How do induction loops work?An induction loop system transmits an audio signal directly into a hearing aid via a magnetic field, greatly reducing background noise, competing sounds, reverberation and other acoustic distortions that reduce clarity of sound.This diagram illustrates how they work.hearing loop diagramAudio Inputs 1, either from an existing audio source such as a P.A. system or from dedicated microphone inputs feed an audio signal into an Induction Loop Amplifier 2. The amplifier drives a current into a Loop 3 or series of loops. As the current flows through the cable it creates a Magnetic Field 4 in the required area careful loop and amplifier design ensures that the vertical component of the field is even and free of dropouts and dead zones wherever the user might be. Inside most Hearing Aids 5, a small coil known as a Telecoil 6 picks up the magnetic field signal, which is amplified into a high quality audio signal delivered directly to the ear of the hearing aid user.

How do induction loops work? - Technical informationNot all hearing-aid users and technicians / system installers can be expected to know the answer. Many have not heard of such things, and do not understand the great help an induction loop can be to users of hearing aids in compensating for their disability. So, the following explanation may be of some help in enabling non-technical persons to understand how an induction loop works.Most hearing aids nowadays have a switch marked M and T. Some even have M, MT and T. The M (microphone) position is for "normal" listening, that is receiving airborne sound via the microphone built in to the hearing aid. The T (telecoil) position is for receiving the sound via an induction coil which is built in to the hearing aid.For the induction coil to provide sound, a magnetic field is needed via which the sound is transmitted. This facility in hearing aids was introduced by a number of manufacturers many years ago and was then known as the "telephone" or "telecoil" position on the hearing aid switch. It was intended to make it easier for the hearing aid user to hear over the telephone, by picking up the sound via the magnetic field generated by the diaphragm coil in the receiver of the telephone.In many locations, telephone handsets now have this required capability. In recent years, however, induction loop systems have begun to be provided in public places such as churches, cinemas and theatres, bank, ticket and information counters and desks. It is even found in the home. In all these cases the T facility is used in to listen inductively, without the interference of airborne background sound. The MT position which is provided on some hearing aids allows listening simultaneously both to airborne sound via the microphone and to inductively transmitted sound via the telecoil.It is well known that when an alternating current is passed through a wire, a magnetic field is generated around the wire. If a second wire is brought within this magnetic field, a corresponding alternating current is created within the second wire. In technical language, it is said that a current is "induced" in the second wire. Hence the term "induction". This particular magnetic principle is the basis on which electrical motors, electrical generators and transformers operate. An induction loop for hearing aid purposes also operates in the same way. An induction loop system consists of an amplifier and a loop. The amplifier can be connected to a sound source such as a TV or radio, a PA / sound reinforcement system or a dedicated microphone.The signal is amplified and fed into the loop cable, in the form of a strong alternating current. The loop itself consists of an insulated wire, one turn of which is placed around the perimeter of the room. When the alternating current from the amplifier flows through the loop, a magnetic field is created within the room. If a hearing aid user switches their hearing aid to the T position, the telecoil in the hearing aid picks up the fluctuations in the magnetic field and converts them into alternating currents once more. These are in turn amplified and converted by the hearing aid into sound. The magnetic field within the loop area is strong enough to allow the person with the hearing aid to move around freely within the room and still receive the sound at a good, comfortable listening level. The performance of these systems is specified in agreed international standards.Some loop layouts are not simple single wire surrounding a room, but the above explanation covers the basic principles.

My Story: Magnetic Communication ProjectGoing wireless the old-fashioned wayBy Philip Kane

Difficulty Level: Beginner-Intermediate
Required Time: 1-2 hours

Several years ago I adapted an idea from an old electronics project book and built a wireless system that allowed me to silently listen to my hi-fi receiver from just about anywhere in my apartment. Although the reception was not hi-fi quality it was more than adequate for my purpose (I used it mainly to listen to the news through headphones while doing other things). My wireless system was based on an old idea that is still in use today. However, unlike last Jameco's feature of make's FM transmitter project, it did not require transmitting the signal over an RF carrier. In fact, I didn't need to build a transmitter at all. My hi-fi receiver was the transmitter and the "antenna" was just a length of speaker wire that encircled my living room.

PARTS LIST

Transmitter
4W - 10W Audio Amplifier (Hi-fi Receiver, etc)
Speaker cable P/N 100280
8 10W (minimum) Resistor P/N 2144593

Receiver (Audio Amplifier Module - Fig. 3)
C1 - 0.22F capacitor P/N 25540
C2 - 0.1F capacitor P/N 25523
C3 - 10uF electrolytic capacitor P/N 94221
C4 - 0.047F capacitor P/N 57621
C5 - 220F electrolytic capacitor P/N 93772
IC1 - LM386 Audio Amplifier IC P/N 24125
L1 - Telephone Pickup Coil P/N 2144585
R1 - 10K potentiometer P/N 29082
R2 - 10 resistor P/N 690380
SPKR1 - 8 speaker IC1 P/N 2099577 A Simple Wireless Audio SystemThe block diagram (fig. 1) shows the basic elements of a very simple unidirectional wireless communication system. Notice that this system contains no RF components. The transmitter is simply an audio amplifier, (or radio, CD player, etc) connected to a large loop of one or more turns of speaker wire that serves as the transmitter antenna. The receiver consists of another audio amplifier and a small induction pickup coil for the receiver antenna. The transmitter signal can be received from anywhere inside of the loop and for a short distance outside of the loop. Block diagram of systemFigure 1: Block diagram of system

In this simple system the audio signal at the transmitter generates a magnetic field around the transmitter loop. This field varies directly with the intensity and frequency of the audio amplifier output. When the receiver coil is introduced into the field, a voltage is induced across its windings. The voltage across the receiver coil varies with the frequency and intensity of the changing field. Orientation of Transmitter and Receiver CoilsThe magnetic field strength decreases relatively quickly as the distance from the transmitter coil increases. Also, the relative orientation of the transmitter and receiver coils determines the strength of the signal at the receiver. For example, minimum coupling occurs when they are orthogonal (at right angles) to each other (fig. 2a). Figure 2aFigure 2b

Maximum coupling, at a given distance, occurs when they are oriented in the same direction (fig 2b). DIY Induction Loop SystemThe TransmitterFor the transmitter, I used an old 10W stereo hi-fi receiver with external speaker connections. The transmitter antenna was a length of speaker cable (if you are using two conductor speaker cables, twist the wires together at each end of the cable). I attached one end of the cable to one terminal of a speaker connection (fig. 3). I attached the other end of the cable in series with the impedance matching resistor to the other terminal of the same speaker connection. The impedance and power rating of the antenna loop should match that of the transmitter. For example, if the transmitter output is rated at 10W into 8 then use an 8 ohm resistor with a power rating of at least 10W or greater. One note of caution, the impedance matching resistor can get hot. Handle it carefully while the transmitter is operating. Keep it away from material that is flammable or can melt easily. Stereo hifi receiverFigure 3: Stereo hifi receiver

The ReceiverThe receiver is an audio amplifier built around an LM386 audio amplifier chip (fig. 4). The version used in this circuit (LM386N-1) has a supply voltage range of 4V to 12V. In this circuit, amplifier gain is fixed and set to maximum by connecting capacitor C3 between pins 1 and 8. Potentiometer R1 controls the output level. Input coupling capacitor C1 should be selected so that, at the lowest frequency of interest, its impedance is small compared to R1 (about 1/10 of R1). If you find that you are getting significant interference from a local radio station (especially when the input is unconnected) try placing C6 between the input and ground but may not be required. Circuit schematicFigure 4: Circuit schematic

The receiver antenna is a telephone pickup coil (fig. 5). Telephone pickup coilFigure 5: Telephone pickup coil

Construction and TestingYou can first assemble the receiver circuit on a solderless breadboard for initial testing (fig. 6). Receiver amplifierFigure 6: Receiver amplifier

A permanent version of the receiver amplifier can be constructed using a general purpose prototyping board with through holes and solder pads. This will help to reduce component wiring. In order to reduce power line hum, RF noise etc., make sure that component leads and connecting wires are as short as possible. The LM386 should be mounted on sockets. To eliminate the possibility of damaging it while soldering. Remember to observe the proper polarity for all electrolytic capacitors.

Note: the National Semiconductor data sheet for the LM386 indicates that high gain applications might require a bypass capacitor between pin 7 and ground. I found that it was not required for this application.

To test the system, start by checking all connections. Make sure that the transmitter and receiver antennas are connected properly. Place the receiver inside the transmitter antenna loop. Connect the pickup coil to the input of the receiver amplifier. Connect an 8 speaker or low impedance earphones or headphones to the output of the receiver amplifier. Turn on the receiver and set the volume control to between one quarter and one half full volume. Power up the transmitter amplifier and set the output to a minimum level. Now, gradually increase transmitter output level until you can hear the signal at the receiver. If you notice power line interference try changing the orientation of the receiver coil.

Phil Kane has been a technical writer in the software industry for more than 10 years. He has also occasionally authored articles for electronics enthusiast magazines.

Jameco welcomes the contributions of its customers. Frankly, we think what you write is more interesting than anything we could write. Share your electronic component story, project, or challenge, and we'll share it with the world. Send your story to MyStory@Jameco.com

Project: Audio Induction Loop Receiver (Part 1)Posted on October 4, 2014 by lui_gough In daily life, you might have come across signs like this, with a picture of an ear, and the letter T on them. Maybe you didnt think any more about them, as an average person with perfectly good hearing after all, they are an assistive technology intended for the hard of hearing.20140829_190123But as an electronics and signals enthusiast, this really piqued my interest. Everyday, we are literallybathed in signals we dont notice, and dont understand! Ive seen these logos on auditoriums, in trains (e.g. picture above), in train ticket windows amongst other places. Ive always wondered how these things worked, and what it would take to receive such transmissions. After some brief research, I got some of the answers I was looking for.Audio Frequency Induction Loop BackgroundThe audio frequency induction loop is a system by which audio can be transmitted to hearing aids through the use of magnetic fields. This system does not modulate the signal on top of a subcarrier, instead relying on an alternating magnetic field in audio frequencies. This system appears to have come about due to the parallel development of telecoils in hearing aids, which were designed to pick up the stray magnetic field leakage from telephone handsets to improve intelligibility for hearing aid users. The T on the signs indicate to users to switch over their hearing aids manually to the telecoil operation position.By leveraging these telecoils for longer distance transmission of audio, it is possible to transmit audio to hearing aids without relying on a bulky receiver, and improve the quality of the audio over that picked up by the integrated microphone. Systems to transmit the audio can be as simple as a loop of wire hooked up to a regular amplifier.Unfortunately, such a system also causes electromagnetic interference by spewing moderate to high levels of electromagnetic fields (as that is how it works). It is also vulnerable to electromagnetic fields, which cause interference and marginal audio quality.What Does That Mean?You know when people say they wished they had superpowers? Well, and I mean this with all the respect, those with telecoils in their hearing aids actually have one that regular humans dont!Normal hearing relies on detecting the compression waves in air. When audio is generated by a speaker, the speaker is converting the electrical waves into magnetic fields which then drive the cone via the voice coil motor to create the compression wave.In a telecoil system, they are shortcutting this, and instead, users with hearing aids can directly perceive alternating magnetic fields the audio range. The induction loop system exploits this, essentially acting as a loud magnetic-field only speaker Because they can perceive alternating magnetic fields in the audio range, they can also hear many annoyances which interfere with the audio such as emissions from switching power supplies, radio frequency transmitters, inverters, etc.Do It Yourself ReceiverGiven the standardization of induction loops here, I would have thought receivers would be cheap, plentiful and designs widely available. Strangely, this was not the case. It seems that the system is generally relegated to assistive uses, and thus non-hearing aid users dont get to benefit from the system.My interest was to not only access such telecoil services, but also to try and perceive the world in the electromagnetic audio region. Think of this as an artistic venture, similar to how the guys who implant magnets in themselves do so to try and perceive the worlds electromagnetic fields except this one is generally painless.Phase One Design Passive CoilDSC_8252DSC_8173Seeing as its an alternating magnetic field, it should be fairly simple to pick up if the field is strong enough. I decided to grab any scrap beefy inductor with many windings (to improve the inductance) and placed it in series with a blocking capacitor (to prevent any phantom power flow which would saturate the inductor). Attach some wires and a 3.5mm plug and youve got something basic.The trick was trying to find a highly amplified sensitive input. I tried mic inputs, and others. But even plenty sensitive inputs, even on the Zoom H1 (which I used to do almost all of my recordings) were very noisy. They are so hissy that I wont even bother uploading any of them.It was clear that some amplification was needed.Phase Two Design Amplified by Rail-to-Rail Op-AmpDSC_7730This time, I decided to get a little more sophisticated, opting for the use of a rail-to-rail opamp to provide amplification (as I abhor trying to build, and carry 15v dual-rail supplies). In this case, the device consisted of a three-AAA carrier with switch, the board with the circuit and the 3.5mm audio plug. The whole device was encased in glue to improve resistance to conducted stray charges which affect performance.It looks pretty simple, but stupid me made so many mistakes along the way. Its clear I didnt do any analog electronics for a long time and Ive literally forgotten some important practical considerations.I started with the Microchip MCP6273 2Mhz Rail-to-Rail Op-Amp, a 1200uH inductor, and an inverting amplifier design with virtual ground provided by a resistive divider. Trying to save some power, I used some fairly high resistances in my resistive divider, which resulted in an unstable virtual ground that caused oscillation. Get it together Gough! You cant screw up an inverting amplifier!After I scoped that one out, thanks to the Picoscope, I still had oscillation of a different sort. I decided to make the gains adjustable by trimpots, and I found the oscillation was pretty bad for most gains. Why? Why!Well, the Picoscope again gave me the answer the coil is a very efficient receiver of broadcast AM transmissions, and these signals were getting into the opamp, distorting non-linearly, and creating the resulting tones. In fact, I only recognized that when, in spectrum mode, I could see the AM sidebands. Then it hit me. So, uh, remember to band-limit your signals. Just because youre interested in one sort of signal, doesnt mean that you wont get others leaking into your receive chain. I decided to go for a simple R-C filter (which doesnt do much, as the cut-off was quite high due to a lack of spare components), and youll see later, I managed to stuff that up too.But it still didnt satisfy me. I tried to push the gain, and at some point, it would just collapse and die. Then I remembered the rule of Gain Bandwidth Product. You would think that the 2Mhz rail-to-rail opamp is fine for an audio frequency project, but alas, when you want 500-1000 fold gain, then the bandwidth drops dramatically. The bandwidth is given for unity gain, duh!Therefore, it is much better to go for a two-stage construction, with the first opamp doing some of the amplifying, and the second doing some more. But I only had a limited number of them at the time, so I decided to conserve.Then after having sorted through all of that, I found out (the hard way) that the circuit was very sensitive to component choice. A design I breadboarded just a moment ago, built using supposedly identical components, was not functioning properly. As it turns out, I was pushing the opamp so much to its limits, that the other unit I picked to build onto the veroboard just didnt have the same characteristics.amplified-designIn the end, I ended up with something that looks like this. But then I realized, I made a royal goof with the R-C input filter, which should have the 2n2 capacitor looped back to the virtual ground point. Ah insert expletive.But funnily enough, this design worked. And it did, in part, because the opamp was pushed so hard that its frequency response fall-off was acting as a natural filter! I think by doing this, Ive managed to refresh a lot of the things I should have known components arent as ideal as you would like them to be.Phase Two Design Audio SamplesI decided to carry this around with me, for a few days, as I went about my regular business and checked what the recordings showed. A lot of interesting sounds were received, with all of them provided as .wav files, as I dislike compression.Riding on a Bus its likely the alternator on the bus engine is putting out these whines.

A different bus this one shows a strange pipping noise as well.

In a car a petrol cars ignition system gives a tick every time the spark plug fires.

Next to a lift the lift inverter produces some rather harsh noises, but the background hum is endemic to the public announcement system at the train station.

Passing train a passing train seems to make a strange buzz, but only at certain carriages. Its likely those carriages carry the chopper/motor drive circuitry.

Under a power line a hum, but not the sort I expected.

A GSM 2G mobile the familiar bipping of the slotted TDMA transmission envelope. How annoying!

Alighting from a Warratah Train with Flashing LED the LED drivers make an interesting noise.

Riding along the rails, and again you get to hear strange noises, some of them alternating, some of them steady tones. These, I believe, are related to audio frequency track circuit impedance bonds I only found out about these when I stumbled across the NTSB presentation about loss of train detection in WMATA. Some others may be related to the RFID detection systems and their power envelopes.

A pelican crossing probably my proudest moment was when I decided to put the transducer up to the vibrating part of the push button padestrian crossing. Normally, when at a crossing with an audio recorder, you get this. Instead, now you can get the signal cleanly, without the ambient noise. Even better, if you employ DSP, you can even clean it up!

Of course, this is not all. By holding the device up to monitors, light switches, power supplies you can tell if theyre on or off, and what sort of loading they are running. Its even possible to hold this up to a phones earpiece and get a recording of the speaker audio (the basic purpose a telecoil should fulfil).Time to visit some real audio induction loop systems see that in Part 2!Phase Three Design Improvements and FixesFor the third design, I decided to opt for a much higher (80Mhz) bandwidth opamp (which is overkill) and not bother with having the two-stage design I would have otherwise gone for. Component values for the R-C filter have been changed to narrow the filter response, and since I was out of 3.6v 3xAAA cell holders, I opted to go for USB, instead using an LDO linear regulator to hopefully remove most of the ripple noise. I left a gain trimpot in there as well, so I didnt have to settle for fixed gain. The design looks like this:amplified-design-updatedUnfortunately, and as I had predicted, the RF noise emissions from the power bank seems to have an influence on this one, causing popping and clicking if you use a poor quality power source. However, it seems the quality power banks do make for a quiet result! I can always build a linear USB power supply if I need to, or adapt the connection.ConclusionAudio frequency induction loops are an assistive technology that allows for the broadcast of audio to hearing aids using magnetic field coupling. Receivers for these systems are not common, however, it appears simple to design and build your own.In the process of attempting to do so, Ive reminded myself of how many basic realities of components I have forgotten, and its been a re-education exercise to some extent. However, I did eventually achieve what I set out to achieve, through sheer persistence and logical troubleshooting. In turn, I have been rewarded with the ability to perceive alternating magnetic fields inside the auditory hearing range.Its important to remember that, as the inductors used are not designed for picking up such emissions, they are probably shaped to reduce stray field leakage which means a low signal collection efficiency which reduces the signal to noise ratio. Real hearing aids are likely to see better quality reception in that regard. Proper hearing aids are now starting to provide digital signal processing noise reduction on these inputs, thus continuous tones, and hiss noise is probably quite significantly reduced to make it more intelligible. Furthermore, they may operate with automatic gain control and bandwidth-limiting systems which would alter what they would perceive.However, by having one of these devices, it is now possible to receive audio from such systems without the associated echo and room noise.

1. What is an inductive loop?
An inductive loop is a wire wound in a rectangular, square, or round shape that is typically sawcut into the pavement. The ends of the wire are brought back to an enclosure, which houses an inductive loop detector module. The detector module powers the loop and causes a field to form around the loop. The loop automatically tunes to a resonant frequency. The detector module monitors this resonant frequency to determine if a vehicle is in the loop area.

2. What is inductance?
Inductance is defined as the opposition to a change in current flow. When a current is applied to a conductor such as a wire, a magnetic field is formed around the wire. If the current source is removed, the magnetic field collapses into the wire trying to maintain the current flow. By winding several turns of the wire into a coil, the magnetic field is intensified, which increases the inductance.

3. How is the vehicle detected?
When a vehicle enters or crosses the loop, the body and frame provide a conductive path for the magnetic field. This produces a loading effect, which in turn causes the loop inductance to decrease. The decreased inductance causes the resonant frequency to increase from its nominal value. If the frequency change exceeds the threshold set by the sensitivity setting, the detector module will output a detect signal.There is a common misconception that an inductive loop requires a mass of metal or ferrous material for detection. Placing a single wire around the perimeter of the loop and shorting the ends together will quickly disprove this misconception. The single wire forming a shorted turn provides a current path for the magnetic field; thus causing a loading effect similar to that of a vehicle. The shorted turn effect of the single wire coil in the proximity of the loop acts much like a shorted turn secondary of a transformer.

4. What is the minimum acceptable loop inductance?
An inductive loop detector will tune to inductance values ranging from 20 to 1000 microhenries. It is preferable that the combination of the loop and lead-in inductance values has a minimum of approximately 50 microhenries for stability. As a general rule, the loop inductance should be equal to or greater than the lead-in inductance.

5. How many turns of wire should be installed in the loop?
The number of turns required in the loop is dependent on the loop size. The loop inductance can be calculated as follows:L=P(t2 + t)/4; WHERE:
L = Inductance (Microhenries)
P = Perimeter (feet)
t = Number of turns The formula can be simplified to: L = PK substituting a constant K for (t2 + t)/4.Filling in the number of turns and calculating K: Number of Turns (t) K (constant) K=(t2 + t)/4
1= 0.5
2= 1.5
3= 3.0
4=5.0
5= 7.5
6= 10.5
7= 14
Example: 4' x 8' loop with 4 turns
L = P K
P = 4' + 4' + 8' + 8' = 24'
K = 5.0
L = 24 x 5.0
L = 120 microhenries Loop Inductance in Microhenries (H)

Number of Turns1 2 34 5 67

P
E
R
I
M
E
T
E
R(FT)10515305075115140

20103060100150230280

30154590150225345420

402060120200300460560

5025 75150250375575700

603090180300450690840

7035105210350525805980

8040120240400 6009201120

904513527045067510351260

10050 150300 500 750 1150 1400

Recommended Number of Turns per Size of Loop

P
E
R
I
M
E
T
E
R(FT)Number of Turns

105

20 4

303

403

502

60 2

702

802

90 2

100 2

Use the highlighted values listed in the table above to determine the number of turns required for a given size loop. Always use at least 2 turns.

6. Does increasing the number of turns in the loop increase the sensitivity of the loop?
NO. Increasing the turns does not increase the sensitivity of the loop. It can improve the efficiency of the loop system (loop inductance + lead-in inductance), if the lead-in length is over 400 feet. The amount of inductance change a vehicle can cause in a loop is determined by the following factors: Amount of Change Vehicle Size
Caused by Vehicle (Loop Size) x (Vehicle Height) The above formula indicates the following:1. Increasing the loop size will decrease the amount of change caused by the vehicle.
Example: If a vehicle causes a 1.0% change on a 6'x6' loop, then the same vehicle will cause a 0.5% change when over one of two 6'x6' loops connected in series.
2. A smaller vehicle will cause less change. A small motorcycle causes approximately 1% to 2% of the change caused by standard automobiles.
3. The higher the vehicle is from the road (loop) surface, the smaller the inductance change.

7. Does increasing the number of turns in the loop increase the detection height of the loop?
NO. Increasing the turns does not increase the detection height. Rule of Thumb: The reliable detection height of a loop is 2/3 of the short side of the loop.
Examples: 6'x6' loop. The sort side is 6 feet. 2/3 of 6 = 4 feet
6'x20' loop. The sort side is 6 feet. 2/3 of 6 = 4 feet.
4'x20' loop. The sort side is 4 feet. 2/3 of 4 = 2 feet 8 inches.

8. How deep should the loop wire be installed?
The deeper the wires are below the road surface the more they are protected from road surface wear and the elements. The top wire should be a minimum of 1 inch below the road surface.

Nonconductive materials such as concrete and asphalt will not influence the loop fields. Installing the loop one inch deeper (e.g. 3" depth instead of 2" depth) would have the same result as raising the vehicle one inch above the pavement surface. To reduce stress and abrasion of the loop wire the 90 corners should be cut at a 45 angle; core drilled (1.5" diameter); or at a minimum, the sharp inner corners should be rounded with a chisel.

9. What type of wire should be used for the loop?
Number 16 or 20 AWG stranded wire can be used. The wire gauge is not critical to proper operation of the loop detector. The wire should maintain its integrity under the pavement stress. Since asphalt is more flexible than concrete, it is recommended that a heavier gauge wire be used for loop installations in asphalt.The main consideration in selecting a wire for loop installations is the type of insulation. Cross-linked polyethylene (XLPE) insulation rated at 600 volts is highly recommended over PVC insulation. Under similar conditions, XLPE insulation will absorb approximately one percent of the moisture absorbed by PVC. When insulation absorbs moisture, loop drift occurs, which if great enough, can cause false detections. XLPE also has higher resistance to abrasion, heat, oils, and gasoline. After insulation, and any time there appears to be a loop related problem, the loop should be tested. Use a MegOhm Meter to test the integrity of the loop / lead-in wire insulation. Readings of 100MO or less indicate possible insulation damage. Use a Multimeter to check the total resistance of the loop / lead-in combination. Total loop / lead-in resistance should never exceed 4 Ohms.

10. How far from a gate should the loop be installed?
As the length of the sides of the loop that parallel the gate increases, the inductance change caused by the gate also increases. The graph shows the inductance change for different distances between the gate and the loop for different sized loops.The closer the loop is to a gate, the more influence the gate has on the loop! Hence, the detector sensitivity must be set lower to ensure the gate will not cause the detector to generate an output when the gate closes.The following rule should be observed: The longer the loop, the greater the spacing must be between the gate & the loop!The inductance change at two feet is one third of the change at one foot. At four feet, the effects of the gate on the loop are minimal.