PI Mono Coil Cable question

[Ferric Toes]

Can someone say how capacitance is measured on these cables? I mean is the shield part of the measurement? How is it measured?
Or are these numbers being quoted just specifications given by the manufacturer?
I found some nice looking cable. Problem is that there is big spool of it. Will I have to cut a piece in order to measure. I don't want to buy expensive cable only to find out it's no good. There is no maker name on it.

You have to be careful, as I have bought several different types of cable that you would think to be good, only to find they were no good. There are some exotic interconnect cables advertised in the audio hi-fi literature. Low oxygen copper, teflon interior insulation etc etc. You would think that if they enhance your audio experience, then improvements would be noticeable and even measurable for detector applications. I tried two types of microphone coax, initially on the basis of its flexibility and advertised low noise performance. Initially it appeared to work fine, but when the detector was used in the field an unsuspected problem arose. The cable was microphonic in itself. Any loose cable that swung or moved, generated noise. When I looked into why this was so, I found that under the braid, between the braid and the soft polythene insulation over the core was a thin layer of semiconducting material. This, for reasons unknown,was the cause of the trouble. The other high end audio coax worked fine in the lab., but in the field was subject to failure due to breakage of the inner conductor which didn't like being swung from side to side or frequently being flexed. This was a strange cable as it had arrows on the outer sheath indicating that the signal should only travel in one direction for best results. I think that a lot of this is just hype so as to justify a high price.
The cable I generally use is 50 ohm RG58 C/U by Belden, which is rugged due to the heavier stranded core, but quite flexible. RG58 by other makers can vary; mainly in flexibility.

Eric.

[waltr]

Yes, I also think there is much hype for some audio cables. If going for audio cables I would look into what professional sound equipment uses.
These cables need to be tough to survive setup/packup for every gig and have good electrical properties.

Good old Belden RG58 C/U is nice but I have been using CAT-5 with good results.

[KingJL]

You have to be careful, as I have bought several different types of cable that you would think to be good, only to find they were no good. There are some exotic interconnect cables advertised in the audio hi-fi literature. Low oxygen copper, teflon interior insulation etc etc. You would think that if they enhance your audio experience, then improvements would be noticeable and even measurable for detector applications. I tried two types of microphone coax, initially on the basis of its flexibility and advertised low noise performance. Initially it appeared to work fine, but when the detector was used in the field an unsuspected problem arose. The cable was microphonic in itself. Any loose cable that swung or moved, generated noise. When I looked into why this was so, I found that under the braid, between the braid and the soft polythene insulation over the core was a thin layer of semiconducting material. This, for reasons unknown,was the cause of the trouble. The other high end audio coax worked fine in the lab., but in the field was subject to failure due to breakage of the inner conductor which didn't like being swung from side to side or frequently being flexed. This was a strange cable as it had arrows on the outer sheath indicating that the signal should only travel in one direction for best results. I think that a lot of this is just hype so as to justify a high price.
The cable I generally use is 50 ohm RG58 C/U by Belden, which is rugged due to the heavier stranded core, but quite flexible. RG58 by other makers can vary; mainly in flexibility.

Eric.

The last 5 mono's I have built, I have been using 75 ohm mini coax (belden 9221) formed into coils. My reasoning for the 75 ohm coax has been the significant reduction of cable capacitance. The reason for mini coax is that I have found it relatively easy to make into coiled coax with the help of an oven. In the future I plan on experimenting with some twin coax with yet higher impedance and lower capacitance. The reason I want to experiment with the twin coax is to keep the coil shielding separate from either of the coil leads... it gives me greater flexibility my TX and RX configurations (been experimenting with floating the coil during RX with a differential front end and an H-bridge TX drive).

I don't think there is a hard and fast answer to the best... it comes down to what parameters are important to fulfill your needs. I have often felt that, if there was a convenient way to support it, good ol' twin lead TV cable would be excellent for mono coil cable.

[dbanner]

I have wondered, apart from the physical challenges, why not just put the transmitter circuit in the coil housing? The Rx front end and remaining circuit about few inches up the stem, out of the coil field(remember shaft at angle to search head, no problem). Using fancy flat shield strip to connect the two parts.
I would like to see entire Rx circuit fit inside stem tube. User interface at the top of shaft where it normally resides, via wireless network. Supplementary power for charging provided by fold out solar panel. Altogether weighing less than 3 pounds. Too much to ask for?
A pulse detector for the 21st century..

[dbanner]

I have seen very expensive audio cables which are only good for use as curtain string.

[godigit1]

Here is a cable I have and have not tried yet for a coil .
Its a belden digital 1800a 110 ohm 24 guage 2 wire with a shield wire.



Ok all thats normal but I started to look at the shield and thats where I was suprised.



It has two individual shields in the foil layer. Both sides are conductive to themselfs but not from side to side of the foil.
Futher looking at it there is a layer of polyester or something between the foil layers.

So the inner shield foil layer would be the only one connected to the ground lead .

My question is is this extra layer of shielding not connected to anything of benefit or it it the extra conductive layer Eric mentions above ?
Another expensive cable that may not perform?

Good thing I got it at a estate sale a partial spool for the price of a foot.
It does work great on my surround set up though.

Here are the specs on the closest Part # I could findhttps://catalog.belden.com/index.cfm...=pd&p=PF_1800B
Thanks.

[KingJL]

Here are the specs on the closest Part # I could findhttps://catalog.belden.com/index.cfm...=pd&p=PF_1800B
Thanks.

Interesting... it may fit my ballpark!!!

[autitch]

Hi brains trust.
I have been wondering for a while if I could use a tinned copper stranded coax cable like the Belden 9221 mentioned above in place of an enameled coated copper stranded coax for a Minelab GPX series PI coil project I’ve been working on. (The Belden 9221 coax would suit the RX for a DD coil I’m working on) All commercially made coils use the enameled copper stranded coax. Are there any specific performance / noise issues or technical reasons which preclude the use of tinned copper coax on Minelab GPX series PI detectors .
AuTitch

[dbanner]

I think the enamel strands are meant to isolate(electrically) each individual strand from one another(deliberately so). Whereas tinned strands are still electrically conductive throughout(which you know). Don't know what difference it makes if the enamel strands are eventually connected at both ends vs tinned strands which are connected throughout.
Also, the strands are twisting along the length(lengthways).
I can imagine some " litzing" effect with the enamel copper strands, which would reduce overall capacitance for a given inductance.
If you mean to make a Rx coil(I hope I understand you correctly) then tinned strands may not be the deal breaker.
Tx coil is different issue. The GPX may not tolerate a coil even with slightly different parameters.

[Ferric Toes]

Have a read of this paper. 'Stranded wire with uninsulated strands as a low cost alternative to Litz wire' by Xu Tang and Charles R. Sullivan. I downloaded a free 7 page copy some years ago but couldn't find a site today, although there are plenty of them that require membership to do so. Very relevant to PI coil and cable design and my personal conclusion was that there is no benefit to using Litz. Tin plated stranded wire does have benefit over solid, in that the higher resistance plating does break up the cross sectional eddy currents in coil wire. The paper explains all.

Eric.

[Qiaozhi]

Have a read of this paper. 'Stranded wire with uninsulated strands as a low cost alternative to Litz wire' by Xu Tang and Charles R. Sullivan. I downloaded a free 7 page copy some years ago but couldn't find a site today, although there are plenty of them that require membership to do so. Very relevant to PI coil and cable design and my personal conclusion was that there is no benefit to using Litz. Tin plated stranded wire does have benefit over solid, in that the higher resistance plating does break up the cross sectional eddy currents in coil wire. The paper explains all.

Eric.

Here's a link with the paper -> https://engineering.dartmouth.edu/in...s/stranded.pdf

[waltr]

Here's a link with the paper -> https://engineering.dartmouth.edu/in...s/stranded.pdf

Good read. Thanks for link.

[6666]

Here's a link with the paper -> https://engineering.dartmouth.edu/in...s/stranded.pdf

Thanks
T

[Polymer]

... I tried two types of microphone coax, initially on the basis of its flexibility and advertised low noise performance. Initially it appeared to work fine, but when the detector was used in the field an unsuspected problem arose. The cable was microphonic in itself. Any loose cable that swung or moved, generated noise. When I looked into why this was so, I found that under the braid, between the braid and the soft polythene insulation over the core was a thin layer of semiconducting material. This, for reasons unknown,was the cause of the trouble. The other high end audio coax worked fine in the lab., but in the field was subject to failure due to breakage of the inner conductor which didn't like being swung from side to side or frequently being flexed ... Eric.

Thanks for posting this Eric!

Who'd have thought that. Microphonic microphone cable. Must be a special 2in1 deal, don't need a mic anymore.

I have had issues with a cable like the one you mentioned. Black "conducting" layer under the braid. It was kinda quirky on my MPP.

Measured resistance of the black layer to braid and in itself. My DMM didn't like it, got jumpy readings from roughly 5k to 200k Ohms.

I can only try to imagine what havoc that black layer creates when hit with high voltage flyback potential.

I have looked at many cables on the internet, the RG58 C/U you mention seems the best for mono coil for now.

There are even "better" coaxes around in terms of thicker inner conductor with many, many strands, tinned and
shield braid copper also tinned. BUT, they ALL have an extra foil shield (Cu or Al) under the braid.

I figure that would be detrimental for PI use. I may be wrong here as I have no experience with this type of cable and none to test.

[dbanner]

Someone should have a try with the cable musicians use to connect between guitar and amp and between FX pedals.
These are extremely tough and are well shielded. The cores are thick and consist of multi strands of copper.
They are very flexible and take to bending and flexing with ease.
They are made to withstand severe abuse.( Think Ozzy rock concert)
They transport signal from guitar pickup coils to amplifier preamp. Should be very good.