How to build a Metal Detector Search Coil

My Name is Ciaran and I have been Metal Detecting for nearly 30 Years. 

So this week lets talk about how to make a coil for your metal detector and why you would do this and potentially why you shouldn’t. 

Hey everyone before we start I want to thank you for listening to the podcast and I hope you enjoy the episode this week  If you want to support the show there are many options available from the links in the episode notes below and if you want to interact with me and the show that information is there too.  

but most importantly If you like this content please don’t hesitate to tell your friends and don’t forget to hit that subscribe button.

Hi detectorists how has the week been nothing major for me this week just a week of plodding about in the rain driving on did a bit of local research in the warmth of the house and tidied up a few piles of finds so not a home run but more of a house keeping week. 

So this week I noticed this question on the forums and I wanted to give my two cents on the topic here and thats about how to make a metal detector coil and why you should do it and why you potentially shouldn’t. 

 Firstly I want to thank everyone for their reviews they have given me over the last 18 months I really appreciate them, 

 they have put the metal detecting show as the highest rated metal detecting podcast online which I am eternally grateful for however I got a review this week that was positive but gave me pause for thought as it prompted me to reiterate the concept of the podcast 

 so the review said something like this I love the podcast however I wont pay for it till it is over 30mins long as 10min of content with 2-3 minutes of intro is not enough. 

 now I could look at this and think it was negative but all it means to me is that this person really likes the podcast and would like more and would be willing to pay for more which is great but it did prompt me to refresh you all about the concept of the show. 

 The concept is simply a clear and  concise answer to metal detecting questions that come up in the hobby it is designed that the listeners can dip in and out when there is a question being discussed that they are interested in. 

 When coming up with the concept I researched and I listened to all the metal detecting podcasts that were available and I was an avid listener of several podcasts across all the genres and knew what I didn’t want to do. 

I didn’t want to have a weekly podcast that rambled on for 90 minutes without direction or purpose with very little valuable information being transferred to the listener. 

I’ll give you an example I listen to a homebrew beer podcast, a very famous one probably the most famous, with the most famous homebrewers on it every week and I noticed two things. 

Firstly they record it in a one 3 hour slot where they drink beer talk about the scene and techniques. 

 Next This 3 hour slot is cut up into 4 x 90 minute episodes and when you listen to each of these episodes you get about 15 mins of actual valuable content, 30 minutes of shit talking all interwoven with about 40 mins of ads and that is not what I wanted for this podcast. 

 Something else I had to consider when coming up with the concept was that I don’t have unlimited time to research, write, record, edit and market each episode. 

I work full time in a high responsibility role within a very large multinational so I have potentially 4-6 hours a week to prep for an episode. 

 ill give you a quick overview of each weeks process and what I do with those hours. 

 I also don’t stock pile episodes as I want to be available to comment on anything exciting that may come up real time like major hoards or product announcements like last week for example so when I publish the last week’s episode I am straight away looking for a topic for the next week 

which I mull and roll about in my head for the weekend during hunts or while I’m catching up on social media.  

 Generally by Monday I have a topic picked. I then do about  

2-3 hours research and maybe testing because remember I’m not an expert I need to answer these questions myself, I then spend about an 2 hours writing up the episode like I said I don’t want this to be a long rambling episode so I script it to keep me on track while being as clear and concise as I possibly can. 

It then takes about an hour to record the episode and about an hour to edit it so about an average of 6 hours work per episode and well over 2000 words wrote every week I believe I have wrote over 200000 words since starting.

I then hit publish generally at some stage on a Friday and the process starts again however I am trying to do some social media push during the week, so I create different artwork for every episode. I try to engage with people on social media and in general, I try to market the podcast in between all the day to day stuff I need to do and believe me sometimes my best work is done on the loo hehehe.

Then there’s the money side of things the podcast costs me about $100 per month to maintain. 

that’s subscriptions, hosting of the podcast and the website. 

 It generates very little revenue through buyme a coffee and patreon which I can tell you means more to me that the money itself, that someone likes the podcast enough to spend some of their hard earned cash on it, it is very humbling to me however I am not in it for the money every cent donated has gone back into the podcast either in equipment or advertising. 

 So you can see if I were to produce a 45 – 90 minute episode it would take me many more hours that would take me away from my family and my job and to be honest my ability to get out metal detecting and don’t forget I love this hobby as much as you guys and this podcast is actually second to that. 

Anyways I hope you can see what im trying to do here with the podcast so lets get on with this weeks topic. 

This week I want to talk DIY Coils 

 So in what scenarios are DIY coils a thing well there are two I can find, one being the whole DIY Detector community and believe me there is a community of people out there who love electronics and love detecting and have brought both of these hobbies  together to build DIY detectors. 

I wont really refer to these here fully as this side of the hobby is also focus on building every aspect of the detector, the control box etc the whole kit and kabudle. 

The other being meteorite hunting or large object hunting with the goal being to only to detect metal not id it etc so the complexity of the coils doesn’t need to be high just metal yes metal no.  

So as you know a metal detector is essentially a radio that simply transmits Electromagnetic inducing Waves which induces eddy currents in a target which are transmitted back out of phase and the receiver coil picks this up amplifies the response and the control box does its magic converting this induced signal to a tone and id. Right! we have talked about this a few times.  

So the transmit coil and the receive coil are essentially coil inductors that need to be tuned to the specific inductance of the transmitted magnetic field.  

Now originally I thought you could just do a quick calculation based on a loop antenna and you would be close enough however you need to use what’s called the Brooks Coil Calculator to calculate the inductance and length based on the size of coil you want.  

Brooks Coil is generally known as a special case of the circular coil inductor of rectangular cross section that is implemented to achieve the maximum inductance with a given length of wire. The coil has a square cross section and the inner diameter is equal to twice the height (or width) of the coil winding.

The inductance for a Brooks coil can be found from the following equation:

L~= 0.025491cN2 uH

where c is the height and width of the coil winding (in cm) and N is the number of turns.

 As you can see it isn’t simple so if you can navigate the equation you will get the inductance of the coil and now you need to consider the impedance or inductive reactance of the inductor/coil which is dependent on the frequency transmitted or received 

and if any of you have dabbled in car stereos or old radios you will know that it is super important to match you speakers impedance to the amplifier or you risk blowing out your amp this is essentially the same and your detector is no different if you happen to create the coil you simple cant plug it in without doing some level of impedance balancing or you risk blowing your detectors amplifier.

You can calculate the inductive reactance XL of an inductor with XL=2PiFL there are loads of calculators online for both these equations. 

Giving you an answer in Ohms. 

The only way of knowing for sure the expected inductive reactance for your control box is to measure the resistance of an know good existing coil and match to that. 

But what if my detector is multi frequency well I wouldn’t be trying a DIY coil on a Multifrequency machine I would try this on an old detector that operates on a single frequency the risk is two great on a newer machine. 

So Say I want to create a 2 meter coil with 100 turns I would end up with a coil, with an inductance of 195.91 mH which if my detector frequency is 17khz this would give me a inductive reactance of 21 kiloohms which will have to be balanced to your control box by adding a resistor in series or parallel depending on the control box. 

Now I don’t know if you want an inductance that high so you may have to reduce the number of turns to better balance the system for example having only 20 turns will result in a coil of 8.6 mH and an reactance of 918 ohms which sound much better so You will have to adjust accordingly.

Now im only talking about a single coil here which will be suited to a PI detector there is a whole other rabbit hole for a double D coil or a duel coils set up, as you will need to balance the coil pairing in those situations which is normally done by twisting your receive coil in a figure 8 pattern resulting in a cancelling of the signals

So you can see it is complicated to do this correctly and with safety for your detector but if you have your coil math done we can move on , 

sticking with the mono coil design you now know the length of your coil and the diameter so now you need to wind it as best you can ensuring no kinks and it is a smooth as possible. 

You would think that’s it, no I’m afraid not, now you need to shield it with either shielding tape which is a tape of metal gauze or you could just use tinfoil but whatever you use you need to ensure the overlap is minimum and that the two ends don’t touch leaving a little gap in the shielding with one end connected to earth or common on your detector this shield is essential as it increases the performance of the coil and protects it from outside interference. 

So now you have three connection ether end of the coil and the shield. 

You need to then put all this is a frame and depending on the size the world is your oyster here but a lot of people especially if they are making a very large coil for hunting meteorites so big they can essentially stand inside it with the coil slung over their shoulder they will make it out of PVC plumbing pipe in a big square with corner pieces glued together with pvc solvent glue. 

Some people cast their smaller coils in resin and attach them to their detectors like normal but that process will require a little of can do attitude beyond the work of a weekend. 

So I didn’t go into control box like I said because there is kits online or ready made control boxes suited to DIY coils etc so that maybe the next step for you if your successful in building your coil.

You have your coil what can you do with it well it’s a coil go detecting with it test it out did it improve on stock coil performance, however there is only one reason to build a super large coil and that is to find large targets while covering more ground. 

Would I recommend you try this out absolutely but I wouldn’t recommend you try it out on your brand new multifrequency detector try it out on one of your old ones and if you want more information there is loads of resources and communities online but the one I like and used most in researching this weeks topic was geotech1.com check them out loads of cool stuff there is you want to make your own coil or your own control box In the future. 

Wrap Up 

That’s it for this week’s I hope you liked this episode of the metal detecting show podcast.

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If you like this content and would like more please don’t hesitate to tell your friends and don’t forget to hit that subscribe button.

Once again, I hope you have enjoyed this episode and we will chat to you all again next week. 

Get out there eyes down and Happy Hunting.