Tank Switching

[W9ZSL]

I just purchased a 1000pf antenna loading variable that should be good for at least 1.2 KV.  I did so because I got a bit of negative feedback regarding the proposed cap for the 813 amp. Gaps are too small.  Better safe than sorry.  Now I need to score some micas to get the works up to about 2000pf. No problem.

Here's the problem. I'll need a shorting-type rotary switch with 3-5 positions and it needs to be robust enough to handle around 350 watts carrier.  I just listed this in the "wanted" files, but I've found that posting in the technical forum often results in sources for parts but other info as well.

[W3RSW]

I wouldn't consider hot switching increased coarse-loading steps with shorting contacts.
I take it your referring to progressively switching in increased value loading caps with the formerly switched and in-line caps staying in parallel.

Also you don't have to have a shorting, load switch. Your search may yield caps that are cheap but robust enough that you can find lots of values that are progressive enough with only one cap per index angle. Small values take a long time to add up anyway.  .

[Opcom]

By shorting, do you mean make before break or do you mean the farther the switch is turned the more of the terminals are shorted together, so as to cut in more and more capacitors?

I use that arrangement in a transmitter doing about 350W carrier. There's a coarse loading and fine loading done this way with two switches and two sets of caps. Switched the loading in increments of 100-400pF hot hundreds of times @ 300-400W carrier with modulation and never evidence of burned contacts. -in all these instances the tuning was already pretty close.

Switches look like those from WWII stuff. bad picture here, top left area, the 'fingers' type of switch. http://bunkerofdoom.com/tuckerkw/rf_deck_rear.JPG  -just to say those have not failed.

Before having made up tuning charts for each band, I always did a low power tune-up with this setup. Just an opinion but burning might be more likely when initially tuning up rather than when fine-adjusting.

[W9ZSL]

I have a couple of choices.  I can either parallel caps progressively to the variable with a non-shorting switch in steps, or I can use a shorting system that parallels the caps.  In other words, if I switched separate caps into the circuit, I'd have to make the caps progressively greater in value...say 100 to 200 to 400 to 800 to 1000 pf using individual caps as opposed to using a 100 added to a 400 added to a 400 via shorting for 900 pf as an example.  The difference is I'd have to score several different values of caps for the former and similar values in quantity, like 3 x 400 pf that I can parallel by shorting rather than add progressively increasing values.

Maybe I'd be better off just using one of the big ceramic switches with 5 or 6 positions to parallel a 100, 200, 400, 500, 800 and a 1000 pf mica with the 1000 pf antenna loading cap.

Or, I can get 3 x 400pf caps and use a shorting switch to parallel them with the 1000 pf loading cap.

[W9ZSL]

No hot switching.

[W3RSW]

Sorry, my edit was way slower than you guys' typing.
get the beefy switch and let it determine what caps you use.
What was that beefy five position mil. Surplus with mounting plate, index and squarish knob that made the rounds not to long ago. $5 to 15 depending on who was selling.
$1.00 at festers...

[W9ZSL]

I have a couple of those.  You can get them for under $10 and I'll probably get a couple more.  I guess it makes more sense to just get a few HV micas and switch them across the 1000 pf variable as needed.  Just thought I'd ask.  I've seen many schematics where the output consists of a variable cap and a shorting switch which parallels equal-value caps with the variable.  By shorting, I mean a continuous adding of caps so each switch position adds that capacitance to the circuit.

[KA2DZT]

1000pfd will work for 40M and up.  80M you'll need more.  With the right tank switch, you can switch in another 1000pf when you go to 80M.  Forget using a loading switch to switch in small amounts of loading capacitance.  That's the crappy way Johnson did things to avoid putting in a proper loading capacitor.  One contact to switch in another 1000pf.

For a loading cap in a higher power xmtr (200-300W) a broadcast type cap is all you need.  I use a 4 gang broadcast cap for all my loading.  I use 2 sections for 40M and all 4 sections for 80M.  Each section is about 550pfd.

The caps I use were pulled from old HP audio generators.  HP used the same 4 gang cap in all their audio generators.

Fred

[W9ZSL]

Hi Fred.  I have one of those big variables but I think I'll use it on another project.  I'll just switch in a 1000 fixed with the 1000pf variable for the 813 amp.

[W9ZSL]

I'm going to set up the antenna tuning with the 1000 pf variable, a SP3T heavy-duty switch that I can use to parallel either a .001 mica at 2.5KV or a 1500 pf mica pair of 1000 and 500 in parallel.  That should do it.