Cable for MPP mono coil - CB coax good enough?

[JOHNCGRAY]

Hello,

I am building a MPP and the simple mono coil will be finished soon.

I am looking at the coax and am I right in thinking the 2 ends of the coil can be connected at the COIL END and in turn be connected to the core of a length of coax. The shield (slug copper tape) can then be connected to the shield of the coax and all this can occur at the coil end. Then the coax leads up to the electronics box where I send the screen to pin 3 of PL2 and the core to both pin 2 of PL2 AND pin 2 of PL4?

I appreciate that this will limit me to a simple monocoil ie not using the 5-pin plugs but this is just my first detector which I am building with my 9 year old so happy to keep this one simple.

Also does anyone out there have experience of using a CB coax cable ... my guess is they are not brilliant. Alternatively for more money I see that there is a 1m Phillips 1000PXT cable with phono end on Amazon for a fiver??

I would really appreciate any help as I am learning --- and this has taken over my life for the last week or so!

John

[WM6]

Don't worry, such antenna cable is more than good enough (if correct connected).

[baum7154]

Hello,

I am building a MPP and the simple mono coil will be finished soon.

I am looking at the coax and am I right in thinking the 2 ends of the coil can be connected at the COIL END and in turn be connected to the core of a length of coax. The shield (slug copper tape) can then be connected to the shield of the coax and all this can occur at the coil end. Then the coax leads up to the electronics box where I send the screen to pin 3 of PL2 and the core to both pin 2 of PL2 AND pin 2 of PL4?

I appreciate that this will limit me to a simple monocoil ie not using the 5-pin plugs but this is just my first detector which I am building with my 9 year old so happy to keep this one simple.

Also does anyone out there have experience of using a CB coax cable ... my guess is they are not brilliant. Alternatively for more money I see that there is a 1m Phillips 1000PXT cable with phono end on Amazon for a fiver??

I would really appreciate any help as I am learning --- and this has taken over my life for the last week or so!

John

---------------------------------------------------------------
Hi John,
John when you say a "simple mono coil" are you building a bunch wound coil or a 3DSS coil on a slotted form? If a 3DSS you must put the outermost winding end on the ground or shield depending on what type of feed line you use.

Will you be using your detector to search for small gold nuggets or just for rings and coins? The reason I ask is that small gold nuggets require short sample delays and the fastest coil possible to allow short sample delay. If you will be nugget hunting I highly recommend using a continuation of your coil wire twisted at a rate of 3 turns per inch as your feed line to keep coil and feedline capacitance as low as possible. Capacitance is the enemy of coil speed. The use of RG58 coax will contribute twice the capacitance of the twisted wire feed. Also keep your feed line as short as practical... I use about 33".

Your coil wires should not be connected together and should be connected separately, one to the coax center and one to the coax shield if you use coax. The coil shield should always be connected to the coax shield.

Good luck,

Dan

[6666]

The type of coax you are probably using is RG58, there are some very cheap cables used in the CB world
you have to look at the connection diagram Fig 14 very carefully to get the connection correctly terminated
there are 2 connections at the coil end, the inner conductor and the shield, at the pcb end there is 2 connections

[Jose]

---------------------------------------------------------------
Hi John,
John when you say a "simple mono coil" are you building a bunch wound coil or a 3DSS coil on a slotted form? If a 3DSS you must put the outermost winding end on the ground or shield depending on what type of feed line you use.

Will you be using your detector to search for small gold nuggets or just for rings and coins? The reason I ask is that small gold nuggets require short sample delays and the fastest coil possible to allow short sample delay. If you will be nugget hunting I highly recommend using a continuation of your coil wire twisted at a rate of 3 turns per inch as your feed line to keep coil and feedline capacitance as low as possible. Capacitance is the enemy of coil speed. The use of RG58 coax will contribute twice the capacitance of the twisted wire feed. Also keep your feed line as short as practical... I use about 33".

Your coil wires should not be connected together and should be connected separately, one to the coax center and one to the coax shield if you use coax. The coil shield should always be connected to the coax shield.

Good luck,

Dan

Hi Baum7154
How does noise affect a braided cable compared to a coaxial cable?

[JOHNCGRAY]

Hi Dan,

The coil is literally that - one piece of wire, 25 turns so the TX and RX coils are the same one coil. I believe this is ok (although basic) for a PI detector. I live in a village around which people are finding bronze age axe heads, small celtic broach (4cm) and roman coins in the fields - my son and I want to do the same! No likelihood of gold I am afraid!

Thanks very much for the reply - I understand except the last sentence. Fig 14 of the MPP build guide shows me that the 2 ends of the coil are effectivly joined together as they enter the PCB (pins 1 and 5 are shown bridged)...

Cheers

John

[JOHNCGRAY]

Thanks for replying so quickly. It is fig 14 that has got me thinking! My coil is one length coiled 25 times and the 2 ends go up to the pcb. Here, according to fig14 they are effectivly joined together. So I got thinking- why cant I join them down at the coil and make that continuous with the central wire of a coax cable and then up to the pcb?

Cheers

John

[geoscash1]

Here is the connection diagram, one wire from coil to pos or plus of control board and one wire to shield or ground of control board. since your using a single mono coil and NOT 2 coils ( tx and rx) you will jumper the rx at the plug on the control board to the tx, still just 2 wires from coil to board.

[JOHNCGRAY]

Umm, so when you say just 2 wires travelling from coil up to the box you mean one of these is connected to the sheild I am wrapping around the coil I made. What is the second wire? I have the 2 ends of my coil ----- are you saying therefore I connect these together at the coil end and connect to the 2nd wire ??

Assuming the upper plug layout in fig 14 is at the box end then this sugests I send down to the coil THREE wires ie one to the shield and one to each end of my single coil?

I think I am not understanding something here and I can hear you lot saying -- its obvious!!

Please educate me further!!

Cheers

John

[baum7154]

Hi Baum7154
How does noise affect a braided cable compared to a coaxial cable?

--------------------------------------------
Jose,

I use only twisted pair feedline on my coils after having tried 93, 75, & 50 ohm coax in a variety of diameters. In my testing there was virtually no benefit in noise reduction with the coax and there was always a penalty in added capacitance with coax. The coil itself is a great antenna for ambient noise anyway. In addition the solder joints connecting coil to feed were detectable at short sample delays. This is why I recommend a continuation of the coil wire twisted to replace the coax feed. I should note that telecommunications technology has used twisted pairs for at least 100 years because of its noise reduction effect.

Regards,

Dan

[JOHNCGRAY]

OK Dan, so if I hear you right - what I do is take the 2 ends of my single coil and continue them up to the control box with the turns as you say. Then at the PCB I connect them to pin 1 and 5 (which are then bridged according to fig14). Then in addition the screen around the coil - I solder a wire to this also down at the coil. I then send that wire up to the control box as well (and connect to 0v)? So all in all I will have 3 wires going from the coil end up to the PCB box? Is this correct?
Thanks again
John

[DOOLEY]

my 2 peneth's worth.



back in my day of CB radio, RG58 was the "average good enough grade"

BUT...............

"MINI-8" is better quality and a better choice.
better insulator,
better shield coverage and quality,
and handles more power.
(in my opinion)

i ONLY use "MINI-8" on Pi's

[baum7154]

OK Dan, so if I hear you right - what I do is take the 2 ends of my single coil and continue them up to the control box with the turns as you say. Then at the PCB I connect them to pin 1 and 5 (which are then bridged according to fig14). Then in addition the screen around the coil - I solder a wire to this also down at the coil. I then send that wire up to the control box as well (and connect to 0v)? So all in all I will have 3 wires going from the coil end up to the PCB box? Is this correct?
Thanks again
John

--------------------------------------------------

John,

Connect one coil wire to 1(receive), the other coil wire to 3 (screen), and the shield to 3 (screen), and jumper 1 to 5 at the connector as in fig 14.

[baum7154]

my 2 peneth's worth.



back in my day of CB radio, RG58 was the "average good enough grade"

BUT...............

"MINI-8" is better quality and a better choice.
better insulator,
better shield coverage and quality,
and handles more power.
(in my opinion)

i ONLY use "MINI-8" on Pi's

-----------------------------------------------

I agree Mini-8 is better than RG58 in several ways. It does have 26pf of capacitance per foot though and twisted 24 awg 600 volt PTFE has 12pf per foot. So if low capacitance and coil speed are important,(and they are not necessarily always important) then a twisted pair is a good way to go. In searching for bronze artifacts that coil speed is probably not an issue.

Cheers,

Dan

[Jose]

--------------------------------------------
Jose,

I use only twisted pair feedline on my coils after having tried 93, 75, & 50 ohm coax in a variety of diameters. In my testing there was virtually no benefit in noise reduction with the coax and there was always a penalty in added capacitance with coax. The coil itself is a great antenna for ambient noise anyway. In addition the solder joints connecting coil to feed were detectable at short sample delays. This is why I recommend a continuation of the coil wire twisted to replace the coax feed. I should note that telecommunications technology has used twisted pairs for at least 100 years because of its noise reduction effect.

Regards,

Dan

Thanks dan
I share everything you say about soldering, it is also difficult to get a coax cable that is not detected, when using fast coils. I am going to try.