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-   -   What type of capacitor for R.F. decoupling ? (https://www.vintage-radio.net/forum/showthread.php?t=140663)

NickG0HIK 19th Oct 2017 9:31 pm

What type of capacitor for R.F. decoupling ?
 
Hi,

I'm trying to rebuild an oldish receiver (1950's), all I have is a more or less bare chassis and the circuit diagram.
There are quite a lot of 1000pf decouplers, Cathode, Screen and heaters. Even though only the screen components will need to be H.V. I presume that they could well have used the same component throughout.

In my stock I have Mustard Polyesters (400v) and polystyrene's (not sure of their voltage rating), which or neither should I choose for use from 20 to 40 MHz ?
I see you can get R.F. ceramics, I presume that they have improved dielectric for lower loss. But they really don't look the part for the vintage of the kit.

Any recommendations and info would be great.

Thanks

Nick

Julesomega 20th Oct 2017 12:35 am

Re: What type of capacitor for R.F. decoupling ?
 
Use polyester: the value has already been chosen, and they are readily available in axial form which will look right. The voltage and temperature ratings can easily be met, and the dielectric loss is quite low enough

Craig Sawyers 20th Oct 2017 6:44 am

Re: What type of capacitor for R.F. decoupling ?
 
You are looking for a capacitor that is still a capacitor at 40MHz - ie that is self-resonant frequency is greater than 40MHz. That means that its inductance has to be less than 16nH, including the leads that connect it to the cathode/screen/heaters. That is a tough ask - 2cm of wire, 1mm diameter has about that inductance, so the wires that connect the capacitor can be the dominant inductance.

Above the self resonant frequency it no longer looks like a capacitor, it looks like an inductor.

NickG0HIK 20th Oct 2017 8:45 am

Re: What type of capacitor for R.F. decoupling ?
 
1 Attachment(s)
These are the capacitors in similar equipment of the same vintage, what are they ?

Nick

M0FYA Andy 20th Oct 2017 9:13 am

Re: What type of capacitor for R.F. decoupling ?
 
Nick, they look like paper 'Metalmites', so probably a bit (or a lot!) iffy by now.
Andy

John M0GLN 20th Oct 2017 10:11 am

Re: What type of capacitor for R.F. decoupling ?
 
1 Attachment(s)
Quote:

Originally Posted by NickG0HIK (Post 984236)
Hi,

I'm trying to rebuild an oldish receiver (1950's), all I have is a more or less bare chassis and the circuit diagram.

I see you can get R.F. ceramics, I presume that they have improved dielectric for lower loss. But they really don't look the part for the vintage of the kit.

Any recommendations and info would be great.

Thanks

Nick

Depending on the type of receiver it is they may not look out of place for the year, I'm working on a 1955 Granco FM domestic radio at the moment and with the exception of 2 electrolytics every capacitor is either a ceramic or silver mica.

The attached view showing them has been downloaded from,

http://www.antiqueradios.com/forums/...p?f=1&t=244660

John

astral highway 20th Oct 2017 10:33 am

Re: What type of capacitor for R.F. decoupling ?
 
Quote:

Originally Posted by NickG0HIK (Post 984236)

In my stock I have Mustard Polyesters (400v) and polystyrene's (not sure of their voltage rating), which or neither should I choose for use from 20 to 40 MHz ?

Silver Mica for these sorts of frequencies. They usually have values stamped on them in pF. They last forever, so NOS are fine, or just ask on the forum, people might have some in their parts boxes.

Argus25 20th Oct 2017 11:29 am

Re: What type of capacitor for R.F. decoupling ?
 
Yes seconded, Astral Highway knows his stuff. By far and away the best 1000pF caps are silver mica. The good ones are often mil spec types, here is what they typically look like (mods please excuse the ebay reference it is just for the photo)

https://www.ebay.com/itm/Cornell-Dub...QAAOSwOVpXW1gL

paulsherwin 20th Oct 2017 11:36 am

Re: What type of capacitor for R.F. decoupling ?
 
Obviously silver mica are ideal, but any cap technology will work for decoupling at those frequencies as the stability of the value isn't very critical.

kalee20 20th Oct 2017 11:37 am

Re: What type of capacitor for R.F. decoupling ?
 
Quote:

Originally Posted by Craig Sawyers (Post 984307)
You are looking for a capacitor that is still a capacitor at 40MHz - ie that is self-resonant frequency is greater than 40MHz. That means that its inductance has to be less than 16nH, including the leads that connect it to the cathode/screen/heaters. That is a tough ask...

I'd mildly disagree with that, actually!

Yes, at 40MHz, resonance occurs with 16nH and your 1,000pF. But who cares about 16nH-worth of resonance? Even with 0.5uH stray inductance, at 40MHz you only have 120 ohms of impedance. And that's not going to be enough to cause instability. In the cathode, it might cause a bit of degeneration; on the screen-grid, with a typical mutual conductance from grid to screen of 0.5mA/V, you'll have a signal on the screen of 1/20 of what you have on the grid. Hardly worth bothering about, even if it is phase-shifted the 'wrong way!'

Put it another way, suppose you have a super-low-inductance 1,000pF capacitor with 1nF stray inductance. (Note that the SRF is 160MHz). At our frequency of interest, 40MHz, the impedance is 3.73 ohms. Now suppose overnight, a little elf comes along and swaps the dielectric in the capacitor for some ultra-high permittivity stuff, so capacitance shoots up to 10F but nothing else changes. Impedance now drops to 0.25ohms, so better 40MHz decoupling than before, even though SRF is now only 1.6kHz!

Ultimately, the reason for lowish values of capacitance is that bigger values tend to be larger, so have MORE stray inductance. So, the initial choice could well have been based on that. Modern polyester caps are much better (the elves have started work!) so something of the same physical size is likely to be your best choice, and the extra capacitance is a bonus. Who knows, the original designers may have hidden some crafty neutralisation by careful component placement, to give most favourable response curve with what they had at the time!

M0FYA Andy 20th Oct 2017 2:15 pm

Re: What type of capacitor for R.F. decoupling ?
 
The problem with typical modern silver-mica capacitors is that they tend to be radial-leaded to sit on PCBs, and don't fit neatly on tagboards or between valve-holder tag and tagboard.
Does anybody make new axial-leaded silver-mica?

Andy

astral highway 20th Oct 2017 2:59 pm

Re: What type of capacitor for R.F. decoupling ?
 
Quote:

Originally Posted by Argus25 (Post 984356)
Yes seconded, Astral Highway knows his stuff...

Thank you, Argus25, nice of you to say so...

Craig Sawyers 20th Oct 2017 3:47 pm

Re: What type of capacitor for R.F. decoupling ?
 
Quote:

Originally Posted by kalee20 (Post 984361)
I'd mildly disagree with that, actually!

You are quite right to disagree! In most of my applications, going inductive presents a real problem - but as you rightly point out the key thing in screen bypassing (for example) is maintaining a sufficiently low impedance regardless.

NickG0HIK 20th Oct 2017 5:54 pm

Re: What type of capacitor for R.F. decoupling ?
 
My collection has quite a few Silvered mica's, but as Andy says, they don't look the part, I would prefer axial types.

I thought that the capacitor technology might have been important, i.e. not to use disc ceramics due to the low "Q"?

And the fact that 1000pf were chosen simply because they could get away with them for the size sound pretty logical, so 10nf maybe the way to go.

Great ideas as usual on here.

Thanks

Nick

G6Tanuki 20th Oct 2017 7:08 pm

Re: What type of capacitor for R.F. decoupling ?
 
If you're talking about needing decoupling in the 20-40MHz range I assume you're working on either first-generation TV or some 'interesting' upper-short-wave-band radios. I'm intrigued!

Truth is, pretty much any wire-ended polyester capacitor available these days will be a lot better than what was used decades back. Silvered-ceramic, "Mustard" caps, or 'tropical-fish' types would be good to go.

paulsherwin 20th Oct 2017 8:14 pm

Re: What type of capacitor for R.F. decoupling ?
 
Q isn't really relevant for this application. The cap is just shorting AC to earth (albeit high frequency AC).

turretslug 20th Oct 2017 8:23 pm

Re: What type of capacitor for R.F. decoupling ?
 
I was intrigued by the frequency range, too- I recall in one of my dad's "pilot's notes" was a summary of SBA receivers of this vintage and similar operating frequency range including a picture of a hand-cranked remote tuning box mounted at one side of the cockpit.

I was a bit sceptical of just how good those olde-worlde paper-and-foil caps would be at these frequencies as well- look for "extended foil" in the specs. of replacement candidates, meaning parallel gathered and connected end-foil and hence low-inductance connections at each end. Not sure how universal this connection method is amongst plastic film caps, or if e.g. Mullard "mustards" are like this.

Radio Wrangler 20th Oct 2017 8:52 pm

Re: What type of capacitor for R.F. decoupling ?
 
For general RF decoupling, ceramics have a lot going for the.

David

NickG0HIK 20th Oct 2017 9:21 pm

Re: What type of capacitor for R.F. decoupling ?
 
3 Attachment(s)
If you're talking about needing decoupling in the 20-40MHz range I assume you're working on either first-generation TV or some 'interesting' upper-short-wave-band radios. I'm intrigued!
----------------------------------------------------------------------------------
I only need to go up to 45MHz because that is the L.O. for the 20 -30 MHz R.X.

It's a ground based GEE R.X. unit.

It came to me as a more or less empty chassis and I thought it might be fun to try it out on H.F.

'LIVEWIRE?' 22nd Oct 2017 9:52 am

Re: What type of capacitor for R.F. decoupling ?
 
Correct me if I'm wrong, but weren't the old style silver mica caps. axial leaded? I certainly have several in values from maybe 100 to 1000pf which are thin rectangular 'plates' 9for want of a better word0 with the two leads protruding downwards from one side.


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