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Metal Detectors & Emi - Electro Magnetic Interference


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Look at the F75. Super sensitive receiver, but near unusable in many urban areas. So Fisher adds digital filtering (DST) and now the detector works far better in urban areas. But lost its edge in remote EMI free locations. That upsets some people because they hate anything that makes it sound like a detector is less powerful. But if the circuit is so over sensitive you have to back all the settings down to get it to work at all you end up in the same place or worse. To me the F75 with DST is a far better detector because it will work most anywhere now but you will now see people seeking out the old versions because they perceive the F75 DST as being neutered. And for all metal use in remote areas it may be the old versions work better for some people.

Some detectors never show any audio side effects from EMI but the interference can act to silently kill performance without you even knowing about it. THis is not uncommon on silent search type units in particular. The interference is there, you just can't hear it, but you are running at sub par performance.

If engineers could design detectors knowing for a fact they would always be operated in areas free of EMI you would see an immediate increase in detector performance. Most detectors are dumbed down to one degree or another due to EMI mitigation necessities. There really is no magic easy answer here.

Keep it in perspective however. The number one problem is not EMI but interference from induced ground signals. The main goal in new tech is to figure out how to best induce detectable signals into the desired targets while not "lighting up" the ground at the same time. Bad ground has far more impact on detector performance than EMI in most situations.

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Just another note to help the thought processes. There are two types of detector sensitivity controls. By far the most common is just an ability to adjust the sensivity on the receiver side. If you get too much electrical interference, the first thing to do is lower the sensitivity. Lowering sensitivity usually reduces EMI effects far faster than it reduces the attainable depth on targets.

Changing coils can also have a huge impact as the coil is the main route into the detector for EMI. Smaller coils almost always are better behaved around EMI. Special figure 8 wound "anti interference" coils can eliminate it entirely. The BigFoot coil is EMI immune as far as a coil can be.

Some detectors like the V3i and DEUS, for example, allow you to increase the transmit power (TX Boost). This can add depth in mild ground of help compensate for depth differences seen at different operating frequencies. Low frequencies like more power. However, this extra power eats batteries faster. It can also result in less depth in bad in bad ground as the extra power just lights up the ground minerals more, to the point where the coils overload and shut down.

In mild ground a trick to reduce EMI is to reduce the receiver sensitivity, which also reduces the EMI. Increasing the TX however adds no EMI but can add back the depth lost by reducing the receiver sensitivity. But again ,this only really works in mild ground.

In my mind I like a detector that has a receiver sensitivity setting such that, when turned up all the way, the detector misbehaves and gets "too sensitive". This indicates to me the engineers are pushing the limits, and so backing off from full settings is needed to ge best results. Unfortunately, too many novices fall prey to max setting syndrome. The detector then acts as if it is defective. Many complaints result. Engineers throw in towel, make sure next detector always behaves well at max sensitivity setting. Honestly, if the detector gets squirrelly at max sensitivity settings, don't complain! That really is a feature in my book. If a detector always runs well at max sensitivity I can't help but feel a little performance has been left on the table.

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3 hours ago, Sourdough Scott said:

I notice this also. I have led lights throughout the house. And when I want to listen to my portable radio, I have to keep the lights off or the radio station is very static.

I also noticed that problem with the radio on my jeep when  the LED light bar on.

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