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

[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]

Derb,

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

[steve_qix]

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.
http://www.atceramics.com/products/100c.asp

[N3DRB The Derb]

Steve, is there a distrib for these that will sell direct, or can you point me to a dealer that will? They sound every good.

Q: will paralleling bypasses to a desired value point further reduce ESR? Likewise, will providing a 8 lane rf highway in the form of soldered silver strap on the chassis side reduce ESR? I would assume such things are much more critical on FETS, but I'd rather apply the best to anything I do in the shack. I have a couple of months time to get everything ready. Once the static is gone I have to make up for lost time operating and I dont want to have to go back inside the gear after each piece is done.

I've really lost a lot of tech knowledge over the last 5 years. I'm sure it shows in various ways but I'd rather just admit I don't know. my experiences has left my memories more like a block of swiss cheese, there's some things that used to be there that are now just holes with nothing.  very common thing after brain surgery and radiation.

[steve_qix]

For Tube transmitters, the 100C or 100E series of capacitors are probably overkill !!  Unless you have an unusual situation (like Tempest or something like that), standard bypassing will probably work just fine.

I like using a few different value capacitors in parallel for serious bypassing of high current circuitry.  This puts several self-resonances in parallel, and can eliminate it.  This is very good practice for high current situations, but can also help stabilize amplifiers, and provide a more consistant reference point.  Anywhere where low impedance shows up, such as tube cathodes or heaters, a few capacitors in parallel works nicely to keep these at RF ground.

With some VHF MOSFET drivers, I've seen 10 different value capacitors in parallel used as a bypass !!  These are 50 ampere drivers looking into a .02uF load operating at 60mHz.  Nothing you're doing here - I used it as an example.

Of course, tube circuitry is very forgiving    I've used .001uF disk ceramic capacitors for bypassing tube cathodes in high power amplifiers (not a good idea, but I did it).  If I used the same caps in the bypasses for a 500 watt class E rig, they would melt down.  I have had orange drops explode in single-capacitor bypass applications in my early class E rigs.  Now, I use 3 of them in parallel (all different values), and everything runs nice and cool.  It really is something to see what high current will do to a "good" capacitor 

Regards,

Steve

[w3jn]

Steve makes a good point - depends a lot on the impedance of the circuit.  A relatively high impedance circuit - the ESR of the cap isn't gonna make hardly any difference.  A very low impedance circuit, say class E rig, well, you need really good caps and very short leads and several in parallel.

The best caps are of course chip caps, having no leads.  You can get relatively high voltage chip caps (several KV, IIRC, in the latest Allied catalog) but of course they're really not suited to tube type point-to-point wiring.

Disc caps are prolly the best for tube bypassing, but not so great for coupling the final to the tank as they don't have a lot of current capability.

[N3DRB The Derb]

I have a crap load of parts coming in for the Gonsets.  the one thing I'm missing is a pair of decent plate chokes. I have to rebuild the second amp to match the first. I keep going back and forth as to if I should try again with the bandswitch. I'm leaning heavy to say F that. it's too weak.

I think I am going to hardwire the tank for 75, put a hole plug where the bandswitch knob used to be. and test into the termaline for loading and wire one or both of the fixed padders in place so it loads as needed into 50 ohms.

tired of f'ing with it. make em make power on 75 and b done. 1000ma's on those 572b's and then drop the drive down apply audio and swing the monky. 225 carrier. put up the W7FG antenna and get on.

Update: I just ordered 2 new plate chokes custom wound from Frontier Engineering offa ebay @ 36 each delivered. I aint playin now. 

http://stores.ebay.com/Frontier-RF-Products

[WA1GFZ]

Many TNX John. I used some of those blue caps you gave me on a project at work.
DDX

[Vortex Joe - N3IBX]

DerbMeister,
                Most of the cathode bypass caps, etc. I see in smallish type transmitters (and receivers) are of the paper or wax impregnated type. I always replace them with "poly" types currently available, or Sprague "orange drops" as others have mentioned. Whilst the paper caps may have lasted 70 years, the poly types will most likely outlast the device they're being installed in. Like "Da Slab Bacon" says: try to keep your leads as short as possible.

Besides having the ability to outlast the device the poly caps are being installed in, I would think that they would be superior due to their electrical properties.

Best Regards,
                  Joe Cro N3IBX