# Maximizing magnitude of fields created by an antenna

I have a small loop antenna ($$\C\$$ = circumference<< $$\\lambda\$$ ). I am interested in the fields created by this loop at a distance $$\D\$$ along the z axis also small compared to $$\\lambda\$$. I am confused with regards to what I should do with respect to the impedance matching of the antenna if my goal is to maximize the $$\B\$$ or the $$\E\$$ field at position $$\D\$$.

I know that, traditionally, impedance matching is meant to maximize the power transfer from my source to the antenna (I assume the source is 50$$\\Omega\$$), however I would think that if I want to maximize the B field at position $$\D\$$ then I probably want to maximize the current flowing through the antenna, so I want to use a matching network to make the antenna impedance look like a short. In contrast if I wanted to maximize the E Field, I should do the opposite and make the antenna look like an open circuit.

Question: Can anyone confirm or disconfirm my intuition ?

In practice $$C=7.2 \mathrm{ cm}$$ $$D=2.2 \mathrm{ cm}$$ $$f=6 \mathrm{ MHz}$$ $$\lambda= 50 \mathrm{ m}$$

PS: I think that exactly on axis the fields created by a small loop antenna might be null, so for all intents and purposes you can assume that I am looking at the fields created by the antenna at $$\(x,y,D)\$$ with $$\(x,y)\neq(0,0)\$$

• It's not an antenna, it's a magnet. Commented Jul 13 at 0:29
• Math mode varies between Stacks; here we use \\$. Commented Jul 13 at 3:09

If the loop antenna is much smaller then the wavelength, it is commonly regarded to as an inductor. It acts more like an inductor.

Yes, you need maximum current fir maximum magnetic field, but the source needs to be capable supplying this current. I suppose this will be handled, the currents can get quite large (or the voltage can clip if there is not enough current budget).

You don't need to pay too much attention to RF part of electronics, because your frequency is quite low. RF part is always there but it's negligible toward magnetic part at low frequencies. By low frequencies i mean frequencies where period of the signal is is much lower then delay in your circuit or in other words, where wavelength is much bigger then size of the system ( which was your opening condition). Think of it this way, let's say we have a transmission line, a cable or trance on the circuit. If itis small enough that the change of sigal is negligible between one end and the other, then there is no need to look into the problem from RF point of view. The circuit will act mainly as purely kirchoffian circuit. If the delay becomes higher or periods of the signal lower, the change of signal one one side will not instantly result in change on the other side creating different conditions that kirchoff equations dont cover. In this cases you need to think in terms of time too, and you need to treat signals as waves in other to capture there nature of needing to propagate through the system. In this cases, as you look at them as waves, you need to consider also matching etc. But you don't have this case. Your case is by your definition "non-RF". so you can apply current directly, use as low impedance source as possible (op amps, transistor circuits..) and drive the coil (not antenna loop) directly. Be aware of two things though:

1. If you intend to do everything of pulse sort the current through the inductor is by nature slow. Also, i assume the source will have some capacitance, resistance etc, even if parasitic, pay attention that your equivalent LCR circuit wont be low or high pass filter cutting away desired frequencies. Based on given case it could be ok.

2. what you will be producing is magnetic field, not electromagnetic radiation. They are linked ofc, but the differences are again due to the prevailing effects as described above. So if you where to use 3GHz on the same antena, you would need to match it, and it would act as RF device but this would also create rf electromagnetic radiation. Which has energy basically E=h*f, where h is a constant. But because you are at lower frequency you are only creating magnetic field, where energy is diminishing with the distance to the power of 2. So don't expect this will be some long range stuff.

• Thanks @ursusd8 for your answer, I am not good with electronics, if I wanted to drive it with an op amp, how would I do it exactly? do you have a reference showing how to properly do this and how to avoid common pitfalls? Commented Jul 14 at 14:59
• I suppose just normal voltage follower, connecting reference to in+ and feedback from out to in-. That is if you are happy with the voltage. You can also add gain. Examples can be found in datasheets of the references. For example, look at page 25 of this: analog.com/media/en/technical-documentation/data-sheets/… Commented Jul 21 at 22:34
• However if you have trouble understanding this schematic i suggest looking for professional to do it for you or start slow. Because these are pretty basic opamp circuits, it will be hard to complete your tasks without understanding that Commented Jul 21 at 22:37