Coil cable woes....

[Anonymous]

Hi,
A warning to fellow coil experimenters. I imported into New Zealand a 100m reel of Belden 9931 cable from the States, at considerable cost to me. I thought it had all the necessary attributes required of a good multi-conductor cable. It has 3 pairs for a DD/mono combination I intend trying, 12 pF per foot, shielded with a weave and foil screen, durable outer sheath, and quite thin.
I was driven to this due to the lack of suitable cable suppliers and stock in NZ. You would need to multiply the total population of NZ twice to equal that of New York City alone.
Anyway, this cable appeared great, I was just about to pot the whole lot, when I noticed the cable was causing a falsing signal if I moved it more than a foot, side to side, absolutely hopeless since this is exactly what we do swinging the darn coil.
Maybe I should have got twisted pairs? Obviously something is moving within the cable when it is flexed, and maybe causing crosstalk. I

 

[Anonymous]

Hi Kev,
I am not sure I understand what you are saying about getting a signal if the cable moves more than a foot. Normally, the cable doesn't move much at all, with respect to the coil. However, both the coil and the wire can move more than a foot together. So, are you saying that if the cable and the coil move together more than a foot, then you get a signal?
If so, does the signal occur while moving or when it comes to a stop? If occurs while moving, does the signal occur all directions of movement?
I am wondering if something else isn't really happening such as you are sampling far down on the delay curve. This might upset the earth field effect circuitry. If this is the case, I would expect the signal to be stronger in one direction than the other.
I would be surprised if moving the coil and lead together produces a signal, especially if the coil lead is snug around the S rod. If it is real loose, then I have experienced a similar problem when the delay is extremely short.
Also, what happens when you increase the delay. Does the problem disappear?
Reg

 

[Anonymous]

Hi Kev,
Sorry to hear about your experience with the cable. It is not easy to find one that does not give false signals when the cable is moved near the coil, particularly at short delays. What happens is that eddy currents are generated in the braid and when the cable flexes, it looks like a moving target. This is even worse if it incorporates a foil shield.
After much searching, I have just sourced some cable that looks ideal. Twin coax with a capacitance of 70pf/m and no reaction with the coil, even with delays of only 5uS. Very flexible, but nice and rugged and with a circular cross section of 6.9mm.
I am going to use it for the DD coils I am starting to make, and possibly also mono coils. Mr Bill has a Goldquest SS on the way to him with a mono coil fitted with this cable for further evaluation. I have grounded the unused conductors also in the mono.
Thing I have learned, is always get a sample to try first. I went through several before finding this one. Now, I have a 100m reel <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">
Eric.

 

[Anonymous]

Hi Reg,
If I place the coil on the ground and grasp the slack cable between the shaft and control box, and move this 6 to 12 inches one way, I get a negative threshold, if I move the cable in the opposite direction I get a positive response.
This surprised me as I expected to get a signal from cable movement near where it exits the coil housing but not metres away from the Tx field. Reject timing doesn't appear to make the slightest scrap of difference.
I've looked to see if I can get some S-Video cable like you suggested, but the only source appears to be pre-made video cables, which tend to be a little on the short side.
Also my wife's watching the credit card like a hawk now, so I hope I'm not going to have to sell some GOLD <img src="/metal/html/cry.gif" border=0 width=40 height=15 alt=":cry">

 

[Anonymous]

Hi Eric,
You've given me some clues which I will work on. The sad part is that most places here are unwilling to part with a sample, a data sheet seems to be the extent to which they are willing to sacrifice.
I guess that if I got at least a few centimetres I could compare how this responds in the field with ones that are no good.
The strange part is the signal occurs when flexing the cable near the control housing not the coil.
I've just had a thought, I may need to recheck my damping as I didn't go back and check the RX after adjusting the TX, so Reg may have a point.
There's some hope yet.
Cheers
Kev.

 

[Anonymous]

Hi Kev,
The audio change you experience is interesting. I suspect the capacitance of the wire is changing but that is only a guess. I am wondering if it is altering as the result of moving closer or farther from the S rod.
Now, I wouldn't expect the wire to move once it is wound around the S rod, except maybe if you are using a hip mount. Since I don't use a hipmount, wire movement is minimal on my setup.
If you are using a hipmount setup, you might try to sleave the wire with something stiff like maybe some wire loom or spiral wrap and see if that helps.
As for the wire I use, I buy a ready made 50 foot S video cable and cut the ends off. I then cut a piece off the cable to the desired length for each coil I make. So, I do buy a pre made cable, rather than the wire by itself. The reason I do this is because I could buy the ready made cables a lot cheaper than I could buy just the wire.
BTW, there is a Mogami cable like I use on Ebay right now and it may go for less than $20 plus shipping. This item ends in about an hour.
I pay about $30 for the same item, new.
Reg

 

[Anonymous]

I added "underwater" to the subject. I normally sit back and learn on this forum, however I might add something to this discussion that I did. I have one of the first Goldquest SS units. I use it in the ocean, the coil is underwater the controls are not, wading only. I found that I got some signal return from the swings (l & R) using the best delay 10uS to I guess around 20uS. I learned from this forum and asking Bill and it was deterimend that the coil wire might be the culpit. So I did this. Coil wire up the shaft of the rod and tied securely. Then I looked until I found 3 or 4 plastic straws that I could split/wrap over each other, then tape the outside till firm. The straws fit inside the anti-tention device on the coil wire lock nut. Photo included...Frank
Hope this give a end-user/tech.non-educated approach to a problem.

 

[Anonymous]

Hi Kev,
I have found that even a short sample (1ft) is useful. I pigtail and solder the braid at one end; just as if I were connecting it to a coil. Then I hold it up against a working PI coil, operating at minimum delay, and see if I get a signal. Ideally one is looking for no response. However, a small response is tolerable, provided the cable does not move relative to the coil, as Reg and Frank have said. Different makes of RG58 coax have widely different results, although I did find that Belden was one of the best.
I did try some low noise microphone cable at one time. The coil did not detect the lap wound screen, and it was good, rugged and flexible cable. On this basis, I bought a quantity, although only about 20m. When I used it, expecting there to be no problems, I found the same as you did. Flexing the cable over a long length gave false signals. In this case, I found that it was due to a semiconducting layer between the lap wound shield and the polythene inner insulator. This appeared to cause a small capacitance change when flexed.
Moral; what is "low noise" for microphone applications, does not necessarily apply to other uses.
Eric.

 

[Anonymous]

Hi Eric,
Thanks for the tip. I did some more testing tonight and it's not going to work no matter what I try . I'll have to put it all down to learning I guess, just as you have also done.
I'm going to put the cable up for auction, may even get my money back if I'm really lucky, as I got it for trade price.
Cheers
Kev.