coax

[authere]

what i am trying to do is make my own coax to suit a coil.I need to know how to lower the ohms& maintain low capacity in the shield. I am using the same conductor in the coil as i do for the coax centre wire, then cover ccw with teflon tape (plumbers ) to about 6 mm (spirally wound). To this i use .15 dia.(m.w.) for shielding & spirally wind on till covered,finally 5 pin plug and
cover with heat shrink.
What i found was the capacity between centre conductor & shield was to high (380 pf/ 5 ft) .To lower the capacity i removed some shielding which lowered it to 150pf,but guess what,the ohms of the coax rose from .1 to .2 ohms (double).This has the effect of instead of .42 ohms for the coil&.1 for the coax (total .52 ohms) to having to lower the ohms of the coil from .42 to .32
.32 coil
+ .2 coax
.52 coil& coax
Note ; minelab coils , .5 ohms
300uh ind.
cap. dependant on coil size
cap. for coax (b.b.sailor) 15/25 pf/ft
I could put on more teflon tape but then the cable doesnt fit into the 5 pin plug.
Maybe i could use a smaller dia. magnet wire for the shield, but would have trouble scraping& soldering.
Does the centre conductor from the coax have to be the same dia. as the coil conductor.
Is there a 2 core & shielded coax that could the job.

' A little knowledge can be dangerous',but fun!

authere,ron

[GT Blocker]

'k I'll jump in....

Hey Authere,

"Is there a 2 core & shielded coax that could the job."

Yes there is..S-video cable, cheap, and easy to find (usually).

Beats the heck out of making cable by hand.
GTB

[Carl-NC]

Why are you making your own coax?

[amtech2005]

coax

I attached a PDF chart for reference only. Something similar to this chart , is what you can search for. The frequecies here are much higher and for different applications. But it will give you an idea of how to pick the best type of cable to connect between the search coil and your metal detector circuit board. Making your own coax-cable may be very difficult. Another option is to experiment with "Twisted Pair" configurations..........Well , I hope this information helps a little for now.......Eugene

[bbsailor]

I attached a PDF chart for reference only. Something similar to this chart , is what you can search for. The frequecies here are much higher and for different applications. But it will give you an idea of how to pick the best type of cable to connect between the search coil and your metal detector circuit board. Making your own coax-cable may be very difficult. Another option is to experiment with "Twisted Pair" configurations..........Well , I hope this information helps a little for now.......Eugene

amtech2005 and all

As a general rule, coax with a higher impedance typically has a lower capacitance per foot.

bbsailor

[authere]

2 core & shield

thanks to gtb,carl,eugene and i think thats joe on the end.
hi,
so far i have had no option but to try and make my own coax because they want me to buy 100m at a time & having no idea wether or not they work, it wasnt going to happen.
I had since come up with another brainwave, to make my own 2 core & shield & blow me down there yous are! I will look into the s video cable @ the car radio (boom box) installers today, and quite frankly i couldnt find a suitable coax(multi strand) with the same dia(mm2) centre conductor so hence im rolling my own& learning.
I am using rg-58 a/u(common) on my smaller coils which i can get from communication shops but the larger dia cable that i needed (?) they dont use, &/or it comes in 100m.
I definetely see the potential in two core, to lower the res. just make the two conductors larger & it lowers the res.This has the effect of allowing the 'coil' to gain resistance.I dont know the optimum as yet only that coil & coax =.5 ohms.
Im still not sure what dia the centre conductors need to be,that is the gp3000 unit has to recognize .5 ohms(?) but the current carried by the coax could have a bearing.I read somewhere where the current drawn by all size coils is the same because .5 ohms resistance & 300 uh is constant* the coax could(?) be constant.?
Sorry, im only a boilermaker/welder!
authere ron

[amtech2005]

coax

Yes bbsailor , is correct ....."AGAIN" !!! The higher the impedance the lower the capacitance. 93-100 ohm impedance shielded wire or coax is available. The higher the impedance the higher the cost !!! "Twin-axial"-a coax made in "u.s.a." is around 100 ohms. Guitar shops also have some specialty wire-products available[between guitars and high-impedance inputs of amplifiers] . Maybe thats why to solve noise or capacitance problems some search-coils have the electronics built-in the coil housing, before the signal hits the shielded wire. And one last huge problem is "Signal Deterioration" and whats called "suck-out" of the signal if part of the shielded cable becomes bent or Kinked !!!!!! I know the frequency of most metal detectors is in the VLF range , but it seems a lot of metal detector manufacturers use real "CHEAP" aluminum shielded wire and even cheap connectors. Proof of this is "corrosion" , and eventually metal detectors finally breaking down...........Eugene