this is an extension of this question:
Why aren't ferrite rod antennas used in WiFi?
if you don't want to go through the previous question, then basically i was asking if we can loop wifi antennas around a ferrimagnetic cores, so as to concentrate the fields and get a higher gain.
the same way in AM radios, we loop wires around a soft ferrite to concentrate the fields, to act as the receiving antenna. greatly reducing what would probably be hundreds of meters long to just a few inches.
well, since then, i've been told that ferrite just can't hold magnetic fields quick enough in those frequencies and they act as noise dampeners. "noise" being the high frequencies in GHz… so i turned to supermalloys.
http://en.wikipedia.org/wiki/Supermalloy
i'm not entirely sure if supermalloys are a good way to go, though… i've skimmed through abstracts of articles of it being used in GHz, but not as cores of loop antennas. then there's also the issue of it being a good conductor, which would lead to A LOT of hysteresis loss…
but putting aside hysteresis loss, would it make a good core??
please point out where i'm wrong here…
Best Answer
No, hysterisis loss is the magnetic material not having a linear BH curve and magnetically saturating when flux density rises too high. If the material is a good conductor then it has high eddy-current losses/problems.
Think about why ferrite is a good for small transformers and inductors - it barely conducts electricity therefore the eddy currents induced in the core are nearly negligible. This is because the particles that make the ferrite are seperated by a non-conducting glue (for want of a better word).
However, as frequency rises eddy currents do become more significant - think of these "particles as exhibiting capacitance to each other then at high frequencies the capacitive reactance is low and the eddy current losses become high again. This is why the frequency versus permeability curve (as shown in my previous answer) has two graphs - one of actual magnetic permeability and one of losses.
Consider an AC transformer - they use laminates and each laminate is insulated from each other by a lacquer - this greatly reduces eddy currents but as the frequency rises, standard AC power transformers become useless - they would need progressively thinner and thinner laminates to avoid the eddy current problems at higher frequencies. This is why ferrite (or powdered iron) is popular for switching power supplies - in effect the "laminates" are microscopic!
Supermalloy has a low resistivity therefore it won't work. The wiki page quotes a resistivity of 6.0 x 10−7 Ω·m and regular iron is 9.71 x 10−8 Ω·m barely 6 times smaller. Here is a table of resistivities for various materials: -
Note that FeNiZn has a resistivity of 1,000,000 Ω·cm compared to Fe which is 10 x 10−6 Ω·cm (sorry for the change in units but basically for Ω·m reduce the of Ω·cm by 100).