How to mount a Cardwell varicap?

[VE3GZB]

I'm not happy with my current PA tank arrangement in my transmitter....the entire cap is live, that's intentional, so I have an insulated coupling and shaft....but the cap's mounting tabs require fasteners with strength - for now I have brass screws holding the frame of the cap to the wooden platform, which makes for some RF loss I'm sure. As well it's only 150pF so I have to pad it to make things work Hi-C on 80m.

I have a better varicap, a Cardwell, 440pF. If I can use it instead, then I won't need an extra padding cap or switch.

But mounting the bugger is an issue! My Cardwell is sort of vaguely similar to this one:



But mine is just slightly different, and has black insulating material to support the stator. It's an older style I imagine. There are no mounting tabs, any mounting is done by the end metal plates to bear the weight. And this is where I have a problem since in my design the whole cap is live, so I can't mount the rotor/end plates to anything that might even faintly try to conduct.

I was trying to peel off the copper from these unetched PC boards this evening, hoping I could use the glass/epoxy board as a mounting material.....good luck peeling that stuff off! I even tried heating them in the toaster oven (my wife might get mad at me for the smell)...no luck! That copper foil is on there firm!

I might pop by the local Home Despot on the way from work tomorrow and see if they sell any plastic sheeting I can slice up.

But anyway, I'm just fishing for ideas on how to mount this so it's relatively firm and ridgid, yet keep it well insulated from wood or anything else.

73s
geo
VE3GZB

[KF1Z]

Look for some Lexan sheet,  at least 1/4" thick.
Attach a piece flat to each end of the cap, and use metal "L" brackets to attach the Lexan to the chassis.

Or, use some metal "L" brackets on the cap, and mount them to ceramic, or PVC stand-off insulators.

Depends on how much room you have, in what directions...



BTW, I use a propane torch to heat the copper foil, then peel... usually comes right off.
But does stink a bit 

[VE3GZB]

Hmmmm.....the economy where I live caters to farmers.....no Lexan on the hardware store shelves.

Is there anything available at a typical small-town hardware store which might substitute?

Thanks,
73s
geo
VE3GZB

[n2bc]

Hi George,

How much clearance between the cap and the chassis?   I have a pretty good selection of ceramic standoffs, some with a hole at right angles at one end.  Or you could fashion a mounting bracket out of 1/2" alum angle and use a 'regular' standoff of the proper length.  Be glad to help out if I can.

73 Bill  N2BC

[KF1Z]

Huh... that seems odd...

But, Plexiglass will work too... it's just a little more brittle than Lexan.


Wait... I have an Idea!

Check the electrical section at the hardware store...  see if they have any of the plastic (pvc) electrical junction box covers... the grey ones...

Or ...even the box itself, then you could cut it in such a way that it has a 90degree bend in it to make for easy mounting.

[VE3GZB]

KF1Z - Hmmm, that sounds like a great idea!! My design is flexible, I can mount the better Cardwell varicap anyhow I like so this might work! Thanks!

n2bc - thanks for your offer!

73s
geo
VE3GZB

[KM1H]

Any shop that repairs windows should have both Lexan and Plexi and a small piece is usually free. Thats where I get all my wire antenna and radial insulator material from.

Here in NH any screen door window has to be non glass by law.

Carl
KM1H

[VE3GZB]

Our screen windows can be repaired at the hareware store or by DIY folks, such as farmers in my area. There's no "screen repair" shop, it's just a small farming village.

[k4kyv]

The cap shown in your photo looks like it was made by EF Johnson.

Cardwells normally have the black insulation that holds the stator plates running along the sides of the capacitor.

There should be three or more tapped screw holes in each end plate.  The best way to mount the cap would be to fabricate a couple of L-shaped brackets, attach these to the end plates using the tapped screw holes, and use ceramic standoff insulators to fasten the L-brackets to the chassis below.  See attached photo: driver plate tuning cap. This is a Cardwell cap.

Or you could use insulating material.  I used bakelite on mine.  See PA grid tuning cap.  This one is an EF Johnson cap, but a Cardwell would work the same way.

Wood might cause RF losses if the frame of the capacitor is hot with RF.  But if the frame is bypassed to ground at RF and carries only DC, RF losses should not be a problem.  About the only occasion when the frame of a tuning cap is hot with RF is when a single section variable is used in a balanced tank circuit, and then both stator and rotor are hot.  If a split stator (dual section) variable is used in a balanced circuit, or a single section cap is used in an unbalanced circuit, the frame and tuning shaft should be at RF ground potential and all you have to worry about is the HV DC.

[VE3GZB]

k4kyv - yes you're absolutely right. You described my cap exactly, I simply couldn't find a photo of it on the internet.

The frame will be hot with RF, so that's why I'm looking for structurally sound yet insulated mounting ideas. I don't have ceramics to do the job but I like the idea of using PVC as mentioned earlier. I'll give it a try and see!

73s
geo
VE3GZB

[k4kyv]

I would first try out the PVC in a microwave oven to see if it heats up.  If so, that tells you that it has high dielectric losses, and might not be any better, or even as good as, wood.  I have never used PVC very much for RF insulation, but I have heard others say that some PVC is more lossy than others.

[KD6VXI]

Hmmmm.....the economy where I live caters to farmers.....no Lexan on the hardware store shelves.

Is there anything available at a typical small-town hardware store which might substitute?

Thanks,
73s
geo
VE3GZB

Tractor Supply or the like MIGHT have it.

Any of the tractor places SHOULD, as they use it for windows in the bigger Deeres, etc.

Make sure you do the microwave test on ANY plastics your going to use.

--Shane

[KF1Z]

I've heard about the microwave oven thing before...

Wouldn't that only prove it's lossy at over 500mhz?


At any rate, ceramic would probably be the best choice over all...

[k4kyv]

I would assume that anything that is low-loss at 500 mhz would pass muster at HF.  The converse would not necessarily be true.

Of course one shouldn't bet one's life on assumptions.

[VE3GZB]

I'll stop by my local Home Despot on the way home from work and see if I can pick anything that might fit the bill.

Failing that, I'll have to use a bunch of Ferric Chloride and etch the carp out of blank PC boards for their fiberglass/epoxy base.

73s
geo
VE3GZB

[KF1Z]

I would assume that anything that is low-loss at 500 mhz would pass muster at HF.  The converse would not necessarily be true.

Of course one shouldn't bet one's life on assumptions.

True...

And also, there are many ceramics that use metal powder in them, or in the glaze that wouldnt be too great to use for rf either!

However I would, of course, assume that any product sold for use as a high frequency insulator would be free of such things!


I'm still not convinced about the microwave oven trick...
considering I can put a fork in mine, and it would take a LONG time for it to get warm..
(have accidentally done that!)
I don't know why this would be,, I remember as a teen, I put one of my mom;s favorite dishes in the old amana radar range...
I blew the gold-leaf decorations right off the bowl!

Man, what a display of fireworks that was..... the sparks in the microwave were spectacular as well   


Perhaps theres some type of sensor that could detect metal now?
Seems if it were sophisticated enough for that...it would just shut off ..


Anyhow


VE3GZB 
 if you run out of luck I'll mail you some Lexan...
Or take N2BC up on his offer of some ceramics.

[N3DRB The Derb]

I love the copper STRAPS in Don's tx. none of this round bus bar stuff. It's got to be flat and wide or it aint happening.

I found a place on ebay where you can get sterling silver sheet and cut your own strap with a good pair of scissors cheaper than you can get ready made copper strap for.

[KF1Z]

I love the copper STRAPS in Don's tx. none of this round bus bar stuff. It's got to be flat and wide or it aint happening.

I found a place on ebay where you can get sterling silver sheet and cut your own strap with a good pair of scissors cheaper than you can get ready made copper strap for.

 

[VE3GZB]

.....About the only occasion when the frame of a tuning cap is hot with RF is when a single section variable is used in a balanced tank circuit, and then both stator and rotor are hot.  If a split stator (dual section) variable is used in a balanced circuit, or a single section cap is used in an unbalanced circuit, the frame and tuning shaft should be at RF ground potential and all you have to worry about is the HV DC.

Yup, I don't have a split stator cap, I'm using a single section cap for a balanced tank circuit.

[VE3GZB]

I think I found my solution at Home Despot!

http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=131358&Ntt=131358&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&recN=51%204294966653&N=0&Ntk=P_PartNumber

In case the link doesn't work, it's ELIANE Maxi tile. I bought Six 3"X6" at 54 cents and Two 8"X12" at $1.19

I popped one of the smaller ones into the microwave when I got home (put it on a plate so if it cracked it'd be easier to clean up). I nuked it first for 3 seconds, then checked it....then 15 seconds, checked it again, then 30 seconds, then a minute, checking it in between.

The dinner plate got warmer than the tile did!

So I've stacked them three on each side, glued together with RTV Silicone. These are like a pedistal for the Cardwell which I've siliconed (by the black insulator) to the top tiles. Then I can silicone everything to a small piece of wood which I can then screw to the breadboard.

I'll use the larger pieces of tile to fabricate an antenna tuner, siliconing the parts to the surface. Then I'll silicone the tile to another piece of wood.

Red Green's secret weapon is Duct Tape. Mine is Silicone! Go figure! LOL!!

I'll be sure to post photos.

73s
geo
VE3GZB

[W9GT]

Hi Geo,

The tile seems to be doing the job for you, but for future reference, I believe Home Despot has plexiglass as well...also many other useful items for radio applications in the electrical and plumbing departments.  I noticed recently that the price of #12 THHN stranded wire for antennas has come down to $40 for a 500 ft roll.  Hopefully, it will come down more....not too long ago it was only $15 for 500 ft.  I have had good luck using PVC (white) for insulators at HF, but have heard that the gray stuff might not be so great.  Have also heard that even white PVC isn't too good at VHF.  YMMV

You are certainly to be commended for your inventiveness and persistence with your home brew projects!  Hope to hear you on the air one of these days soon!

73,  Jack, W9GT

[KM1H]

Hmmm, a small farming community yet it has a Home Depot nearby? Out this way that might get the night riders in motion . Many of the very small towns are inbred and dont take kindly to city folk putting family out of business

The Johnson design TX caps have been sold under the Cardwell name for over 30 years under the same Johnson part #. Cardwell makes very few of their original designs except special $$$ order.

Carl
KM1H

[KF1Z]

Hmmm, a small farming community yet it has a Home Depot nearby? Out this way that might get the night riders in motion . Many of the very small towns are inbred and dont take kindly to city folk putting family out of business

The Johnson design TX caps have been sold under the Cardwell name for over 30 years under the same Johnson part #. Cardwell makes very few of their original designs except special $$$ order.

Carl
KM1H

Yeah, and now it's no longer Cardwell.

Now they are   Viking Technologies Ltd. 

Guess it's been 5 or 6 years?

[KM1H]

http://www.cardwellcondenser.com/PAGES/history.html

True but the brand name still exists.

Carl
KM1H

[KF1Z]

The Johnson design TX caps have been sold under the Cardwell name for over 30 years under the same Johnson part #. Cardwell makes very few of their original designs except special $$$ order.

Carl
KM1H

You certainly have the "$$$" part right.  !!!

I got a scary quote from them a couple years ago... was cheaper to buy brand new vacuum variables!