Tx and Rx coil inductance ratio

[Nandor]

I forgot it and maybe did not understand it well, but now it's all clear to me thank's dbanner.

[Nandor]

I just made some experiments with different Tx coils and finally found out why different frequencies have different coil values. It was interesting to see that the oscillator cant run if the tank capacitor is too high in value but then the Tx inductance also has to be changed and then power consumption is also very important, look:

This is the Raptor's oscillator with my homemade TGSL coil


Tx-12.85kHz 14.8Vpp 1.56mA


R1-1K R1-680ohm
Vpp-9.44v Vpp-14.5v
A-1.3mA A-1.8mA


L-Tx 1mH~~R=3.7ohm R1-100ohm


TX-8.5kHz 13.1Vpp 6.45mA
Tx-10kHz 14.8Vpp 6.06mA
Tx-13.9kHz 16.4Vpp 5.11mA
Tx-15.7kHz 17.2Vpp 4.80mA


Tx R=25.8ohm~5.96mH


Tx-13.9kHz 14.8Vpp 0.94mA

As I lowered the Tx frequency the power consumption went higher. Also the sinewave got smaller so it needed more current and voltage

[Nandor]

That was the information I was looking for!!!
Thank you very much Carl.

[dbanner]

I'm reminded of just why measuring instruments typically have high input inpedance, precisely to avoid loading the circuit being measured, and in so doing, avoid incorrect readings.

[Carl-NC]

I've tried different value resistors for TGSL front end. As the resistor values gets smaller than the original 4k7 the Rx signal gets more noisy and less in amplitude with the same gain ratio of 47x aprox. I tried 1k-47k, 2k2-100k but the best was with 4k7-220k.

Does the coil gets loaded if the resistor value gets smaller, could somebody explain this?

If the TGSL runs at 10kHz then a 6.5mH RX coil has a ~400 ohm impedance (2*pi*f*L). You want the input resistor to be at least 10x this to minimize loading. 1k will load it and cause a signal reduction.

Lowering the input R also increases its current noise (4KT/R) that gets reflected into the coil impedance, which might explain why it looked noisier.

[dbanner]

I'm no expert, but I think it's has to do with the coil impedance. Otherwise you "load" the coil and change its characteristics, adversely affecting the search loop.

[Nandor]

I have many Tesoro schematics, as you say they are almost the same. Some time in the past I was browsing the TGSL topic and it was a post about coil loading, but then I did not understand electronics as much. I would like to find out what it does mean and how to avoid it.

[dbanner]

I think 5.1K @1% was the stated value on the bill lahr schematic(tesoro golden sabre). TGSL schem was derived from original, but a few part values were altered slightly.
All these tesoro circuits ( silver sabre, golden, pantera, el dorado, royal sabre, bandido) use 5.1K. The lobo is different because the inductance of the RX coil is different.
The rx inductance for original coil is about 6.2mH, uses a 5.1K resistor. The TGSL build document states the rx coil at 6.5mH, using a 4.7K resistor. So there you have it.

[Nandor]

I've tried different value resistors for TGSL front end. As the resistor values gets smaller than the original 4k7 the Rx signal gets more noisy and less in amplitude with the same gain ratio of 47x aprox. I tried 1k-47k, 2k2-100k but the best was with 4k7-220k.

Does the coil gets loaded if the resistor value gets smaller, could somebody explain this?

[ifrenide]

Hello hyina, I think 21" coil weigths : 21"/30"*1095grams=766.5 grams . for 41" coil it is: 41"/30"*1095grams=1496.5 grams .I just guessed !

[Hyena]

What is the weight of these beautiful monsters?

30 inches / 75 centimeters; Approx 1095 means grams?

What is the weight of 21" coil and 41" coils?

[ifrenide]

When someone states that big coils are unpractical that tell's me one thing - the person behind the statement knows little or nothing about big coils, how do they work, how to use them and what they are good for.
The Nexus Exselsior was sold to only few customers, because I do not want to market this model. It takes months to manufacture one and it is more profitable to make and sell smaller detectors. That detector was developed by request from customers in the first place. So do not be in a hurry to believe in wrong conclusions.

The bigger coils do exhibit more sensitivity to both ground and targets, however they also exhibit higher signal to noise ratio. This means that the ground noise by proportion is lower compared to the target signals, thus the better depth of bigger coils.
Big coils are less affected by the reactive component(ferro-magnetic) in the soil and more vulnerable to the active component (conductivity) of the soil in VLF only. This is why they work better in highly mineralised soil conditions compared to smaller coils and are somewhat more noisy on muddy soil, next to small coils.
However big coils are big and always get the deep stuff, no matter what. Well providing the operator knows why did he get the big coil in the first place and how to use it.

Since you use cars for comparative example let me give you one.
Ferrari is not much good off road, but it gets 300km/h easily where it counts.
Same with big coils. They are unbeatable where it counts.

1-you told me that I don't know how big coils work,you can just say how VLF coils work , you specified them here as they work differently.do you think that work with a different way?.
2-you think I don't know how to use a big coil. so if we have a big coil .I can't use it like usual ones the bigger coils have special manipulation? .my brother we use all coils the same way,we keep a small space between the coil and the ground and we seep it "explanation for aliens, I think they don't know how to use a coil".
3-what they are good for? when I meet Aliens ,I will tell them" its complicated you have spend years and years to understand why we use a big coil".
4-you said" I do not want to market this model. It takes months to manufacture one".I don't think that you are manufacturing a rocket .takes months,come on man.
5-you said :" it is more profitable to make and sell smaller detectors".all this detectorists.they don't know what is the good thing for them unfortunatelly.hey man there something let them go to choose smaller coils. you have to find it. I can't repeat it.
6-you said"That detector was developed by request from customers in the first place". I think this company should take the initiative and lead customers,and don't be passive regarding inovation.
So do not be in a hurry to believe in wrong conclusions from you,to you .
7-you said:"The bigger coils do exhibit more sensitivity to both ground and targets" this is a passive point of view "this is what happen",the truth is that we can be active and treat this problem with electronics and "change what's happening".this is the golden rule . vintage detectors haven't a lot of features which exist now days.
8-you said:"however they also exhibit higher signal to noise ratio. This means that the ground noise by proportion is lower compared to the target signals, thus the better depth of bigger coils."So you think that detection depth of big coil is a result of that the ground noise by propotion is lower tha target signals, are you serious? not eddy currents are propotional to electromagnetic field power.of course with taking care about parasitic factors,and how good our electronic system ,in terms of effeciency?
9-you said:"Big coils are less affected by the reactive component(ferro-magnetic) in the soil and more vulnerable to the active component (conductivity) of the soil in VLF only. This is why they work better in highly mineralised soil conditions compared to smaller coils and are somewhat more noisy on muddy soil, next to small coils.
However big coils are big and always get the deep stuff, no matter what. Well providing the operator knows why did he get the big coil in the first place and how to use it."you are talking about a result and not about the reason "Big coils are less affected by the reactive component(ferro-magnetic) in the soil and more vulnerable to the active component (conductivity) of the soil in VLF only " this is a result and not a reason.try to find, the reason behind that.
Finaly the example of cars is to highlight the portability,and not the speed.
nice to contact you nexus.we are brothers .

[Nexus]

Is this a dual field coil?
Sure looks that way. ive thought this is the only way to do a monster coil.

It is a concentric coil with TX outside and RX inside. It works like DD on GPX, just the RX is much smaller. The purpose is to have the power output of a Mono coil and some discrimination abilities as DD. For a given size Nexus concentric coils are also more sensitive to small objects compared to Mono and DD equivalents when used on ML GPX detectors.

[godigit1]

Is this a dual field coil?
Sure looks that way. ive thought this is the only way to do a monster coil.

[Nexus]

Nexus
my brother.your coil is an exeption and you said : "that was made only for few customers" you know why , because it is unpractical and sometimes you can find some people like testing and trying some stange stuff "the world's biggest VLF coil" we love unusual things. you said :" like Carl mentioned the increase of sensitivity is not only to targets, but to ground as well" I respect him, of course he's the lion here . but I want tell something mybe I'm wrong. If if we get a high sensitivity for both ground and target, we stand and watching them? no,we will solve this problem by using electronics" make a good ground balance system".
in theory making a big VLF coil is a true fact .un practical if we take in consideration sweep speed of coil and human force to match this speed, and there is a problem when we want to take this coil with as "it's too big ".we need a truck to make metal detecting so! .Without talking about effeciency. it's like you bay a bus instead, of car. and you bay it for personnel use . you can use it but, look around you .only cars is used for personnel purposes.

When someone states that big coils are unpractical that tell's me one thing - the person behind the statement knows little or nothing about big coils, how do they work, how to use them and what they are good for.
The Nexus Exselsior was sold to only few customers, because I do not want to market this model. It takes months to manufacture one and it is more profitable to make and sell smaller detectors. That detector was developed by request from customers in the first place. So do not be in a hurry to believe in wrong conclusions.

The bigger coils do exhibit more sensitivity to both ground and targets, however they also exhibit higher signal to noise ratio. This means that the ground noise by proportion is lower compared to the target signals, thus the better depth of bigger coils.
Big coils are less affected by the reactive component(ferro-magnetic) in the soil and more vulnerable to the active component (conductivity) of the soil in VLF only. This is why they work better in highly mineralised soil conditions compared to smaller coils and are somewhat more noisy on muddy soil, next to small coils.
However big coils are big and always get the deep stuff, no matter what. Well providing the operator knows why did he get the big coil in the first place and how to use it.

Since you use cars for comparative example let me give you one.
Ferrari is not much good off road, but it gets 300km/h easily where it counts.
Same with big coils. They are unbeatable where it counts.