# Ferrite rod antenna explanation

Discussion in 'Radio and Wireless' started by michael1978, Apr 9, 2018.

1. ### Audioguru

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Sep 24, 2016
This is an extremely simple radio with only ONE LC tuned circuit.
If the tuned coil has many more turns than the coil that feeds the transistor then the bandwidth is narrower resulting in less interference from other radio stations but high audio frequencies will be reduced. Then YOU must decide if you want a narrower bandwidth for less interference or a better high frequency response. YOU know how many AM radio stations you have in your city that will cause interference, we don't. YOU know if you like listening to RAP instead of real music, we don't.

This radio regenerates and the ratio of the coils will cause it to be very sensitive and probably break into noisy oscillation or not.

2. ### duke37

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Jan 9, 2011
L1 is the resonant circuit. The other windings are much lower impedance because of the fewer turns. A regeneration radio has positive feedback which modifies everything. I cannot help with how to determine the correct values.

3. ### michael1978

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Mar 17, 2012
AuduGuru i am reading one book he say is rf transformer, is true?

4. ### michael1978

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Mar 17, 2012
thanks Duke can you help me just for coupling inductor, one example please

5. ### duke37

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Jan 9, 2011
A mains transformer has a primary winding and a secondary winding. The coupling between them is enhanced with a metal core. The coupling approaches 1.

The ferrite cored aerial has a three coils each coupled to the others and the coupling will be dependent on the permeability of the ferrite and the position of the coil. The coupling can be found by measuring the inductance of the two windings when aiding and opposing. My guess is that in the case of the ferrite rod aerial, the coupling will be quite low. Look up the details of transformers and their coupling and leakage inductance. The equivalent circuit with the three inductances and all the leakage inductances will be very complcated.

6. ### michael1978

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Mar 17, 2012
HI, i read rf component like i understand rf transformler, is for example if 400ohm input primary at the secondazy to the base for example 50 ohm theyy tell you how to calculate, how many winding primary and seconday, but is this the formala

7. ### Audioguru

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Sep 24, 2016
Of course the ferrite loopstick antenna is an RF transformer. The winding that is tuned with a variable capacitor has a high impedance at the tuned frequency and has many winding turns and the other windings have fewer turns so that they are lower impedance and do not load down the bandwidth of the tuned winding.

8. ### duke37

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Jan 9, 2011
You do not quote any formula.
If the coupling is tight, then the turns ratio will be Sqrt(400/50) or about 3.
The number of turns will be dependent on the core and the power to be transferred.

michael1978 likes this.
9. ### michael1978

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Mar 17, 2012
thank AudioGuru

388
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Mar 17, 2012
Thanks Duke

11. ### michael1978

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Mar 17, 2012
Hi, Duke one more time, ferrit rod does have a primary and secondary coil, so seconday coil is coupling inductor(because i am reading rf componnets book) and the book tell about primary winding and secondary winding(connected to transistor), and he tell how to calculate turns and so ...

12. ### michael1978

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Mar 17, 2012
Hi AudioGuru, so the ferrit rod is like transfoirmer, and secondary of copling is connected to amplifier right

13. ### Audioguru

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Sep 24, 2016
Can't you see that Q1 is an oscillator? The output of Q1 feeds L3 through the regen control and Q1 is an inverting common emitter circuit.L3 and L2 are a center-tapped transformer that increases the signal level 2 times (the turns ratio) and also inverts the signal and feeds it into the input of Q1. Then two inverts makes Q1 an oscillator but the regen control is set so that it almost oscillates which makes its gain high at the tuned frequency.

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14. ### michael1978

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Mar 17, 2012
so for example ferrit rod loopstic als is a transformer(that i want to know is transformer) thanks a lot man

15. ### duke37

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Jan 9, 2011
Transformer.
I think it all depends on what is meant by a transformer. The mains power transformers have windings on a common core and a coupling factor between windings near 1. Intermediate transformers (IF) have windings with much lower coupling, perhaps 0.01. A radio transmitter produces a magnetic field and a receiver detects this. Is this a transformer? Perhaps a transformer has a confined magnetic field.
In the case of the loopstick, the windings are coupled at perhaps 0.5 (I have not measured this) so will act as a transformer if power is supplied to one winding and some power is taken out of a secondary. The winding going to the transistor base is often overwound the primary to get closer coupling.

16. ### Audioguru

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Sep 24, 2016
I have never seen a Mickey Mouse AM regenerative simple radio circuit like this one with the loopstick antenna made into a tuned oscillator. Instead I have seen many normal AM superheterodyne radio circuits that have a loopstick antenna with only a tuned winding and a secondary winding that feeds the RF amplifier without a center-tap that inverts the phase.

This simple circuit doesn't have an RF amplifier, does not have a tuned IF amplifier, does not have automatic-gain-control (AGC) and its detector diode does not detect the AM signal.

17. ### michael1978

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Mar 17, 2012
This one on the left is transformer?, 3 and 4 main LC TANK of primary and 5 and 6 are secondary? right?

18. ### Audioguru

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Sep 24, 2016
This circuit has more parts than the simple circuit in your first post. It has two RF amplifier transistors (Q3 is like Q2 in your first post) so both invert the signal and the center tapped winding on the loopstick in your first post is not used to invert the signal. It is also a regenerative circuit but it should have a regen control to avoid oscillations.

Loopstick winding 3 and 4 have many turns for a high impedance and is tuned. Loopstick 5 and 6 have only a few turns for a low impedance to feed Q1. R14 applies positive feedback for Q1 to Q3 to almost oscillate.
Since it regenerates then it does not need an AM detector diode.

19. ### michael1978

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Mar 17, 2012
Oh my dear frind, i have all explanation, i just want to know is transformer of not, for the rest i print it, how it works...
only for ferrid rod loopstick did not say nothing, my question was is that transformer?
thank a lot,.....have a good day

20. ### michael1978

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Mar 17, 2012
Grretngs AudioGuru, i see you post a lot, you are cool man ;-)