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From the reading I have done, it seems like a ferrite rod antenna is mainly used to receive long and medium waves (500Khz to 1Mhz) but I don't understand how they differ from old fashion wire/monopole/dipole antennas?

This calculator shows a simple monopole antenna for 1Mhz frequency would be 468 feet which is of course too long to be practical. Is a coil antenna essentially the same length of wire rapped around a rod, maybe just to make it shorter? or it works on a totally different concept? if so then why can't that "concept" work for say 100Mhz frequencies and we need to use a whip antenna instead? also what is so special about ferrit and why can't it be a some other metal?

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A "loopstick" antenna is more or less equivalent to a loop antenna with diameter equal to length of the rod. Because it is an electrically short antenna, it has either very low gain, or narrow bandwidth.

Ferrite is fundamentally different because it has magnetic permeability, extending the magnetic path length of the otherwise much smaller winding.

The wire length of the winding is also much less than 1λ (a resonant loop), not just because it's electrically short to begin with, but additionally due to the added inductance contribution of the ferrite rod.

In circuit, the consequences of these properties is dealing with a low SNR and probably narrow bandwidth. Since atmospheric noise (global lightning and etc.) is considerable at LW frequencies, not much SNR is actually required of the receiver, so quite small antennas are effective (10s of cm). The downside of the low bandwidth is, active tuning may be required (such as a varactor diode or variable capacitor to tune the loopstick jointly with the rest of the radio).

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  • A ferrite rod antenna utilizes the magnetic part of the electro-magnetic radio wave to produce a received output voltage. This can then be amplified as you require
  • Ferrites are generally good in the DC to 10 MHz range but have increasing losses as frequency rises hence, they are not as good at 100 MHz as they are at 1 MHz
  • But, at 100 MHz a monopole antenna is barely an inconvenient length and outshines a ferrite rod (performance wise) quite significantly
  • Above 1 MHz and below 100 MHz you can use a ferrite rod antenna and get decent performance but, they are only usable for receiving. They make poor transmitting antennas because they barely produce an electric field (the other half of the electro-magnetic radio wave needed for a long-distance transmission)

Is a coil antenna essentially the same length of wire wrapped around a rod

You can make a coil antenna that has dimensions suitable for generating both the magnetic and electric fields. This would produce a decent EM wave but, it wouldn't use a ferrite rod.

what is so special about ferrite and why can't it be a some other metal?

It needs to be ferromagnetic to concentrate the prevailing magnetic field and increase the signal. However, a simple block of iron would not be very good because it conducts electricity and, if used like a ferrite rod antenna would be poor as a receiver due to eddy current losses. Ferrite material is an insulator broadly speaking.

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Is a coil antenna essentially the same length of wire rapped around a rod, maybe just to make it shorter? or it works on a totally different concept?

Certainly, the object of a coiled-up antenna is to make it physically shorter than a straight wire (whip) antenna. Coiling up a full-length whip to make it shorter doesn't make an efficient radiator.
Engineering a coiled-up whip to ensure that it presents a similar impedance to the transmitter as a quarter-wave whip still results in a less-efficient antenna.
Totally different concept? Yes, you can view the antenna shown below as a lossy transmission line or possibly as a self-resonant coil. It is not broad-band - its depleted efficiency is only maximum over a small frequency span.

For example, shown below is a shortened UHF antenna. No ferrite is used, (only air) because all ferrites are lossy in the UHF frequency range. One end of this antenna is left open, while the other end is driven by the transmitter. Not shown is the ground counterpoise, which is an important part of the radiating system, requiring even more space:
coiled antenna radiator
It is important that the immediate space around this antenna be kept free of other objects. Even so, it is a less-efficient radiator than a full-size whip, which is about 4.5 times longer.
A receiver using this antenna suffers because noise generated by amplifiers masks weak signals that are made weaker by antenna inefficiency. For a transmitter, you could always drive it with more power to compensate for inefficiency.

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