Fast coil from coax

[jakub]

I was reading somewhere that gas injected polyolefin has a better dielectric constant then teflon. Its what they use in coax.

In a week and a bit (when I get back to Oz) I am going to remove the outer layers from a coax and get to the foam coated wire in the middle, keeping the foam insulation on. This I suspect will make a slightly fat but otherwise very good very fast coil.

A few things bother me though.. What does 75ohm impedance mean? It's not the resistance per meter / foot or whatever, is it?

Should I get a stranded core or a single core? Plated with anything or not?

Will keep you posted on how it goes, theoretically and practically I think it ought to turn out rather well...

[WM6]

A few things bother me though.. What does 75ohm impedance mean? It's not the resistance per meter / foot or whatever, is it?

.

75 Ohm impedance is characteristic impedance (Zo) of cable or something like "total resistance" (R + C + L component of cable) regarding measured sinus signal (where a source of signal is coupled to a 75 Ohm load with your selected cable) in absence of refletions from 75 Ohm coupled load at other end of cable. Measured lenght of cable (at absence of reflections) depend of wavelenght and is not fixed by whole meter or foot. This is SWR maesurement very known in Amateur Radio by using mostly homemade SWR meters.

If you remove outer cable layer (metalic shielding inclusive), 75 Ohm impedance is no more important but only thickness and dielectric characteristic of insulative foam around center wire.

Such spiral wound coil is very good solution if you are looking for "quick" (low parasitic C) coil.

[Leto]

I was reading somewhere that gas injected polyolefin has a better dielectric constant then teflon. Its what they use in coax.

In a week and a bit (when I get back to Oz) I am going to remove the outer layers from a coax and get to the foam coated wire in the middle, keeping the foam insulation on. This I suspect will make a slightly fat but otherwise very good very fast coil.

A few things bother me though.. What does 75ohm impedance mean? It's not the resistance per meter / foot or whatever, is it?

Should I get a stranded core or a single core? Plated with anything or not?

Will keep you posted on how it goes, theoretically and practically I think it ought to turn out rather well...

Very bad solution for sure... insulation is surely superior but wire in coax is not pure copper!! I made such coil once for 320uH and resistance I got is 2.5ohm!!
Do not know about your PI, but 2.5ohm for 0,75mm wire is way too much..
Forget about coax with 75(50..)ohm impendance .. for sure

You can try with microphone cable, some have same gas injected insulation but pure copper core, can be pretty costly sport destroying 20m of mic. cable.

[WM6]

Very bad solution for sure... insulation is surely superior but wire in coax is not pure copper!! I made such coil once for 320uH and resistance I got is 2.5ohm!!

Hi Leto I don't know what sort of cable you use, but most of UHF cable have center wire out of pretty pure copper wire (who need additional attenuation for weak UHF signals?). TV cable too. So this is not "very bad", but one of the best solution. Have to bee wrapped in spiral (not as yours) and adapted to amplifier stage by damping resistor (scope needed).

[Leto]

TRY IT YOURSELF...

Hi Leto I don't know what sort of cable you use, but most of UHF cable have center wire out of pretty pure copper wire (who need additional attenuation for weak UHF signals?). TV cable too. So this is not "very bad", but one of the best solution. Have to bee wrapped in spiral (not as yours) and adapted to amplifier stage by damping resistor (scope needed).

Actually it was cheapest 75ohm coax from hardware store..
And yes I thought so too - should be pure copper but on contrary most UHF cables for wide use have inner conductor made of CCS or copper covered steel (the centre of inner conductor actually play little role in transmission via coax as skin effect kicks in at those frequencies..)
Should point out that such wire is usless for PI..
So one should buy more expencive type of coax.. (silver plated copper or pure copper)
but than again it's better to use stranded wire with those diameters of conductor in PI coils: I would never use solid wire thicker than 0.4mm with lower delays as the wire itself becomes the "target" .. this has been discused many times on this forum..
Also the thickness of the insulation is a bit of a problem: posted picture is three layer spiral as was impossible to achieve desired inductivity in single layer.

WM6 you should first try it yourself (as I did), and than post results:
You claim it is the >one of the best solution<
I say to you hardly.

Regards

[WM6]

Leto I don't know what resistance you expect from PI coils (you say 2.5 Ohm is way too much)?
Most of my PI coils are about 2.5 Ohm (even 3.5 Ohm) and work excellent.

http://www.geotech1.com/forums/showp...54&postcount=1

[jakub]

OK, I am going to give it a try with the following info:

-keep the internal wire under 0.4mm
-never use iron core

I would like to make it a well aligned vertical bundle that is, rather then starting flat and layering the coil horizontally, I would like to wrap it around a cylinder, say 5 layers high and make it that way. I think the fields would be better aligned in the direction they are supposed to go (up and down rather than sideways)