Why does the magnetic loop antenna have a coupling coil

[arildj78]

I'm trying to wrap my head around the magnetic loop antennas. I understand how it resonates with the parallel capacitor, but why is it coupled to the receiver with a small coil? Wouldn't this be a step down transformer? Could you not hook your receiver straight into the primary loop?

Arild

[T3sl4co1l]

Impedance matching.  The point of the capacitor is to load the loop so it becomes resonant; the loop's reactance is much less than the impedance of free space, and intercepts less than a wavelength around, so its radiation impedance is even higher (or lower) still, compared to the impedance of space.

Whether it's higher or lower, depends on the equivalence being measured.  The series and parallel equivalents are related by the resonant circuit: EPR = Zo * Q, or ESL = Zo / Q.  Zo is the characteristic impedance of the tank, Zo = sqrt(L/C), while Q is the quality factor.

If we connected our transmission line in series with the loop, we would ruin the Q, and get very little signal out (the radiation resistance, as ESR, might be a few ohms).  Conversely if we connect in parallel with the capacitor, the same happens (radiation EPR might be ~kohm?).

However if we use a smaller second loop, we can tap off some of the larger loop's energy, maintaining its Q (actually, dropping it by half, so that the transmission line loads it equally to the radiation resistance -- this comes from the power matching theorem).  The equivalent circuit is a nonideal transformer, two dissimilar inductors with some coupling.  We again draw the equivalent circuit, which with the right ratio and coupling, gives an ESR of 50 ohms at the feedline -- just what we need.

In practice, it may well be that it's not easy to calculate the dimensions of that transformer (the secondary inductance and coupling factor), in which case additional matching components may be desirable.  At this point there isn't really much difference between using an incorrect sub-loop and matching, versus matching the main loop directly.  Or the match is considered "good enough" and that's that.

These loops are usually used for receiving low frequencies, where atmospheric noise dominates over antenna losses, even for very compact antennas, or poorly matched ones.  A sensitive receiver may still be required, but these are easily constructed with modest transistors, or even op-amps.  A better match is desirable for a transmitter, however, as is a larger antenna in general.

Tim

[radiolistener]

it is needed in order to feed magnetic loop with RF energy. It requires impedance match between feeding line and antenna. Coupling coil plays role of impedance matching circuit. You can use other type of impedance matching circuits to feed magnetic loop. For example, you can use gamma match instead of coupled coil.

Here is illustration of these two types of impedance match with schematic equivalents:



Without this impedance match, RF energy will be reflected back from connection point and only a small part of the energy gets into the antenna. Reflected RF energy will come back to the transmitter, where it will be summed with a new portion of output energy and can damage transmitter (because amplitude will be ~2 times higher than normal operation).

There is device which role is to measure forward and reflected power relation - SWR meter. When SWR meter shows SWR=1.0, it means that all energy gets into antenna. When SWR is higher than 1, it means that some part of energy is reflected back (bad impedance match). Higher SWR means higher reflection. Normally SWR range 1.0 - 1.5 is accepted. If SWR is higher than 1.5 it may damage your transmitter, so you're need to add better impedance match. You can use antenna tuner for that. But note, if you place tuner on the transmitter side, it's efficiency will be very low (due to very high loss in the feeding line). You're need to place it near feeding point of antenna.

Step-up and step-down transformers is an example of impedance match circuit. Just square voltage ratio of transformer and you will get impedance transformation ratio. For example, if you have 220V to 12V transformer, it's voltage ratio is 1:18.3, it means that impedance ratio of such transformer is 1:336. So, you can use it to match 1 Ohm output with 336 Ohm load or vice versa. But each transformer has limited working frequency range, so you cannot use usual mains transformer for antenna, there is needs RF transformer which is intended to be used at high frequency.

[StuartA]

Many popular designs for receive-only mag loops are of the "active" type, with a pre-amp connected directly on the loop. That's particularly good if you need a down-lead of any great length and also deals with impedance matching. In the UK, the Wellbrook and Wellgood designs have attracted much interest.