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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th Feb 2020, 8:01 pm   #1
G6Tanuki
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Default Coil former trick.

I needed to make a coil for an antenna-tuner; looking around I saw an old cartridge that had once held the clear silicone-seal used on shower-cubicles, kitchen-worktops etc.

It was 50mm diameter - a quick consult of the RSGB handbooks from the 1960s gave me the inductance tables to wind the right number of turns.

I cut the threaded nozzle-stub off the end, then drilled a 6mm hole through the hole and into the plastic plunger that had been pushed down to the end of the cartridge to expel the last of the sealant, so I could use a setscrew, washer and nut to hold it down to the chassis. Little holes drilled through the cylinder let me tie-off the coil ends (20-gauge wire). It's a bit fiddly reaching down into the cylinder to loop the ends through the holes though.

The result works well: I wondered if the plastic might be RF-lossy and the whole thing would melt when squirted with RF, but several minutes of 100 Watts of carrier on 3.7MHz didn't seem to hurt it at all.
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Old 5th Feb 2020, 9:07 pm   #2
Al (astral highway)
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Default Re: Coil former trick.

Quote:
Originally Posted by G6Tanuki

The result works well: I wondered if the plastic might be RF-lossy and the whole thing would melt when squirted with RF, but several minutes of 100 Watts of carrier on 3.7MHz didn't seem to hurt it at all.
Nice one ; it’s satsifying to previsualise things and then see them work out like this

What is a setscrew?
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Old 6th Feb 2020, 2:01 pm   #3
kalee20
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Default Re: Coil former trick.

Good post - I'd actually expect it to work rather well! The sort of plastic used is rather Polypropylene-ish, which happens to have low losses.

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Originally Posted by astral highway View Post
What is a setscrew?
A straight piece of rod with one end formed into a head, with the remainder of the rod having a helical thread running completely along its length?

As distinct from a bolt, where the thread (strictly) runs only part-way along.
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Old 6th Feb 2020, 2:48 pm   #4
G6Tanuki
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Default Re: Coil former trick.

I'm now pondering whether I can use a couple of these sealant-cartridges as waterproofing covers for antenna-traps (with a piece of 32mm plastic waste-pipe fitted inside to serve as the coil former).

I've got plenty of them to experiment with!
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Old 6th Feb 2020, 8:17 pm   #5
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Default Re: Coil former trick.

I assumed those tubes were made of polyethylene.

They seem to be too tough to be Polypropylene.
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Old 6th Feb 2020, 8:32 pm   #6
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Default Re: Coil former trick.

Nice one- I'm always encouraged to hear of a bit of imaginative re-purposing. There must be zillions of these tubes that end up in builders' skips and thence to land-fill. I recall thinking that the ribbed ceramic formers in old hair-driers might make a basis for TX tuning coils- though the ceramic might of course be chosen for cheapness, rather than low RF losses!
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Old 7th Feb 2020, 12:58 am   #7
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Default Re: Coil former trick.

I have tried electrical conduit and plumbing plastic pipe as formers. I found them hugely lossy, even at low frequencies. Both orange and grey. ( the two types we get in aus, orange underground and grey internal )

Joe
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Old 7th Feb 2020, 12:19 pm   #8
Dave757
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Default Re: Coil former trick.

Hi,

Isn't the normal test to put a sample of the plastic into the microwave
along with a separate cup of water to act as a dummy load, and see if
the plastic heats up within 2 or three minutes?

Kind regards
Dave
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Old 7th Feb 2020, 1:00 pm   #9
Silicon
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Default Re: Coil former trick.

Electrical conduit and some plumbing pipes are made of uPVC.

PEX plumbing pipes are apparently made from Polyethylene with cross-link bonds.
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Old 7th Feb 2020, 7:31 pm   #10
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Default Re: Coil former trick.

i heard from tesla coil community that black PVC pipes contain carbon and very lossy. White or grey PVC pipes are less lossy.

White HDPE plastic bottles or pills containers are excellent as RF formers
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Old 7th Feb 2020, 7:34 pm   #11
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Default Re: Coil former trick.

That I can well believe.

Some years ago I was slightly involved in a system which involved measuring currents in the nanoamp range. It was found that black plastic project boxes were noticeably more leaky than white ones.
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Old 8th Feb 2020, 7:00 am   #12
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Default Re: Coil former trick.

Quote:
Originally Posted by TonyDuell View Post
That I can well believe.

Some years ago I was slightly involved in a system which involved measuring currents in the nanoamp range. It was found that black plastic project boxes were noticeably more leaky than white ones.
Presumably they use carbon to colour the black?

Peter
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Old 8th Feb 2020, 7:37 am   #13
regenfreak
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Default Re: Coil former trick.

They add 2% carbon as UV blocker or stabilizer for outdoor black PVC pipes. Old PVC can absorb moisture.

Dialectric loss tangent of different materials:

https://www.rfcafe.com/references/el...-strengths.htm
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Old 8th Feb 2020, 8:25 am   #14
joebog1
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Default Re: Coil former trick.

Can I just add, WHAT happened to Polystyrene coil formers?
Varnished paper?
Air coil?
Ceramic?

It's like US, YES, YOU and I. Things of the past. It's easier to Wi-Fi it at some gigahertz.

Your old mate, Joe.
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Old 9th Feb 2020, 4:47 pm   #15
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Default Re: Coil former trick.

Several years ago I repaired the turntable carrier on my Panasonic microwave oven.
I used a whte (translucent) plastic number plate bolt with a black nut.
Within about three days use, the black nut was scrozzled, and had to be removed. The bolt was completely unscathed.
The black filler used in the nut must have been very lossy.
Eventually I acquired an all-white nut and bolt, and it gave no further trouble. Tony.
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Old 9th Feb 2020, 7:41 pm   #16
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Default Re: Coil former trick.

It was probably carbon-loaded to get the black colouration of the plastic.

David
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