Help with 58khz coil

[Dan71]

Its definitely going to be a thick coil with 1,200 turns on a 10 x 100mm ferrite rod
Also I have seen pictures of these devices with a lot less copper wire such as the one here:

from what I know, it only has 1 layer and its on a ferrite rod
Do you know how they used less copper and achieved the same effect?

 

[JimB]

1200 turns with 1mm dia wire will be enormous.

Do you know how they used less copper and achieved the same effect?

I don't know.
I assumed that what you wanted was a coil which was resonant at 58kHz.
Maybe the one with the thick wire is just a non-resonant coil and they batter it with a very high current.
I am just guessing here because I have never had anything to do with this type of equipment before.

JimB

 

[Dan71]

Okay, thanks Ill have a look and get back with results and so on.
Many thanks for the help.

 

[KeepItSimpleStupid]

From what I could tell, the sensor is basically a diode and another antenna. So, you have a modulated carrier at one of the ISM frequencies and if the modulation is detected, the alarm goes off.

I had an early RFID card and there was a diode(s) and 4 antennas. The F was swept and it causes a dip in signal strengthe at the reciver. That was translated into a "code".

At that point in time, the card I had in my pocket set off the anti-theft devices. Easy to defeat, if you know where the tag is.
One company put the tag in the ferrite in the line cord. Another company put it inside the power tool.

 

[Dan71]

From what I could tell, the sensor is basically a diode and another antenna. So, you have a modulated carrier at one of the ISM frequencies and if the modulation is detected, the alarm goes off.

I had an early RFID card and there was a diode(s) and 4 antennas. The F was swept and it causes a dip in signal strengthe at the reciver. That was translated into a "code".

At that point in time, the card I had in my pocket set off the anti-theft devices. Easy to defeat, if you know where the tag is.
One company put the tag in the ferrite in the line cord. Another company put it inside the power tool.

Hi. I was trying to create a jammer for the devices for experimental reasons, not the tag itself
Thanks.

 

[Dan71]

If the system is 58Khz ( there are 2 main types, 58khz aka AM and 8.2Mhz aka RF) you can pulse
an electromagnetic field specifically, it will jam the system and prevent it from detecting
the tags..
though some systems have jammer detection and of course there are ways to bypass that

I also couldnt take the ferrite out of the tag as it would be powerful enough.
Thanks.

 

[unclejed613]

RF begins where ever you want to start using it (even audio type frequencies, or sub-audio)

ITU regulations for radio services begin at 9khz... there are radio systems that operate below 9khz, and they are unregulated (and very difficult to get any usable range out of with any reasonable amount of power. the US Navy had an ELF system that operated on 76hz during the cold war. spelunkers have built radio systems for communicating between a cave and the surface that operates in the 1-3khz range...

 

[unclejed613]

Hi. I was trying to create a jammer for the devices for experimental reasons,

EAS Anti theft systems, like the ones you see when you enter
the store which beep,

i used to live down the street from a store that had a rfid anti-theft system, and as it turned out it operated on 13Mhz, not 58khz....

 

[Nigel Goodwin]

spelunkers have built radio systems for communicating between a cave and the surface that operates in the 1-3khz range...

Cave rescue also use low frequency radio systems, as I recall laying long wires on the surface above the caves.

We actually know the one of the guys who runs the local cave rescue, and there's been some discussion about him bringing one in to see if we could make something similar - we've helped him with various things so far (at work we both used to be cavers).

 

[Dan71]

i used to live down the street from a store that had a rfid anti-theft system, and as it turned out it operated on 13Mhz, not 58khz....

Hi. There are many systems out there..
The main company making 58Khz systems is Sensormatic
Their systems are actually very popular around where I live
Though of course RFID is starting to be used

 

[JimB]

There was an LF radio system developed by John Hay (G3TDZ) which gave good results in the limestone caves of North Yorkshire.
I think it may have used SSB on around 76kHz (from bad memory).

Having just called it a radio system, I have a feeling that the transmission medium was more magnetic than electro-magnetic.

JimB

 

[Les Jones]

Hi Jim, I did a bit of Googling about that system. I didn't find full details but these two link may be of interest.
https://rsgb.org/main/blog/news/gb2rs/headlines/2018/07/13/ham-designed-comms-support-cave-rescue/

https://hackaday.com/2018/07/11/ham-designed-gear-used-in-thailand-cave-rescue/

Les.

 

[JimB]

Yes, that's the guy.

JimB

 

[Pommie]

Hi Jim, I did a bit of Googling about that system. I didn't find full details but these two link may be of interest.
https://rsgb.org/main/blog/news/gb2rs/headlines/2018/07/13/ham-designed-comms-support-cave-rescue/

https://hackaday.com/2018/07/11/ham-designed-gear-used-in-thailand-cave-rescue/

Les.

Two very interesting articles, thanks. The trapped boys got a lot of airtime here in Australia but not one mention of the technical expertise needed to communicate.

Mike.

 

[unclejed613]

https://caves.org/section/commelect/spelonic.html this magazine/journal has lots of schematics for radio equipment from 1khz to 80khz to choose from... and it also should be helpful to Nigel as well...

 

[Dan71]

I had a play with the calculator, and after a bit of trial and error, I got this:

View attachment 132062

1200 turns of 30AWG wire, in 12 layers on a 10mm former gives 6.157mH

But we wanted 9.47mH the crowd shouted...

The calculation is for an air-cored coil.
When we put the coil on to a ferrite core, the inductance will double (or maybe more).
Actually the inductance will vary depending on the position of the coil on the rod, more inductance with the core in the middle of the rod, less inductance with the coil at the end of the rod.

I know that 1200 turns is a lot of wire and takes a lot of winding.
I use a lathe and a home made turns counter when I have to make a lot of turns.
It can be done by hand with a bit of patience. Been there, done that, got sore fingers.

JimB

Hi, I made the coil today and it works quite well except that it doesnt have much range.
if I upped the volts would that give it a little bit more range?

Also many thanks for the help.

 

[unclejed613]

if I upped the volts would that give it a little bit more range?

a coil on a linear form is not going to have much range. if you might have noticed, the coils in the store are in containers that are 3ft high x 2ft wide x 4 inches deep... the coils are "pancake" coils where it's on a flat surface and one end of the coil is at the center, and is laid down in a spiral and the other end of the coil is at the outside... very similar to the antenna coils of the RFID tags

 

[Dan71]

a coil on a linear form is not going to have much range. if you might have noticed, the coils in the store are in containers that are 3ft high x 2ft wide x 4 inches deep... the coils are "pancake" coils where it's on a flat surface and one end of the coil is at the center, and is laid down in a spiral and the other end of the coil is at the outside... very similar to the antenna coils of the RFID tags

Hi, It is not my point to create a huge coil like the stores use.
I wanted to create a jamming device for that frequency and I was able to.
I upped the voltage and it gave me the results I needed.
Thanks.

 

[JimB]

I upped the voltage and it gave me the results I needed.

It is good to hear about a bit of success. Thank you.

JimB