Shielding of Coils

[Ray-NM]

Not sure if I read it on this forum or elsewhere, but can you build a coil then use a close wrap of wirewrap wire all the way around and connected to ground and use it for the faraday shield. I ask this since I thought it may be another way to shield it.
Thanks
Ray

[bbsailor]

You could probably make a shield using wire wrap, however you would need a lot of wire to make a close wound shield around the circumference of the coil. Some PI coil builders have mentioned using bare wire from wire wrap to do a few spirals around an aluminum shield to make the ground connection using black tape to secure it tight. Others have used nickel conductive paint to coat the plastic spacer and used the bare wire wrap as a way to make the ground connection.

What you use as a coil shield depends on the minimal delay that you are seeking. Faster delays require a shield that has less eddy currents capable of being generated in it. Thinner shielding is better for fast coils. Many things have been used for coil shields (listed below).

Household alum. foil
Opened and unwound electrolytic cap plates
3M copper fabric tape (recommended by Reg) (good shield)
3M Lead Tape (Eric Foster) (good shield)
Alum. tape (2" wide) as recommended by Carl on the Hammerhead coil (see his article)
Mylar, conductive, decorative ribbon
Copper foil

What you are trying to achieve is a low shield to coil capacitance of about 40 to 50 pf per foot of coil circumference (some of my best). When you wave a target under the coil listen to the response and lower the delay using a gold ring or nickel. If the target suddenly stops responding, you have reached the lower limit of that coil. The fix is to reduce the total capacitance and readjust the damping resistor value. Total TX capacitance is caused by the following things main things.

1. The coil winding-to-winding capacitance (1 to 1.2 Mhz self-resonance with no shield.
2. Coil-to-shield capacitance.
3. MOSFET output capacitance (COS) rating. About 50 pf is a good low nember.
4. Coax wire capacitance. 16 to 25 pf per foot is common. Lower is better.
5. RX input circuit stray capacitance and input circuit component loading (relatively fixed by the circuit design).

The value of the damping resistor reflects the total capacitance seen by the coil in the TX circuit. Higher is better, meaning there is less capacitance to damp thus a higher value of resistor will achieve critical damping and make a more potentially sensitive coil.

DD coils can be made more sensitive because the TX and RX circuits are separated. The RX coil does not see the MOSFET output capacitance. My own crude tests indicate that for each 100 pF I can eliminate form the TX circuit, as seen by the coil, I can speed the delay up by about 1uS. I would like others to verify these crude findings?

You need to have an LC meter to make relative measurements to see the effects of the coil-to-shield capacitance changes you make in you coil design.

bbsailor

[Tinkerer]

Conductive paint for shielding

In the third world we have little access to sophisticated materials. Often we have to be inventive and do with what we have at hand.
I found some graphite powder and some zinc powder in my old stuff and made some experiments to produce a conductive paint out of it.
The first 15 test samples were made with acrilic resin base as a binder.
The resulting paint varied from extreme brittle to tough, but the conductivity was very low.
Then I made 15 samples each, using polyester resin and epoxy resin, diluted 10:1 with thinner. this gave a fairly solid coating that adhered well on sanded PVC tube.
Filled only with graphite, the resistance of the coating ended up being about 200 Ohm between two points touched with the probes, about 1" apart.
Surprisingly, when I added the metallic (not oxide) Zinc powder to the mix, the resistance increased significally.
I wonder how much capacitance this shielding will add to the finished coil and how efective the shielding will be.
Tinkerer

[detectoman]

i nedded one efective shield for one gigant coil bfo

hi friends i build one bfo, of six transistor, i install one coil of 40x60 cms
i have one problem, is the extreme sensitive the coil in big sensitivity volume potentiometer to ground effects
if i like my bfo great depp, actually it detect one ring of car to, 1,30 m
i can stabilizer in low sentitivity,i succes! , i have poor deep, 50 porcent, i not problem of stabilizer, only in position hig the detector is very instable o desestabilized, i install faraday shield of aluminium wrap, foil and blind copper shiel i not have good results of succes, i nedd one shield stabilizer for this bfo, i build whit my modifications
this aparatus, is low consumer of energy batery, in 9 volts
i like your help my thanks
excusme for my englisk


si alguien sabe escribir en español por favor denme una direccion para el problema yo necesito un efectivo faraday shield o un artificio para anular las corrientes de inductancias, para mi bfo , en el problema de desestabilidad que tiene el coil de rastreo, que es cuadrado de gran tamaño, y tiene gran sensibilidad, pero necesito estabilizarlo, ya que necesito llevarlo a una misma altura para que no varie la aguja del marcador, y eso es muy cansado, necesito una buena solucion , si alguien sabe mucho le agradecere

saludos a todos, detectoman jose

[bobsyn]

hi friends i build one bfo, of six transistor, i install one coil of 40x60 cms
i have one problem, is the extreme sensitive the coil in big sensitivity volume potentiometer to ground effects
if i like my bfo great depp, actually it detect one ring of car to, 1,30 m
i can stabilizer in low sentitivity,i succes! , i have poor deep, 50 porcent, i not problem of stabilizer, only in position hig the detector is very instable o desestabilized, i install faraday shield of aluminium wrap, foil and blind copper shiel i not have good results of succes, i nedd one shield stabilizer for this bfo, i build whit my modifications
this aparatus, is low consumer of energy batery, in 9 volts
i like your help my thanks
excusme for my englisk


si alguien sabe escribir en español por favor denme una direccion para el problema yo necesito un efectivo faraday shield o un artificio para anular las corrientes de inductancias, para mi bfo , en el problema de desestabilidad que tiene el coil de rastreo, que es cuadrado de gran tamaño, y tiene gran sensibilidad, pero necesito estabilizarlo, ya que necesito llevarlo a una misma altura para que no varie la aguja del marcador, y eso es muy cansado, necesito una buena solucion , si alguien sabe mucho le agradecere

saludos a todos, detectoman jose

Pleace, send me this BFO shem.


Thanks
bobsyn17@yahoo.com

[detectoman]

bobsyn:
i not have diagrams of my modifications,in my pc, i have my camera whitout wire conection , i can,t send you information, soo, in little time i send the group picture of my detector bfo whit coil of 60x40 , and to you i write at your mail soo
un saludo desde mexico buscador detectoman

[bobsyn]

bobsyn:
i not have diagrams of my modifications,in my pc, i have my camera whitout wire conection , i can,t send you information, soo, in little time i send the group picture of my detector bfo whit coil of 60x40 , and to you i write at your mail soo
un saludo desde mexico buscador detectoman

O.K. DETECTOMAN

TANKS

[detectoman]

my camera web have not the interface conections, for install the photos of my detector in my pc, this wire conection is not avaliable, i go buy other photo camera in short time
excusme for these problem, i can,t send your these pictures

regards detectoman

[Unregistered]

see this one for good BFO that many here have build
http://home.clara.net/saxons/bfo.htm
very stabilty
very much we hope you like it

[detectoman]

these detector de home clara is the detector i build, i add modifications in the coils,and circuit, one large quad, of 40x60 cms , i not count before the turns,i can,t information ar, not, the rounds , the detector capture one object, of 50 x50 cms, at 1.50 mts, of depp,in soil of river, i shell,the coil, whit two round of aluminium ,tape,for control of heat, my detector is very sense and to big sensitivity switch, the is not stable, i nedd one very good faraday shield of artefact, for stabilize
thans, in two days i send photo of my detector bfo

detectoman

[detectoman]

my bfo structure in pictures

friends of group, these is my bfo what i build, whit coil induction of 60x40 cms , have two rounds of aluminium tape , i not is suffcient,is quad coil , and some modifications of internal circuits, my problem is the excesive trouble of reactions of ground capacitance, is very sensitive and i can,t one shield efective, no sucess whit tape aluminium, no sucess whit secundary paralell coil, i nedd one efficient shield of yours experiencie

aqui les presento las estructuras de mi bfo que yo modifique, el bfo tiene cambios en el circuito tanto de grandes capacitores, como de blindaje, y de manipulaciones en la bobina de referencia, la bobina de rastreo fue embobinada sin la precaucion de contarle las vueltas del alambre, y basandome solo en equilibrar el enrollado de la bobina de referencia, y le fue puesto un marcador muy sensible, afortunadamente el bfo eficiente, solo la extrema sensibilidad al cambiarle en otras oscilaciones por medio de resistencias, pero en baja sensibilidad si actua el shield de aluminio
espero alguien tenga un tip eficiente para estabilizarle en alta oscilacion
un saludo a todos
detectoman mexico

[digger barns]

I did not quite understand your problem. You may expecting to much from this detector, and the benefits of the faraday shielding. This is a BFO design and it will always drift a bit. Your big coil will be very sensitive to temperature changes and this will make the detector drift. This will not change by fitting Faraday shield. Also the beat note will alter as you raise and lower the coil to the ground, and a Faraday shield will make little improvement to this. The faraday shield will stop effects of wet grass ect. I must say I envy the excitment you must be getting from messing about with this. I wish I could go back and do this again. Digger

[chemelec]

If you want Stability on searching the ground for objects, and Probably even better sensitivity, I would suggest you build a PI type Detector.

[Qiaozhi]

BFO Detector

friends of group, these is my bfo what i build, whit coil induction of 60x40 cms , have two rounds of aluminium tape , i not is suffcient,is quad coil , and some modifications of internal circuits, my problem is the excesive trouble of reactions of ground capacitance, is very sensitive and i can,t one shield efective, no sucess whit tape aluminium, no sucess whit secundary paralell coil, i nedd one efficient shield of yours experiencie

I agree with digger_barns and chemelec. Now you've spent some useful time investigating the capabilities of a BFO, forget about it and build something else. BFOs are nowadays only considered as toys. Their main problems are instability, drift and lack of sensitivity. If you're detecting in the river, then a P.I. detector would be a good choice.

Good luck!

[detectoman]

i like experiment whit the bfo,s the bfo,s have future for my , are very versatiles, on lot of simple diagrams,the pi are very consumer and battery drain ,have problems whit ferrous targets and other, bfo,s not problems, is excelent, pinpoint ,pi is big problem,some times, are very minucious in the circuits buildings and the test of waves is critical,i am one beginer not experiencied in complejs print circuits electric , in the future i build also one pi , to bfo, i like put one sistem, of secundary oscillating circuit ,and coil , on coil induction for stabilized the reactions of ground thanks excusme my troubble english

hola amigos si saben escribir en español por favor haganlo, miren, yo estoy investigando sobre los bfos, a los que les tengo mucha confianza que se les pueda sacar mas adelantos a ellos, los bfo son muy versatiles y aguantadores a los cortos, estos bfos con los nuevos componentes ahora disponibles creo que se les puede sacar mas ventajas, y ponerse a la altura de los mejores detectores de induccion , pulsante, los bfos de pueden hacer discriminadores, su unico problema son las corrientes a tierra , yo pienso que ese es un problema que se puede solucionar entre todos , yo le he sacado mucha eficiencia a mi bfo , y creo que se le puede aun sacar mas, haciendole nuevas inovaciones, el unico problema en verdad duro es la inestabilidad en grandes bobinas de rastreo, y al darle mas sensibilidad, tambien creo que se pueden hacer bfos de dos cajas, bueno ese seria otro proyecto pendiente, no se si al hacerlo de dos cajas tuviera los mismos problemas de rechazo a tierra
para mi lo que pasa es que registra la tierra o dea el planeta en conjunto como si fuera un solo objeto metalico , entonces lo que se debe hacer aqui, es darle familiaridad con la tierra, o sea una especie de conexion intermedia desde la bobina de rastreo hasta la superficie de la tierra, otra solucion pienso seria amarrar a tierra una conexion gracias

detectoman