Why does 4:1 loop stick work better

[prabhatkarpe4]

I made a loopstick for my AM radio and I used enameled copper wire of 1 mm (approx) thick to make the loops on a ferrite rod of 1 cm thick and 14 cm long. I made the main loop with the number of turns 4x greater than the coupling loop and that gave a combination of 4:1.

I even tried the combination of 1:1 and 1:2, but none of them work that great.

I also noticed that only a wire greater than 19 (AWG) gives good resonance.

So, I would like to know why the magic is behind using 4:1 and 19 AWG?

I have attached the setup diagram for reference.

[wasedadoc]

The 4:1 ratio may be optimal for your specific combination of main loop and input characteristics of your receiver.  Does not mean that 4:1 would be optimal  for a different main loop or different receiver.  So nothing "magic" about 4:1.

[prabhatkarpe4]

But it's not a static loop---it has a variable cap to adjust with any other radio recdeiver. But why the 1 mm (19 AWG) works better than the other thinner wires?

[wasedadoc]

But it's not a static loop---it has a variable cap to adjust with any other radio recdeiver. But why the 1 mm (19 AWG) works better than the other thinner wires?

The variable capacitor tunes the main loop to the frequency you want to optimise.  It's main purpose is not to adjust to the specific AM receiver on the coupling loop.

[A.Z.]

But it's not a static loop---it has a variable cap to adjust with any other radio recdeiver. But why the 1 mm (19 AWG) works better than the other thinner wires?

The variable capacitor tunes the main loop to the frequency you want to optimise.  It's main purpose is not to adjust to the specific AM receiver on the coupling loop.

and that should give you an idea about the OP...

[radiolistener]

So, I would like to know why the magic is behind using 4:1 and 19 AWG?

A parallel elsie (LC) circuit has a high impedance at the resonant frequency, while your receiver has a low impedance. This leads to an impedance mismatch and poor signal power transfer from the elsie circuit to the receiver.

A 4:1 transformer performs impedance matching, which results in the receiver getting more power from the elsie circuit.

Using thicker wire for the elsie inductor coil reduces heating power losses due to wire resistance. The elsie circuit requires low-resistance wire to achieve a higher Q-factor. A higher Q-factor provides a higher noise-free amplitude gain but reduces the bandwidth.

An ideal parallel elsie circuit would have infinite impedance, infinite Q, and zero bandwidth. However, since no wire has zero resistance in real world, you can't achieve infinite Q, so amplitude gain is reduced, and the bandwidth is more than 0 Hz. The optimal bandwidth for an AM receiver is about 20 kHz, but tuning the antenna for each frequency is required, so there is a compromise between antenna bandwidth and Q-factor.

A full-size half-wave dipole antenna typically has a Q-factor of around 10-20. High-Q magnetic loop antennas can have a Q-factor of up to 2000, which, for a carrier frequency of 7 MHz, corresponds to a bandwidth of 3.5 kHz, which is sufficient for a voice reception.

Q = 1 / BW

BW = BW_Hz / fc

BW - relative bandwidth
BW_Hz - absolute bandwidth in Hz
fc - carrier frequency in Hz

combine it we get:

Q = fc / BW_Hz

For example:

1) BW_Hz = 3.5 kHz, fc = 7000 kHz:

Q = 7000 / 3.5 = 2000

2) BW_Hz = 500 kHz, fc = 7000 kHz:

Q = 7000 / 500 = 14

As you can see, this is why a usual half wave dipole at 7 MHz has bandwidth about 500 kHz and Q about 14 and small magnetic loop antenna at the same 7 MHz has bandwidth just 3.5 kHz and Q about 2000.

[prabhatkarpe4]

But is that a 4:1 loop stick or a 16:1?

I came to know from someone that this one is 16:1.

[wasedadoc]

But is that a 4:1 loop stick or a 16:1?

I came to know from someone that this one is 16:1.

For an ideal transformer the impedance ratio is the square of the turns ratio.

[A.Z.]

an example, let's say we have a transformer with the windings being 6 turns and 2 turns, we divide 6 by 2 and obtain 3, we calculate the square of 3 and obtain 9, so our transformer has a 9:1 transformation ratio

ok, oversimplified, but hopefully clear

[radiolistener]

But is that a 4:1 loop stick or a 16:1?

I came to know from someone that this one is 16:1.

1:4 turns ratio = 1:16 impedance ratio

But in real world RF transformer practical limit is about impedance ratio 1:9.
Higher impedance ratios are not generally practical because of bandwidth and distortion limitations due to physical properties and limitations.

[prabhatkarpe4]

There are some confusions here. First of all, what is the difference between 4:1 and 1:4 ratios?

Another confusion is that I came to know that the higher impedance side should be connected to the transformer side that has higher number of turns. But in my experiment, I connected the the 200 ohms twin line connector of my AM radio with the 24 turns of the transformer and the loop antenna with the 96 turns side. But reversion the sides doesn't give any signal output.

The resistance of the loop antenna is very less, and I am sure it's less than 200 ohms.

So, why the setup is working in opposite order?

[wasedadoc]

You have not understood that the dc resistance of the coil in the tuned loop is low but the impedance of that loop (coil and variable capacitor) at the resonant frequency is high.

[ahbushnell]

You have not understood that the dc resistance of the coil in the tuned loop is low but the impedance of that loop (coil and variable capacitor) at the resonant frequency is high.

The resistance will be higher at higher frequency because of skin effect.

[iMo]

The Q of a ferrite rod antenna could be around 100-200 (it depends on the Q of the tuning capacitor as well).
For frequencies up to say 500kHz people use "litz wire" because of the skin effect.
For higher frequencies the type of wire is not so important (litz wire gets less effective) a thicker and gold/silver plated would be nice to have, sure
The usual ferrite rods do not work well above 2-3MHz, however.
The lighter the coupling the better selectivity (because of a lower loading of the primary LC), the turn's ratio of 4:1 looks a bit odd, as I can remember from my childhood the ferrite rod antennas I made had, say, 80-120t primary (the LC) and, say, 5-10t the coupling (low Z input) aprox 1-2 cm (ie. movable) off the cold side of the primary LC.

[prabhatkarpe4]

the turn's ratio of 4:1 looks a bit odd, as I can remember from my childhood the ferrite rod antennas I made had, say, 80-120t primary (the LC) and, say, 5-10t the coupling (low Z input) aprox 1-2 cm (ie. movable) off the cold side of the primary LC.

If your choosing 120 turns for the main loop and 10 turns for the coupling loop then it's going to be 120/10 = 12^2 = 144:1 ratio. 144:1 is a bizarre ratio because it doesn't fit within the limit of 16:1. I guess 16:1 is the highest possible ratio of any loop antenna or a transformer and beyond that is useless or unnecessary.

[iMo]

the turn's ratio of 4:1 looks a bit odd, as I can remember from my childhood the ferrite rod antennas I made had, say, 80-120t primary (the LC) and, say, 5-10t the coupling (low Z input) aprox 1-2 cm (ie. movable) off the cold side of the primary LC.

If your choosing 120 turns for the main loop and 10 turns for the coupling loop then it's going to be 120/10 = 12^2 = 144:1 ratio. 144:1 is a bizarre ratio because it doesn't fit within the limit of 16:1. I guess 16:1 is the highest possible ratio of any loop antenna or a transformer and beyond that is useless or unnecessary.

Look at the ferrite rod antennas used in the commercial radios - what is the number of turns and their ratio.
BTW. - I am not talking here the "loop antenna", but the "ferrite rod antenna".

[prabhatkarpe4]

A typical ferrite loop stick in a radio receiver doesn't come with two loops. So, there is no ratio. I am talking about a small loop antenna concept using a ferrite rod.

[Andy Chee]

A typical ferrite loop stick in a radio receiver doesn't come with two loops. So, there is no ratio. I am talking about a small loop antenna concept using a ferrite rod.

A typical ferrite loop stick in a radio receiver definitely has two coils. So, there is definitely a ratio.