coil wire?

[satdaveuk]

Capacitance per meter is inversely proportional to the cable impedance, and as far as low capacitance goes, a 100ohm mic cable has less capacitance than 50ohm RG58 over the same length. However, you have to check the microphone cable voltage rating first because it is a PI rig, and you may have arcing.

In my projects (which are not PI rigs yet) I prefer microphone cables as well because of their flexibility, and heat shrinking tubing at the ends to reduce wear.

Thanks Davor
Talking of mike cable ive got some thick heavy two core screened cable which i bought years ago to wire the stage up in a school for PA equipment, will dig the drum out and get a number off it, thats flexable, pleased the subject was brought up.

This is the cable and plugs that I use for IB format maybe ok for PI because there fairly thick cables.

[sinclairuser]

Hi Ally
I been using RG58 in past project, are any of the cables you mention performing any better far as performance?

Regards

Dave

not really dave the mic cable is softer than rg58 (mil) no difference in performance for me
anyway sorry for not replying sooner but im away this week and the wifi on this site is PANTS!
cant say for standard rg58 i only use military spec coax(normal rg58 is softer i think)

[satdaveuk]

Doing some checks of different cables ive got in stock and use for work the TX100lsf 75ohms TV distribution cable comes in at a very good spec inner/outer solid copper capacitance 53pf per meter.

Also opened up a faulty Labgear 20 meter HDMI lead with EMI filters both ends, can see why these cables are so exspensive with 4 shieldings on the paired cables which are insulated with something that is certainly not pvc and 3 shieldings on the remaining, will find out more.


The other cable is telecomunications which was given to me by a engineer on site where I was working its external 40 cores of solid copper strand, got about 10-12 meters of it, maybe coil making material.

[Tinkerer]

Capacitance per meter is inversely proportional to the cable impedance, and as far as low capacitance goes, a 100ohm mic cable has less capacitance than 50ohm RG58 over the same length. However, you have to check the microphone cable voltage rating first because it is a PI rig, and you may have arcing.

In my projects (which are not PI rigs yet) I prefer microphone cables as well because of their flexibility, and heat shrinking tubing at the ends to reduce wear.

Audio cables are designed for frequencies up to 20kHz. Above this frequency they act as low pass filters. For PI, where we have frequencies of up to mega Hz, it does make a difference.

Tinkerer

[Kroton]

I think the best choice is twisted pair.

[Ferric Toes]

I tried low noise microphone cable a few years back. I liked its flexibility and the fact that you could not detect the outer shielding where it enters the coil. Some r.f. coaxes with braided shield give false signals when flexed down near the coil entry. Some makes of RG58 do this, while other makes don't. This is noticeable when using sample delays below 15uS, but upwards of this figure the effect disappears. The low noise mic cable with lapped shield solved this problem but threw up another one in its place, in that it gave a false signal wherever you flexed it. I concluded that it was caused by the semiconducting layer under the screening which gives low noise for microphones but high noise for PI. You have to wind it tightly up the shaft to prevent any movement of the cable when swinging.

Eric.

[bklein]

I come across a lot of USB3.0 cables at my work. Two shielded twisted pairs plus unshielded Vbus/ground and ground/shield wire. May be cheaper than HDMI from pc swapmeets or online.

Barry

[SVEN1]

I just ordered some Ancor Marine Coax for my PI. Going to give it a try over standard RG58 with a multi strand center core, not tinned that I have been currently using.

[Davor]

RG59 has less capacity per length.

[Ferric Toes]

RG59 will give problems. It has a copper coated steel inner conductor which will give a strong and varying signal when it is near the coil. It is also a thin conductor and will break under flexing.

Eric.

[Davor]

I once worked on some EMC compliance testing where I learned the harder way that there are big differences in coax cables even with the same designation. We constantly had some ghosts that would not disappear regardless of the filter. By pure chance someone suggested using a different set of cables for a spectrum analyser and puff! - the ghosts were gone. The darn cheap cables were picking up the rubbish from the nearby equipment.

I did not know about the steel core in RG59. Is it also present in RG58?

[Ferric Toes]

No, RG58 is always with tin plated stranded wire core. As I mentioned before, it does vary between manufacturers but only in its mechanical characteristics and tightness of weave in the outer braid. I still have a reel that I decided not to use as the internal polythene insulation is too stiff making it less flexible than one would like. If the outer braid is too tight then it presents a better eddy current path down near the coil so that the cable is then detectable. Mind you, we are using such cables in an application for which they were not designed. From memory, Belden RG58A/U was about the best. Reg Sniff also investigated suitable cable and found one by a company called Mogami to be very good. Minelab use a specially made Litz type coax where each strand in the core and outer is individually insulated with a solderable coating. Still a dickens of a job to solder though, but it does ensure that the cable is not detectable by reason of induced currents in the braid.

I was also involved in EMC compliance testing of industrial PI detectors, both self test and test house. For one special application I used triax cable with inner braid for electronics ground and outer braid for general ground to the metal case and mains psu ground. Coil was also double shielded. The problem was not emissions but making the unit immune to persons using mobile VHF comms in close proximity. Maybe Moodz differential coil would have solved the problem.

Eric.

[ppodg]

1. Some versions of rg59 do not have the steel core.

2. I haven't seen any 'steel' rg58 so far, but some have solid core.

3. The old belden 8421 (guitar players love it) seemed optimal for the pi - for many reasons. The piece I used many years ago was steel-free.

[Davor]

The problem was not emissions but making the unit immune to persons using mobile VHF comms in close proximity. Maybe Moodz differential coil would have solved the problem.

Differential coil by itself? No. VHF wavelength is comparable with coil sizes, so whatever works in VLF must not necessarily work for VHF. Whatever comes as a differential mode signal in VLF may come as a common mode signal in VHF. There are simple ways to make detectors more immune to VHF though.

I'm thinking of making my own Litz cables for my future builds. It is not that much of a trouble since there is not much cable to weave. I'm very much into building unshielded coils (the FKK coils ), and for me that means differential mode all over.

[Ferric Toes]

Now this is something I find puzzling. I have this week built a differential coil of the Moodz type – twisted pair, bifilar wound, centre tap to ground etc., and modified a circuit board to have a differential front end. All works well, except that it is no quieter in terms of the pickup noise on the coil, than a similar size monocoil. All the pickup is sub 1Mhz noise as can be seen on the scope. We have high power transmitters in the UK from 100kHz through to 210kHz. There are also two French broadcast stations round about 190kHz. I have a communication receiver which covers 75kHz – 200kHz on its lowest frequency band, so I can hear what is going on. Hold the detector coil vertical and rotate it and you can get a null or a max, so you can use it to find the direction of a station. Even sometimes see the modulation and that it coincides with music or speech from the radio. Now this type of signal pickup is the magnetic component of the electromagnetic wave and appears as a differential signal on both a standard coil or a differential coil. Only the electric field would be common mode. Shielding the coil with copper fabric emc tape, or even lead foil makes little difference to this type of pickup. I shielded the differential coil with no difference in noise level. You could of course use solid copper tape, but then all you would see is the eddy current decay in the tape. The only ways I know of drastically reducing VLF noise is to use a figure 8 type coil or a differential coil with a second receiver stacked vertically above the lower TX/RX.

This is not to say that a differential coil does not have other positive benefits but I cannot see that it can reduce noise within the band that mostly affects PI detectors i.e say 1Mhz – 1Hz where the coupling to the coil is the magnetic component.

Eric.