build a loop of a few turns of normal electrical wire or thick magnet wire, 50-100 cm diameter. I would avoid resonating the loop with capacitors, instead I would build a low pass filter followed by a low noise preamplifier, both designed for very low impedance. You can find nice examples in "The Art of Electronics", where inexpensive BJTs like the ZTX851 are successfully used for low noise @ low impedance, for example for ribbon microphones (paragraph 8.5.9).

In the past I have designed an ELF system to communicate from the inside to the outside of a 30 mm thick steel pipe (commercial equipments existed for the purpose, but were not sold alone). In that case the working frequency was around 20 Hz and I used linear solenoids having cores made with packs of transformer-grade steel sheets. Coils were of ~1k turns of thick magnet wire and -yes- in that case resonance was necessary both in TX and in RX sides. I also considered the rotating magnet as a transmit option, but in the end discarded it, because it was easier to use each coil both for TX and RX.

[coppercone2]

3cm of steel is pretty nuts

would like ultrasonic not be better there? It seems a little wasteful to use magnetic field to communicate through a standard magnetic shield

[RoV]

3cm of steel is pretty nuts
would like ultrasonic not be better there? It seems a little wasteful to use magnetic field to communicate through a standard magnetic shield

Ultrasound works well if you have access to opposite sides of the steel wall. But if you are at a distance, and there is air inside, it becomes difficult. The specific problem can be seen here https://www.ppsa-online.com/papers/17-Aberdeen/2017-09-TDW-slides.pdf (this is not the system I worked on). They claim being able to track a pig with 80 mm pipeline wall depth.

[coppercone2]

oh ok, I was imagining some static object inside of a particle accelerator or something

[Kleinstein]

For receiving the magnetic antenna with a magnetic core (e.g. ferrite) is an option. For the 1 kHz range this could be steel, like electric steel, ideally not that thick in the dimentions (e.g. 0.3 mm laminations).
AFAIK the length of the rod about corresponds the loop diameter of a just copper wire loop.

The problem with high voltages only applies to sending, because of the low efficiency. For receiving the problem is more that the signal is relatively low impedance and low voltage unless the Q factor is high.

Many of the experiments at these low frequencies would likely be in the near field anyway, as the near field is quite large (e.g. 150 km for 1 kHz). The characteristics (directionality, efficiency) for the near field can be different from far field.
It is more like measuring the AC magentic field, not really an EM wave.

[Marco]

I'd like to attempt to do something similar to this paper - 'Experimental Generation of ELF Radio Signals
Using a Rotating Magnet' where they generate low frequency RF by spinning a permanent magnet.

They are also generating low frequency RF, but they are detecting a low frequency magnetic field. Their receiver is at over a 1000 times less than the wavelength, why the hell would you assume the electric field plays any part at all? Pure inductive coupling.

An electromagnet could produce the same levels of RF and magnetic field as that turning magnet BTW. Also the receiver could be a lot smaller if they used a ferrite core. This paper is just a reminder that tuned circuit inductive coupling is detectable over surprisingly large distances.

[Wallace Gasiewicz]

Rov:
How well does the magnet radiate?
Is it good for longer distances or is the field relatively contained?

wally

[RoV]

Rov:
How well does the magnet radiate?
Is it good for longer distances or is the field relatively contained?

wally

I didn't make tests with the magnet, only with the solenoids of the photo in my previous post.
Regarding distance: I was focused on pipe thru-wall transmission, so I didn't test communications at some distance in air without obstacles. However, this is something that can be tested in simulation: I have used the free tool FEMM and Comsol.