Low PF Capacitor Home Brew
 

If you need a low PF capacitor, there are 3 ways I have made them.
1. Twist 2 wires together and cut down until you get the correct PF rating.

2. Use a length of coax cable. The type of coax and pf/ft varies with the type (and sometimes the mfg.) It also varies with the percentage of shield. Mfg. specs. will give you pf/ft rating.

3. Use zip cord, speaker wire or similar twin wires and cut to length for the correct pf you need. You can cut a ft., meter, or whatever unit you want and measure the PF for the length, then cut it to size for correct pf.
Added bonus: I have seen twin wire cord used as spark gaps in TV sets too.

This type of capacitor is great for tweaking that last little bit to get an oscillator on freq. down to a gnats hair.

Re: Low PF Capacitor Home Brew
 

Manufacturers sometimes use homebrew capacitors as well. ISTR that Philco used an insulated wire wrapped around the tag of the tuning capacitor on models such as the 444.

Cheers

Mike T

Re: Low PF Capacitor Home Brew
 

During my early career in video Broadcast, the modulator (using MC 1495 Ic's) card used this method. The technique was to twist some Kynar wire together and then cut it down to reduce the capacitance until the vectors were in the boxes.

Problem was when you tried to neaten up the arrangement and secure it the capacitance often changed. Depending on which way it went, you might have to de solder the Kynar and start all over again....... Then you used to go off for a tea break and come back to find everything out of spec - happy days - not >((

Re: Low PF Capacitor Home Brew
 

I did exactly that in my first job in the early 80's. There was a video amp, and to tweak up the pulse response I used the twisted wire method.

Craig

Re: Low PF Capacitor Home Brew
 

When you need a 1kV 27pF for a late night emergency fix for an LCD monitor backlight, and you've only got some PTFE covered wire:-

Attachment 169681

A 'temporary' fix, still in place three years on...

Cheers

Re: Low PF Capacitor Home Brew
 

The twisted wire capacitors are also called 'gimmick' capacitors.

Re: Low PF Capacitor Home Brew
 

One presumes that if you use enamelled wire twisted together the capacitance can be fairly high (relatively)

Re: Low PF Capacitor Home Brew
 

And the breakdown voltage disappointingly low?

Re: Low PF Capacitor Home Brew
 

Yes there is that but on a quick check online it seems that it might not be as bad as it sounds, at least for receivers:


http://www.magnetwire.biz/soderon155_data_sheet.pdf

I do notice though that with the standard stuff from RS (for example), the voltage rating is not stated.

Re: Low PF Capacitor Home Brew
 

Interesting that its breakdown voltage is higher at higher temperature.

Certainly looks good for 300v or so operating voltage. I wonder what the dielectric properties of the insulation are like?

Re: Low PF Capacitor Home Brew
 

The coax "gimmick" capacitor is quite well-known in ham radio circles: one popular use is as the resonating capacitor in antenna-traps, where the coax 'capacitor' can be conveniently stuffed into the inside of the coil-former once you've GDO'd the trap to the required frequency.

Re: Low PF Capacitor Home Brew
 

Spot the gimmick cap in this WW2 dinghy homing beacon transmitter.

Attachment 169926

Re: Low PF Capacitor Home Brew
 

I presume that TX is "Walter" somewhere I have two brand new triodes as shown in your example and often wondered what the unit looked like out of its casing.

Are you able to post another photo,nice implementation of a gimmic capacitor in the tank circuit (I presume).

Re: Low PF Capacitor Home Brew
 

Yes, a Walter. Another photo and full info here: https://www.royalsignals.org.uk/photos/AN_CPT-2.htm

gmb

Re: Low PF Capacitor Home Brew
 

Most kind of you,no idea how that reference had slipped me. I bet it would have been quite effective, but do wonder how well a dunked Airman,possibly with bad injuries would have coped deploying it.

Thanks for posting the link.

Re: Low PF Capacitor Home Brew
 

on a similar vein,ive used broadcast variable capacitors for mag loops before,milled a case out of nylon with a perspex cover then filled it with oil,gets the kv rating up and nearly doubles the capacitance,sort of poor mans vacume variable!.

Re: Low PF Capacitor Home Brew
 

ISTR the SP Radio range of transmitters (the earlier versions) used many 'home made' capacitors and have a vague recollection of seeing the equipment they used to make them when I visited their factory in Aalborg???

Or was that all just a dream?

Re: Low PF Capacitor Home Brew
 

I seem to remember Spencer West using lots of twin wire low pf capacitors on their printed circuit boards in the distant past.I can't imagine how this could be any cheaper than using standard prefered value components of the time but then labour was cheap.Perhaps they were snipped and adjusted in circuit.Peter.

Re: Low PF Capacitor Home Brew
 

It too me ages to find a particular low value capacitor in the main bard in the Telequipment D61a oscilloscope. It turned out that to was using the twisted wire trick. I think one of the calibration steps involved tweaking it by twisting or untwisting as required.