What about Nexus search coil frequencies?

Topics related to Nexus metal detectors here, Off topic posts will be removed.
NEXUS Videos
Post Reply
Nexus
Posts: 93
Joined: Thu Apr 03, 2014 10:58 pm
Has thanked: 44 times
Been thanked: 5 times

What about Nexus search coil frequencies?

Post by Nexus » Thu Feb 02, 2017 7:39 am

An explanation of why there are no high frequency search coils for most Nexus models.

Many have been led to believe that high frequency search coils are the best answer to hunt for small non-ferous objects. What are the technical facts?

Benefits and drawbacks from using high frequencies.

1. High frequency transmissions will induce stronger Eddy currents in metal targets at LOWER power consumption levels, but at appropriate HIGHER power output lower frequencies can do just the same. So if the power consumption is not the main issue, which is the best frequency choise for all terrain search? The answer is – low frequency. Why is that?
– Because at low frequencies the ground interference is much lower and as a result the detectors will be more stable at considerably higher depth penetration. This is direct result from the fact that high frequencies induce Eddy currents in the soil much more than low frequencies do.
– Low frequency detection is far more succepable to metal targets with a long time constant, such as hoards and other medium to large non-ferrous objects. This means that at low frequencies these kind of targets are possible to find considerably deeper.
– Compared to high frequencies Low frequency systems are also more accurate in identifying small and medium Iron objects at any depth.

So if we are to summerize the above in a few words it would look like this:
- High frequency – high ground interference vs. Low frequency – low ground interference.
- High frequency – lower ground penetration in normal and mineral soil conditions vs. Low frequency – better ground penetration in normal and mineral soil conditions.
- High frequency – high sensitivity to small objects at low power output levels vs. Low frequency – high sensitivity to small objects at HIGH or VERY HIGH power output levels.
- High frequency – reduced sensitivity to hoards and larger non-ferrous objects regardless power output vs. Low frequency – best sensitivity to hoards and larger non-ferrous objects regardless of power output.
- High frequency – greater loss of depth in wet soil conditions vs. Low frequency – lower loss of depth in wet soil conditions.
- High frequency – high interference from salt water vs. Low frequency – lower interference from salt water.


Ok then! Is there any benefit from high frequencies at all and what is it?
- Good sensitivity at relatively low power consuption.
- Faster recovery speed. At high frequencies the search coil will store inherently much less energy, which always tends to oppose the useful target signals thus allowing better recovery speed and greater rate of finding non-ferrous objects next to Iron.
- High frequency search coils can be lighter as they can work fine with smaller gauge wires, which is not the case with low frequency search coils.

Can faster recovery speed be achieved with low frequency search coils? Definitely yes. How?
- By reducing the output power of the transmitter if possible.
- - By reducing the Threshold level to a point where some 20-30% of the detecting range will be lost. But what about depth penetration? Well! In Iron infested areas there is no such thing as depth penetration. In such trashy conditions the only thing that counts is a finds rate, which will come mostly from the top couple of inches of the soil, therefore great sensitivity and depth penetration are of no use in those conditions.
- By reducing the input sensitivity of the detector if possible.
- By utilizing a concentric search coil (if possible) instead of DD. Concetric search coils are inherently the fastest in terms of recovery speed, because their loops are working on the same axis.

What is the main advantage of high frequencies from a marketing point of view?
- They are the best choice should any manufacturer choose to impress the audience with high in air sensitivity, long lasting batteries, possibly lighter search coil weight and good sensitivity to non-ferrous objects smaller than any coin out there, such as tiny jewellery and gold nuggets where the conditions would allow their detection in first place. All in all it is easier to impress people at a first glance with the high frequency detectors and they are easier to manufacture.

What are the main advantages of a HIGH powered low frequency search coil?
- It will go deeper always. On what targets? Just about any except the tiniest bits and bobs, which in most cases are of little use compared to good quality hammered coins for example.
- It will identify Iron more accurately than a high frequency search coil.
- It will be for ever more stable on all ground conditions.
- It will be less affected by hot rocks interference.
- It will penetrate better all red bricks and other ceramic materials.

The above information helps explain why the HIGH powered Nexus MP is as good as it is claimed to be.

- The MP (since 1.09.2016) can find all hammered silver coins and also all other coins considerably deeper than any alternative high frequency detector.
- The MP can find any hoard or medium to large non-ferrous object much deeper compared to any high frequency detector.
- The MP is inherently more stable in all ground conditions compared to any high frequency detector.
- The MP is the most successful IB detector for work in high mineral soil conditions.

Filternozzle
Posts: 387
Joined: Wed Apr 17, 2013 12:08 pm
Location: UK
Has thanked: 5 times
Been thanked: 16 times

Re: What about Nexus search coil frequencies?

Post by Filternozzle » Thu Feb 02, 2017 8:17 am

Many thanks Nexus, I now understand why Nexus use lower frequency coils. I wasn't aware that HIGH power output had such an affect on Low frequency coil performance. It all makes sense now. A full days detecting with the Nexus MP requires two battery packs because of the HIGH power being used. This HIGH power is not only giving extra depth but also sensitivity to small low conductors such as hammered coins.

For myself the benefits of having to use two sets of batteries for a full days detecting is well worth it.
Updated NEXUS MP & Minelab GPX-5000 ::g

WVAM
Posts: 374
Joined: Fri May 20, 2016 9:13 pm
Has thanked: 5 times
Been thanked: 49 times

Re: What about Nexus search coil frequencies?

Post by WVAM » Thu Feb 02, 2017 9:35 am

Thanks Nexus, very informative post. I'm in the "what machine to choose" quandary" and searching for actual data. As yet I can't get any from the various companies. A bit of perspective first: I recently put my mountain bike on a diet. Spending £1 to loose 10g seemed reasonable and I shed nearly 1Kg for ~£100. The next step would require spending £10 for each gram, or thereabouts. At that point I drew the line!
Also, I've done a lot of work with visible light and that always suffers from the inverse square law.

So, can you add some perspective on the increase in depth vs. frequency vs. cost. And, does power/frequency/depth also contain an inverse square effect, or similar. I realise that any coin just out of reach is a coin not found but if an extra cm in depth costs £1k then it might change decisions.

Thanks again for the post.

Nexus
Posts: 93
Joined: Thu Apr 03, 2014 10:58 pm
Has thanked: 44 times
Been thanked: 5 times

Re: What about Nexus search coil frequencies?

Post by Nexus » Thu Feb 02, 2017 11:33 am

WVAM wrote:
So, can you add some perspective on the increase in depth vs. frequency vs. cost. And, does power/frequency/depth also contain an inverse square effect, or similar. I realise that any coin just out of reach is a coin not found but if an extra cm in depth costs £1k then it might change decisions.
Depth vs. frequency vs. cost - these 3 have little in common and are mostly down to a decision from the particular manufacturer.
Depth vs. frequency - these 2 are strongly related as I have explained in the article above.
Extra 1 cm depth does not have to cost anything extra at all, however considerably greater differences do not come cheap in the designs. The MP is not about such small difference and it is only marginally more expensive than Deus and also cheaper than CTX3030, but from these 3 models only MP can reach down to 12" for my hammered silver Edward III. The other two will not get it even at 10". I know this for a fact.
On a public test the Deus (with all coils tested) was the only detector that failed to detect (in all of its programs) 1 kg of small Roman coins at 20" in the ground, where the MP can do this at 24" in most soil conditions.

Deus is 1500 Euros, the MP is 2000 Euros and the CTX is 2500 Euros.

So what is left for you to decide is what detector do you want? Popular! With many features! Definitely digital! Super light! Underwater! And so on and so on. Or just one that can find the things you are after with considerably greater probability.

I personally believe that the inverse square law have little to do with metal detectors.

Power-frequency-depth are very much related, but I do not see any square inverse law in that relation.
If depth is a fixed value less power will allow lower frequency to reach that value without much trouble from ground minerals.
If the frequency is high there is a good chance that the fixed depth value is not going to be reached (if too deep) regardless the power level. Not just that, but the ground interference will raise out of control if the output power is too high at high frequencies.

This is why the best choice should one desire to get at depth both small coins and all of the other objects will always be a high power output at low frequency.

WVAM
Posts: 374
Joined: Fri May 20, 2016 9:13 pm
Has thanked: 5 times
Been thanked: 49 times

Re: What about Nexus search coil frequencies?

Post by WVAM » Thu Feb 02, 2017 2:50 pm

The simple questions are,
1. Are there any other detectors working at low frequency and high power and if so which ones?
2. What is considered "high power"?

And I can't get the new Nexus website to load properly.

Nexus
Posts: 93
Joined: Thu Apr 03, 2014 10:58 pm
Has thanked: 44 times
Been thanked: 5 times

Re: What about Nexus search coil frequencies?

Post by Nexus » Thu Feb 02, 2017 3:04 pm

WVAM wrote:The simple questions are,
1. Are there any other detectors working at low frequency and high power and if so which ones?
2. What is considered "high power"?

And I can't get the new Nexus website to load properly.
I am not aware of other detectors working at low frequencies and high power output.
We measure the power output of any TX by e specialized tool and compare the distances relative to absolute amount measured. So we consider a high power a measurement that exceeds at least by few times the average measurement for most detectors.

The web site accessibility is a bit out of our control as we are using services from web hosting company.

User avatar
liamnolan
Moderator
Posts: 9110
Joined: Sat Jul 18, 2009 11:06 am
Location: Norwich - originally from Dublin, southern Ireland
Has thanked: 107 times
Been thanked: 132 times
Contact:

Re: What about Nexus search coil frequencies?

Post by liamnolan » Thu Feb 02, 2017 7:52 pm

Some excellent product description there, most of it over my head technically but its information such as this that helps us all find the right machine for the right conditions.
Great topic, Liam OooO
Deus, WSi's - In the end we will regret the chances we didn't take, the relationships we were afraid to have and the decisions we waited too long to make .. Secretary Irish Metal Detecting Society IMDS

WVAM
Posts: 374
Joined: Fri May 20, 2016 9:13 pm
Has thanked: 5 times
Been thanked: 49 times

Re: What about Nexus search coil frequencies?

Post by WVAM » Thu Feb 02, 2017 9:12 pm

further to my earlier question to Nexus about whether the inverse square law applies to detecting, it's actually worse. This is on the First Texas website from their engineer David Johnson:
"Getting extra depth out of a VLF, multifrequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That’s the basic reason why depth increases come so slowly in this industry."

Liam, I trust you are paying attention :D

User avatar
liamnolan
Moderator
Posts: 9110
Joined: Sat Jul 18, 2009 11:06 am
Location: Norwich - originally from Dublin, southern Ireland
Has thanked: 107 times
Been thanked: 132 times
Contact:

Re: What about Nexus search coil frequencies?

Post by liamnolan » Thu Feb 02, 2017 9:31 pm

Chris, I fell asleep after that first sentence zzzzzzzzzzzzzzzzzzzzzzzzzz :D I-)
I know that my rate of digging is directly related to the time since my last cuppa ..
Detectors these days are really so advanced. I will not go off-topic but I had the GMP and now the Deus and both machines have been fantastic if set up correctly.
I expect not many users actually really know how the machines perform under all conditions, most are "switch on and go" types that suit Mr & Mrs Average detectorist, Liam OooO
Deus, WSi's - In the end we will regret the chances we didn't take, the relationships we were afraid to have and the decisions we waited too long to make .. Secretary Irish Metal Detecting Society IMDS

WVAM
Posts: 374
Joined: Fri May 20, 2016 9:13 pm
Has thanked: 5 times
Been thanked: 49 times

Re: What about Nexus search coil frequencies?

Post by WVAM » Thu Feb 02, 2017 9:46 pm

liamnolan wrote: I know that my rate of digging is directly related to the time since my last cuppa ..
Detectors these days are really so advanced. I will not go off-topic but I had the GMP and now the Deus and both machines have been fantastic if set up correctly.
I expect not many users actually really know how the machines perform under all conditions, most are "switch on and go" types that suit Mr & Mrs Average detectorist, Liam OooO
Completely agree ::g

Nexus
Posts: 93
Joined: Thu Apr 03, 2014 10:58 pm
Has thanked: 44 times
Been thanked: 5 times

Re: What about Nexus search coil frequencies?

Post by Nexus » Thu Feb 02, 2017 10:06 pm

WVAM wrote:further to my earlier question to Nexus about whether the inverse square law applies to detecting, it's actually worse. This is on the First Texas website from their engineer David Johnson:
"Getting extra depth out of a VLF, multifrequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That’s the basic reason why depth increases come so slowly in this industry."

Liam, I trust you are paying attention :D
Raising the power output is not the only way to increase depth.
Depth is most efficiently increased by increase of search coil size, because the field transmitted by a larger TX coils decay much slower than the field of smaller coils. This is the very reason why larger coils can detect objects further away with the same TX power compared to a smaller search coils.
Detection depth can also be improved by a carefully arranged relation between output power, sampling frequency, recovery speed of the DC amplifiers and amplification of the electronic circuits.
In short terms to double the in ground depth of any detector;
- The effective surface area of the search coil have to be increased 64 times or the equivalent diameter of the search coil have to be increased 8 times. This is the way with the least potential increase of ground interference.
- Or the electronic amplification have to be increased 64 times. This would require increase of the quality of electronic circuitry in the same rate and almost certain decrease of recovery speed. This is the way of the worse increase of ground interference.
- Or the power output of the TX (for known fixed coil size) have to be increased 64 times. In this way ground interference will increase with lower increments to power output increase.
- Or a reasonable combination of all mentioned above.

Change of frequency can never lead to double depth increase (except for tiny bits) , but it can lead to marginal increase of depth for some targets at the expense of depth for other targets.

The FTP statement mentioned above can not get any more wrong than it is.
The very reason for the limits of depth increase is not the power, nor any other electronic problem or solution, because worse limiting factor for all detectors depth penetration is the ground it self.
By electronic means detectors can exhibit many times the demonstrated sensitivity so far, BUT this will also come in package with equivalent amount of ground interference increase.
For this reason manufacturing detectors that can pick up a single coin at 30" (with small search coil) in air is pointless, not only because of the accompanying unmanageable increase of ground interference, but also because of the limits of how deep a given target can be detected in the ground in the first place. And there are really stiff limits for that too.
So the only way ahead is to reduce the ground interference as much as possible, while improving depth capabilities. True depth penetration can not be achieved in any other way.

Also be aware that increase of in air sensitivity have nothing to do with increase of in ground depth, because there is a relation to that too.
If the in air sensitivity is increased by 100% the in ground depth will be increased by only 50% providing the increase sensitivity did not lead to a dramatic increase of ground interference.

The real reason for lack of considerable increase of depth in this industry is the fact that the ground interference phenomenon hasn't been beaten yet, only partially controlled.
Last edited by Nexus on Thu Feb 02, 2017 10:27 pm, edited 1 time in total.

Nexus
Posts: 93
Joined: Thu Apr 03, 2014 10:58 pm
Has thanked: 44 times
Been thanked: 5 times

Re: What about Nexus search coil frequencies?

Post by Nexus » Thu Feb 02, 2017 10:21 pm

liamnolan wrote: Detectors these days are really so advanced.
Actually they aren't. If they are how come no one gets twice as deep with the same size coils as all of the rest?
What did get more advanced is only the digitally controlled adjustments, which suit better as you call them Mr&Mrs Average.
There is also this improvement with the dual or multi frequency technologies, but again only to suit better Mr.&Mrs Average as with this technology they can have more fun digging coins and not so much pull tabs. But in terms of more depth there is nothing.
If you get one let say Nautilus that was first made about 35 years ago you will see that the only thing less advanced in it is the adjustments (possibly the chunky box too), but in terms of real performance it puts to shame each and every advanced digital detector.
Behind every digital screen is hidden a good old fashion classic metal detector with a search coil, transmitter, receiver and some electronic circuit, which tells to the loudspeaker when to scream.

Post Reply

Return to “NEXUS Forum”