What's the best type of cap to use for RF bypassing, other things being equal?

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N3DRB The Derb:
I need to go back to school on a subject I used to know about. The info is no longer there in the noggin.

Is there any advantage to types of caps used to RF bypass something, a grid, a cathode?

for example, would a silver mica be superior to a regular ceramic disc or vice versa? Why?

is this frequency dependent?

thanks for the lesson. :)

Ed - N3LHB:

Per the 1965 handbook, page 170 paragraph under Plate Blocking and Bypass Capacitors:

"Plate Blocking and bypass capacitors should have low inductance. Between 3.5 and 30 Mc a capacitance of .001 uf is commonly used. The voltage rating should be at least 50% above the peak supply voltage.

Disk ceramic capacitors are to be preferred as bypass capacitors, since when they are applied correctly (see TVI chapter), they are series resonant in the TV range and are thus very useful in filtering power leads."

What I have observed is most stuff made since the inception of the ceramic disk uses them for RF bypassing. Also, ceramic disks have the smallest amount of series inductance of any capacitor made.   

N3DRB The Derb:
thanks ed. I don't have any printed info anymore on such things. Need to start a new collection of handbooks and such. I'll lay in a stash of 2~3KV+ disk ceramics in popular values. I have a good stash but they only go up to 1 kv. I cant believe how many parts one needs to have a decently equipped bench. Thousands of various things.

At the radio store we had around 50 parts racks, the big solid metal kind. thousands of drawers.

The Slab Bacon:
I typically always use ceramic disk caps (keeping the leads absolutely as short as possible) for bypassing lower voltage / lower power applications. Besides being being series resonant at vhf frequencies, as Ed said, they also have minimal inductive properties. I have had a few applications where I had to use a .01 along with a .001 to keep the rf from sneeking out.

High voltage ceramic disks are getting a little hard to find (and expensive when found) in values higher than a few hundred pF.

I am now finding it necessary to use ceramic doorknobs for plate choke bypasses as 5 and 6 kV ceramic disks are a little rough to find these days.

                                                             The Slab Bacon

The best bypass capacitor is the capacitor (or capacitors in parallel) with the lowest ESR (inductance and resistance).

Bypassing becomes a serious art with the class E rigs, where capacitors will heat up (and fail) due to ESR.  This property also shows up in higher impedance tube transmitters, with RF leakage around the bypass, and other ill effects.

Orange drops are typically very good !

The ATC (American Technical Ceramics) 100C and 100E series of capacitors are the best I've ever used for anything where RF is involved.  These capacitors are FAR superior to doorknob capacitors where RF current and low ESR is concerned.  They cost approximately 1/2 to 1/3 what a doorknob would cost.

Click here for more info on the 100C series.  The 100E series (larger capacitors) are also available.


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