Help with 58khz coil

[Dan71]

Hi. Im pretty new to coils and specifically tuning them, I was looking for some help on
how I would go about tuning the ferrite core antenna to transmit at 58Khz..
Any help would be appreciated.
Many thanks.

 

[alec_t]

Welcome to ETO!
58kHz is in VHF broadcast band 1. Where is it legal for you to transmit on that frequency?

 

[Nigel Goodwin]

Welcome to ETO!
58kHz is in VHF broadcast band 1. Where is it legal for you to transmit on that frequency?

Having a senior moment

58 KILOHertz is a very long way from band I

It's ultrasonic, and presumably still illegal to transmit RF on?.

 

[alec_t]

Oops! Yes, senior moment

 

[JimB]

It's ultrasonic,

While the frequency is low, this is not ultra sonic.
Sonic implies a pressure wave, like sound.

This is low frequency RF, ie an electromagnetic wave.

However, I suspect that the OP is not going to transmit very far past the end of his workbench using a "ferrite core antenna" at 58kHz.

What is a ferrite core antenna?
Is that a ferrite rod antenna, or as I believe it is referred to in the USA a "loopstick" antenna.

More information is needed from the OP, what is he really trying to do?

JimB

 

[Nigel Goodwin]

While the frequency is low, this is not ultra sonic.
Sonic implies a pressure wave, like sound.

This is low frequency RF, ie an electromagnetic wave.

The frequency is ultrasonic, which was the point - that it's not VHF.

However, I suspect that the OP is not going to transmit very far past the end of his workbench using a "ferrite core antenna" at 58kHz.

What is a ferrite core antenna?
Is that a ferrite rod antenna, or as I believe it is referred to in the USA a "loopstick" antenna.

I read it as 'ferrite rod', and it's not too far off the bottom end of LW - if you put two LW coils on the same ferrite rod you should easily be able to tune down as low as 58KHz. Even adding extra capacitance to a single long wave coil should get it down there.

And of course it's pretty close (60KHz) to the MSF time transmitter that used to be at Rugby.

 

[JimB]

The frequency is ultrasonic,

Can you explain at what frequency ultrasonic ends and radio begins?

JimB

 

[Nigel Goodwin]

Can you explain at what frequency ultrasonic ends and radio begins?

JimB

As you well know, RF begins where ever you want to start using it (even audio type frequencies, or sub-audio) - the use of 'ultrasonic' was simply to signify easily where abouts in the spectrum it is.

 

[JimB]

There are formal definitions for the various frequency ranges as applied to radio etc...

3kHz to 30kHz Very Low Frequency (VLF)
30kHz to 300kHz Low Frequency (LF)
300kHz to 3MHz Medum Frequency (MF)
3MHz to 30MHz High Frequency (HF)
30MHz to 300MHz (Very High Frequency (VHF)
300MHz to 3000MHz (Ultra High Frequency (UHF)

Using the term ultrasonic to describe something which is electromagnetic in nature is misleading.

JimB

 

[Dan71]

Hi. Im trying to make the antenna to transmit at 58Khz as I was doing some research on EAS Anti theft systems, like the ones you see when you enter
the store which beep, I needed it at that frequency as I was working on a jammer purely for testing and not actually shoplifting..

From what I can tell, the system creates a sort of electromagnetic field which it pulses on, off

It doesnt have to be a long range antenna and probably should not be.

Also, JimB Yes it would be a Ferrite rod antenna, Sorry I was a bit sleepy and mistyped
Thanks.

 

[Dan71]

Also it would mostly be generating an electromagnetic field, there would be a field effect transistor which would be controlled by a microcontroller to
open and close quickly which would be wired to the ferrite rod antenna.

 

[JimB]

OK Dan, that is making sense now, I understand what you are trying to do.

A ferrite rod type antenna should work OK for that application.

To get an idea of the size of coil you would need, consider a simple long wave transistor radio,
the lowest frequency is about 150kHz, this is tuned with a capacitor of about 310pF.
If we do the maths, that gives an inductance of about 3.67mH

You want to tune to a frequency of 58kHz, so the inductor and the capacitor need to be scaled by a factor of 150/58 = 2.58
This will give an inductance of 9.45mH and a capacitance of 800pF to resonate at 58kHz.

Now the big question is, how many turns of wire will be required to give that inductance?
Quick answer is I don't know, I would have to get the books out to find the formula for a multilayer coil.

Does that help?

JimB

 

[Dan71]

Hi. Yes that helps.
Where should I look for how mant turns of wire I would need?
Thanks.

 

[JimB]

Try this:


I put
"inductance of a multilayer coil"
into google and the link above was about the first in the list.
I have not tried it yet, I may try it later.

JimB

 

[Dan71]

Thanks

 

[JimB]

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

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

 

[Dan71]

Hi. Thanks for the reply
Will it change the properties depending on the wire diameter? and diameter of ferrite rod?
Thanks.

 

[Dan71]

as I have 10mm diameter copper wire

 

[JimB]

Will it change the properties depending on the wire diameter? and diameter of ferrite rod?

The quick answer is yes.
The wire size alters the overall dimensions of the coil and hence the inductance.
The diameter of the rod makes a difference, the bigger the rod the more "magnetic stuff" and the greater the inductance.
If I were making this, I would use a former on to which the coil would be wound.
The former would be of a suitable diameter to slip over the rod.
I would not wind the coil directly on to the rod.

I have 10mm diameter copper wire

That is copper plumbing pipe!

JimB

 

[Dan71]

Hey, Sorry that was wrong, Mistyped its actually 1mm diameter.......