Why is a slightly longer loopstick more sensitive for AM (medium wave)?

[Tycho_Brahe]

I'm going to expose my ignorance here, but what the heck.   I've been reading a lot of reviews of portable Shortwave & AM (medium wave) radios recently and regarding the AM band, a frequent comment is that the loopsticks (ferrite coils) are small/short, causing sub-par medium wave performance.

I've read that the iron core inside the coil can even extend way longer than the edges of the coil itself and can help with sensitivity.  Or even if the iron core didn't extend past the sides of the coil, it would still help if the entire loopstick was longer.

So ignoring the existence of antenna boosters and mutual coupling and all that (to keep this simple), my question is - Why does a longer coil and/or longer iron core inside the coil increase sensitivity?  The loopstick is in effect the antenna.   If I'm remembering correctly, a larger core would increase inductance (more magnetic permeability).   But why would a greater inductance help with sensitivity?  Whether you have a radio with a large or small loopstick, you're tuning the same signal so I'm assuming the inductance would need to be the same.   And if you did want greater inductance, if you wanted a different way to fit a very small coil inside a portable radio, couldn't you also increase inductance by increasing the number of turns?   I also wondered if a larger coil or core might simply capture (due to surface area) more of the signal but when a radio with a 3.85" loopstick has a much better sensitivity than a radio with a 2.69" loopstick that doesn't add up to me (seems like such a small difference wouldn't help much). 

Anyway, just want to better understand the whole topic.  Thanks!

[bob91343]

An antenna is not a lumped component, with inductance and so on.  It's a distributed device.  Its purpose here is to intercept signals.  To do that most effectively, it should be large, just as a camera or telescope lens needs to be large to intercept more light.

Someone asked me, what is a good antenna that is small and doesn't have to be raised high?  I answered that a good antenna has to be large and high.  So your answer is that the antenna core, where most of the flux is, must have a large dimaension so that the impinging radio waves can set up currents in the coil.  The wave travels at the speed of light and the antenna relies on the wave having a different phase when it exits the antenna volume, from its phase when it enters.

[RoGeorge]

But why would a greater inductance help with sensitivity?

The increase in inductance when an iron core is inserted is just a side effect.  That's not the reason why a longer magnetic core helps.

An intuitive explanation is that electromagnetic waves will prefer to travel through materials with high magnetic permeability (big value for μ).  It would be "easier" for them to travel through a high μ material than to travel through air.

Other said, it is like the ferrite inside the coil will "channel" or "collect" more magnetic lines from the air, making them to prefer the "easiest" traveling path, which will be through high μ materials rather than through "normal" space that has a much lower μ called μ₀.

Something like this:



Source:  http://loop-antennas.ceyhunsezenoglu.com/en/latest/magnetic-core-loops/introduction.html

The magnetic rod longer than the coil will help keeping the "captured" magnetic lines parallel inside all of the coil's turns, so each turn will produce the maximum voltage.  If the ferrite rod is too short, then the first and the last turns of the coil won't see "all" the "captured" magnetic lines (the magnetic lines spread away at the ends of the rod), and thus the first and the last turns will contribute with less voltage to the total signal than they will contribute when the rod is longer and all the lines are passed through each and every turn of the coil, including the coil turns situated at the extremities of the coil.

Technically it is different, but also very abstract and complicated to get a visual grasp or intuitive understanding.  I don't know enough electromagnetism to give a proper mathematical explanation.

[radiolistener]

The ferrite rod increase magnetic coupling between inductor and a free space. The power of electromagnetic waves is distributed over a free space. So, when you increase dimensions of the core it leads to capture more power from a free space. Because core takes up more space, so it can capture more power from that space. For example the twice longer core can capture almost twice more power from a free space.

But the things a bit more complicated, because the ferrite rod core dimension also affects Q factor and inductance, so it also affects resonant frequency and amplitude gain at resonant frequency. Since you will use less or more winding wire with different core dimension, it's resistance also will be changed and it also affects Q factor and as result amplitude gain.

[Tycho_Brahe]

Just wanted to say many thanks to all of you for your explanations.   They do make sense to me (though, not totally - I think my brain isn't smart enough to understand it all).  I'd like to do some experimenting with antennas, etc. to help me understand, but unlike 35 years ago when I lived on a farm, there's so much EMI around that it's depressing.  And shortwave is nearly dead.  Thanks again.