# What's the methodology of MDF magnetic field antenna?

I get a MDF 9400 magnetic field antenna from school.
I am not able to disassemble this flat antenna to take a picture.

1. I am not sure if this antenna detect only magnetic field.
2. I am not sure if output value of A/m.
3. I am not sure this antenna could be connected to keysight spetrum analyze.
4. I am not sure if this antenna emit only manetic field.
5. I am not sure if magnetic-iron affect this antenna.
6. I don't know the usage difference between loop antenna and this antenna.

I know the loop antenna (kind of magnetic field antenna) consists of a wire loop with an AC current flowing in the loop.

1. What's methodology of MDF magnetic field antenna?
• you have access to a Keysight spectrum analyzer - so I'll assume you've studied physics or electrical engineering. What remains unclear about 1, 2, 3, 4.? Feb 14 at 17:21
• Those are 7 questions in one... Feb 14 at 17:22
• @MarcusMüller,You raise many comments but make us confused.I even don't know why do you revise gain my question asked 2 days before. Feb 14 at 17:26
• The edit (as you can see when you click on the edit information) only introduces numbering to your 7 questions, to make it easier to read and answer them individually! Feb 14 at 17:33
• There is a slight irony in the fact that if you study for the Amateur Radio qualification in your region you might already have a good working knowledge of this stuff. Part of the qualification is introduction to all sorts of practical antenna information. This would really mesh nicely with whatever other studies (formal or ad hoc) you happen to be doing.
– user21417
Feb 15 at 12:52

The antenna portion is an inductor, likely a multi-turn coil of circular (or a polygonal approximation to circular) loops of some conductive material (such as wire). An inductor picks up changes in any magnetic field lines going through the open area within the loop(s).

Transmitting RF EM waves causes alternating changes in magnetic field lines to radiate away from the transmitting antenna. If you aim the plane of the circular loops of an inductor at an RF transmitting antenna, the magnetic field lines can go though the area enclosed by the loop or loops. Any changes in these magnetic field lines going through the loop, due to EM field propagation though the loop, will cause current to flow in the inductor.

These currents can be amplified to make them easier to measure.

You can vary how much current a planar loop-like inductor picks up from a changing magnetic field by changing the loop orientation so less or more of the field lines go though the open area in the middle of any loops. Any changing magnetic lines going across, rather than through, the loop's opening causes less, if any, current to flow in the inductor. So you end up with a somewhat directional receive pattern.

• It's a magnetic field antenna,should output value of A/m?Can I connect this antenna to keysight spetrum analyze?Is this antenna only useful for near field? Feb 12 at 18:25
• Output may depend on the pre-amp’s gain (manufacturers spec, if any, otherwise do your own calibration) and bandwidth. Depending on gain, might even be useful for DX SWL. Feb 12 at 19:30
• Any usage difference between MDF antenna and loop antenna? Feb 12 at 21:55
• @kittygirl From the link you provide in your question: "Design: Loop". It's a kind of loop antenna, which almost always needs active or passive tuning for selectivity, and probably with a high "Q".
– user21417
Feb 15 at 12:49

It's a circular loop antenna with a built-in preamplifier for weak signals. when electromagnetic waves hit it, alternating current flows out the wires to a radio receiver that you plug it into.

For small loops like this one: When the flat face of the antenna is facing toward the signal source, the amount of current is a minimum. When the loop is edge-on to the incoming waves, the signal is a maximum, or nearly so.

For large loops the operation is different: When the flat face of the antenna is facing toward the signal source, the amount of current is a maximum. When the loop is edge-on to the incoming waves, the signal is zero, or nearly so.

• The loop directivity is the other way around - sensitive on the edge, null in the middle. Its a small loop. Feb 12 at 3:44
• It's a magnetic field antenna,and can be used to transmit signal.Does this antenna emit only manetic field?Does magnetic-iron affect this antenna? Feb 12 at 18:30
• Antennas with pre-amps generally cannot be used for transmit (any Tx power will blow up the pre-amp’s circuitry.) Feb 12 at 19:28
• @tomnexus, sorry, I assumed large loop, will edit. -NN Feb 12 at 20:53
• @hotpaw2,MDF 9400 has no pre-amp Feb 12 at 21:52

The magnetic and electric fields of a signal actually can not be separated for a radiating wave. So the antenna absorbs and emits both together.

Antennas can be considered as transducers that convert AC signals into electromagnetic waves and vice versa.

The effective diameter seems to be about 0,1 meter so it is small all the way up to 400 MHz. I am pretty sure it has a single turn with a fairly large effective wire diameter. It could be something made from copper foil. The impedance at the low end is very low so the input impedance of the amplifier has to be extremely low. For a zero ohm amplifier (noiseless feedback) the current into the amplifier is proportional to the magnetic field and independent of frequency. A "normal" amplifier with an input impedance of 50 ohms will see a voltage that is proportional to the frequency up to the corner frequency where the inductive reactance reaches 50 ohms. As a transmit antenna the passive version is useless at low frequencies, but it might work reasonably well above 100 MHz if you connect a suitable, low loss impedance matching network.