Metal detector cable lenght

[jladre]

Hi! Its been a long time when i read to this forum that average lenght of md cable is 1.5 meters. My question is, is there negative effect if i used a shorter or much longer one?

[WM6]

No coil performance difference if you put 115 or 150cm.
Difference is only with manufacturer budget (Ukrainian NEL once charged me 35€ more for 30cm more length of coil cable for his Hunter coil).

[jladre]

No coil performance difference if you put 115 or 150cm.
Difference is only with manufacturer budget (Ukrainian NEL once charged me 35€ more for 30cm more length of coil cable for his Hunter coil).

Thanks for enlightenment wm6!

[baum7154]

In a PI detector you should keep the feed line length as short as practical to minimize capacitance to allow shorter sample delays important when detecting small gold targets. The feed line is a major contributor of undesired capacitance in a fast coil/feed line system. I typically use 33 to 35" feedlines with my fast coils. In fact I only use a continuation of the coil wire twisted at 3 turns per inch as a feed. This eliminates solder at the coil and minimizes capacitance as well. See the Chance PI Cool thread in this forum.

regards,
Dan

[jladre]

In a PI detector you should keep the feed line length as short as practical to minimize capacitance to allow shorter sample delays important when detecting small gold targets. The feed line is a major contributor of undesired capacitance in a fast coil/feed line system. I typically use 33 to 35" feedlines with my fast coils. In fact I only use a continuation of the coil wire twisted at 3 turns per inch as a feed. This eliminates solder at the coil and minimizes capacitance as well. See the Chance PI Cool thread in this forum.

regards,
Dan

Thanks for that information baum7154!!

[WM6]

In a PI detector you should keep the feed line length as short as practical to minimize capacitance to allow shorter sample delays important when detecting small gold targets. The feed line is a major contributor of undesired capacitance in a fast coil/feed line system. I typically use 33 to 35" feedlines with my fast coils. In fact I only use a continuation of the coil wire twisted at 3 turns per inch as a feed. This eliminates solder at the coil and minimizes capacitance as well. See the Chance PI Cool thread in this forum.

regards,
Dan

This is very true in case of Pi and coaxial feed line.

If you use dual, enough separated feed lines, added parasitic capacitance by feed line is so low,
that 25cm (9") more or less cable length has no practical impact in PI sampling delay possibilities.

[baum7154]

The feed line capacitance of 24 AWG 600 volt PTFE insulated stranded silver plated wire twisted at 3 turns per inch runs about 1pf per inch so for a 33" long feed the capacitance is about 33pf.

[SVEN1]

Keeping it shorter is better for a PI.
I have made a couple PI's for really tall guys with long arms. 33-35" was too short.
So I bumped up the cable length to 44". That satisfies the tall people.
I saw no performance difference from 33-35 and 44 inches.
For under the armcuff control boxes, 44" length seems ideal without binding.
All my fast PI mono coil cables are now 44" long.

[baum7154]

Hi Sven,

nice build pictures.

what at cable are you using? What is the capacitance per inch of that cable?

Dan

[Koala]

Are we sure there is no performance loss in a VLF. Everything else the longer the sensor is from the controller the weaker the signal and more prone to interference. Maybe the difference is too small to be noticeable. Thought that's why some manufactures put the Preamp in the coil.

[WM6]

I prefer hip-mount search position of detector housing. Most nowadays detectors are equipped with too short cable for hip-mount, so I decided for cable prolonging for couple of detectors (even for those not really cheap as AT Gold). Detectors was thoroughly tested before and after at my testing plant for different testing targets. No difference sensing before and after prolonging.

Of course I can agree with theory that 30pF more on prolonging account in case of Pi can imply to sampling delay in some PI designs. But do we really have such designs in which we can sample in nanoseconds region? Which are those designs? If we are talking about Surf PI, Barracuda, Delta Pulse, HH, Garry PI, Minipulse, Sandbanks and other "classical" designs, in practical world, 30pF more stray capacitance, given with cable, mean nothing, if your completed coil is properly adapted to PI electronic input by carefully selecting damping resistor. I am usually using cable wit capacitance between 60 and 120 pF at 120cm (about 47"). Once tested very low C antenna twin lead with PI without significant improvement. Only significant improvement was achieved by using proper damping R on different coils.

[SVEN1]

Hi Sven,

nice build pictures.

what at cable are you using? What is the capacitance per inch of that cable?

Dan

Originally used Belden RG58A/U Coaxial Cable
20 AWG stranded (19x33) .035" tinned copper conductor, polyethylene insulation, tinned copper braid shield (95% coverage), non-contaminating PVC jacket. Overall Nominal Diameter: 0.195 in. , M17/155-00001
But found it too stiff.
http://www.ebay.ca/itm/Belden-RG58A-...item51a7d50e68



Then started using Marine Coax sold by West Marine, Anchor Brand.
Really flexible
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?productId=35292&catalogId=10001&lan gId=-1&storeId=11151&storeNum=50157&subdeptNum=50187&cl assNum=50192



The coax I use now is the same but, has a polyurethane jacket similar to what Eric Foster used.
RG58 POLY.pdf

Hi Sven,

The orange cable is a specially made RG58 coax. It is made for the offshore industry, specially for underwater use, with a polyurethane outer jacket rather than pvc. This is much stronger and hard to penetrate by contact with sharp edges. Water at pressure would very quickly siphon along ordinary coax if there was even a small cut, and enter the coil windings. Salt water would write off both the coil and the cable and even enter the electronics unless a water blocking connector was used. Of course the orange coax is considerably more expensive, but I thought it was worth it as even for shallow water work it gives added protection.

The cable I used is by JDR Cable Systems, part no. CAF9007. http://www.jdrglobal.com/
I haven't bought any for some years and although I can find the company on the internet, I can't find the product. Maybe it has been discontinued. The 58cu by Ancor may be as good and it maybe worth while asking for a sample length. Both inner and outer conductors must be tin plated copper - the inner conductor stranded, not solid.

Eric

[baum7154]

My posts #48 & 49 of the Chance PI Coil thread show that a high performance 3DSS 8" round coil has 35pf of inter winding capacitance (1.47 MHz SRF) and adding 33" of twisted feed adds another 33pf of capacitance, nearly doubling the coils capacitance. This has the effect of reducing the self resonant frequency (1.05 MHz SRF)by about 29%. In my estimation this is a significant effect and for this reason the feed line should be kept as short as practical if you are optimizing for small gold detection.

Regards,

Dan

[Davor]

Here are my 3 cents...

Any coax capacitance per meter length may be calculated from:



For all practical purposes the lower capacitance per length is achieved by picking a coax of high impedance (Zo) and with lower dielectric constant, e.g. a foam.

[kt315]

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