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Magnetic loop antennas--far field

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Advanced Member level 6
Dec 24, 2004
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New England, USA
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I am trying to come up with a compact antenna in the ~5 MHz region. A ferrite loaded loop antenna seems to be a good way to go. But a number of papers I have read say that "magnetic loops only work in the near field", as the magnetic field does not propagate.

However, I see loop antennas like this Terk one that work fine for AM broadcast band reception from stations hundreds of miles away.

Also, I know historically that these types of loop antennas were popular with Ham radio reception over very long distances.

Can someone explain why there are two diametrically opposed opinions of the loop antenna?

Using ferrite will place a limit on the amount of power you can transmitt from an antenna.

I am thinking 2 watts max (battery operated), and typically a lot lower. Am worried about the propagation range though.

What is the FCC going to think of this?

The usfull range is going to depend on a whole load of things. The HF part of the spectrum tends to be noisy. The background noise at the receiver location is going to make a big difference. A few years ago my local amateur radio club put up a 40m wire antenna from the top of a tall building. Operators thousands of miles away could hear us but we could not hear them because the building was full of computers.

A receiver with a 30Hz wide filter for morse code will be able to discern and on or off carrier at greater distance than an AM signal will be receiveable.
Some of the new data modes using DSP will work even better.

2watts into a 1m loop resaonably high above the ground will be receivable at least five miles away with a good receive and a resonably large receiving antenna.

Either part 15 or part 90, should be ok with the FCC. And I agree, it is pretty noisy, but I too have a narrow effective bandwidth. I am just really trying to get the size down so someone can carry it/wear it, etc.

I don't think there is any chance of the FCC giving you a license for
5MHz so part 90 seems to be out.

Part 15 requires intentional radiators to be below 30uV/m at 30m at
frequencys between 1.705Mhz-30MHz

An online calulator says that comes out to about -75dBm from a dipole receiver antenna.
I'm guessing 40dB path loss with an smallish loop at 30m

Even with an inefficient tx antenna you should be thinking about
tranmsitter power in the region of microwatts not watts to be compliant
with part 15

I was told that 5.075 - 5.450 MHz was available for part 90 licensed operation. ¿Si o No?

perhaps this article will be interresting for you .
ARDF was very popular in days of USSR . though they usually used bigger loop antennas for direction finding. The name of the game was "fox hunting " and it is still there.
Have a look to antenna - this construction uses ferrite one for direction finding in 3.6 MHz band.
Tx output power is restricted to watts -dont remeber exact number. Radio coverage was about few km's. You can find more on subject and get idea on feasible ferrite antenna working range.

**broken link removed**

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