Installing Wire Loops for Amplifiers—Single, Double or Parallel Loops—Which?

Installing Wire Loops for Amplifiers—Single, Double or Parallel Loops—Which?
Installing Wire Loops for Amplifiers—
Single, Double or Parallel Loops—Which?
1. The reason why we always recommend installing a double wire loop is
flexibility after installation. With a double wire loop installed you can use
the configuration that will give the best result at any given location, and
can even change the amplifier if necessary.
2. Connecting both wire loops in series will create a 2-turn loop. A 2-turn
loop will give twice the magnetic field strength compared with a 1-turn
loop with the same wire size. In practice this means that a 2x14 gauge
wire loop will result in a 6 dB stronger field than a 1x14 gauge wire loop,
assuming that the voltage of the amplifier can supply the loop with the
same amount of current.
3. What you want to avoid is getting the voltage "stuck" in the amplifier and
not "released" in the loop (resistance) as a current. If the voltage stays
in the amplifier, the amplifier will get hot which is bad. However, if the
resistance of the wire is too much, the voltage will not be enough to
create the necessary amount of current, and both the magnetic field and
the high-frequency response will not be according to the standard.
4. If the looped area is small in relation to the amplifier’s capacity, use a 2turn loop to create a greater resistance, which avoids the voltage
“staying” in the amplifier. However, if the looped area is close to the
capacity of the amplifier, the resistance of a 2-turn loop may be too high
for the voltage to overcome, thereby limiting the current in the loop. In
this case, connect the wires in parallel to create a single turn loop which
results in essentially a larger wire size and less resistance.
5. The resistance of the wire that the current has to "travel through" is not
only built up by the static resistance, but also an important component
called inductance. Resistance and inductance together are referred to as
impedance.
6. The impedance of a wire is a combination of resistance (direct current,
DC) and inductance (alternating current, AC). The resistance is constant
whatever the frequency, and is measured in ohms. The resistance in
normal wires varies approximately from 0.3 ohms to 2 ohms. The
inductance, however, varies with the frequency of the signal. At low
frequencies, the inductance is lower than at higher frequencies. At very
low frequencies the inductance is zero or close to zero and the
impedance is equal to the resistance. As the frequency increases, the
inductance will eventually become larger than the static resistance,
thereby increasing the impedance beyond the level of the resistance.
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Although a 2-turn loop may deliver the right amount of current at 100
Hz, it may not be enough at 5,000 Hz, as the inductance "kicks in". This
might be another reason to use a 1-turn loop. Therefore, it is vital to
measure the magnetic field at different frequencies to determine if you
need to connect as a 1-turn loop rather than as a 2-turn loop.
7.
You need enough voltage to overcome the impedance and push the
current through the wire. If, however, the voltage is high and the
impedance is low, you will have excess voltage that will not be needed
to create the current needed. This extra voltage will then be wasted as
heat in the amplifier. The ideal situation is to have the exact amount of
voltage to overcome the impedance, but not so much that it will “stay”
in the amplifier.
8. Thus, our general recommendation for all standard (PLS) loop amplifiers
is to use a twin wire (two wires), then you can either:
A. Connect the wire as a 2-turn (two wires) loop.
B. Connect the wire as a 1-turn loop (and not use the second wire).
C. If the desired magnetic field level is not reached, especially in the
higher frequencies, combine the leads and connect in parallel as a 1-turn
loop.
First, connect the wire as a 2-turn (two wire) loop (A). Measure the field
strength. Then disconnect the 2nd wire (B) and test as a 1-turn loop
(one wire) and measure field strength again.
If the 1-turn is stronger, leave the 2nd wire unattached.
You can also attach the ends of the 2nd wire to the ends of the 1st wire
thus creating a “parallel connection” which still connects to terminals #1
& #2. Then compare to the stronger of A and B. The best connection is
the strongest one—A, B or C.
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