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Author Topic: 813 Amp Antenna Load Cap  (Read 4709 times)
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W9ZSL
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« on: April 24, 2015, 05:31:44 PM »

OK, I'm getting down to the gritty-nitty.  Got this loading cap today.  The gap looks to be .031 and it's about 1100 pf.  The rotor is really free-wheeling.  It's tight front-to-back along the axis, I don't want to mess with its tension so I'll need a vernier.  That was in the plan anyway.  With 2KV on the plate of the single 813, my Tune cap is double-ganged... 150 x 2 @ 4500 volts.  I'll include a HV switch to short the 2 gangs together.  Tank is Air Dux  So far so good.

Question:  I'll need to switch in some fixed padding caps across the loading variable.  I calculate a max of 2500 pf would do the job.  To be safe, what voltage would you recommend for the extra 1500 pf and should they be mica?


* Tank.JPG (771.94 KB, 2048x1536 - viewed 319 times.)

* Ant Load.JPG (808.7 KB, 2048x1536 - viewed 275 times.)
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kw7y
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« Reply #1 on: April 24, 2015, 05:54:11 PM »

With 400W into 50 Ohms the peak voltage is 200V. With 150 Ohms it would be 350V. So I think a 500V Silva Mica would work great. Available from Mouser for about $4.

- Paul
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w8khk
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« Reply #2 on: April 24, 2015, 06:29:22 PM »

You might find this document helpful:  Not real easy to read, but lots of info on the 195-1 and 195-2.

http://bama.edebris.com/manuals/illumitroniceng/airdux
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Rick / W8KHK  ex WB2HKX, WB4GNR
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W9ZSL
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« Reply #3 on: April 24, 2015, 07:19:22 PM »

OK.  I'm not great with math.  I wanted to be an astrophysicist at one time.  Fail. I found a source for some 560 PF micas at 1KV.  I can get 8 of them so I can come up with a combination to hit the 2500 pf in steps with a SP5T military ceramic switch with room and poles to spare.  I'm sure this is a 195-1 and should be good for 400 watts input.  The #2 is silver-plated and good for up to a KW.  SHOULD THE FIXED CAPS BE MICA?
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w1vtp
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« Reply #4 on: April 24, 2015, 08:25:06 PM »

This is what we use in our legal limit finals.  You won't anything else

https://www.fairradio.com/catalog.php?mode=viewitem&item=969


* loading cap.jpg (24.94 KB, 482x378 - viewed 279 times.)
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W9ZSL
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« Reply #5 on: April 25, 2015, 03:33:29 PM »

This one is over 2500 pf and it was my original choice.  I'm just not sure if the plate spacing is enough.


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PD0RTT
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« Reply #6 on: April 25, 2015, 04:39:27 PM »

One millimeter is 1kV for a spark gap, normally a capacitor from an BC radio is good enough for 500V of Voltage.
This means that under carrier conditions (e.g. 1/4 of power) 250V over 50 ohms is (sqrt)2.250= 177.5V (eff)2/50= 630Watts.
For higher voltages means more spacing.

73'Martin
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w1vtp
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« Reply #7 on: April 25, 2015, 05:54:32 PM »

This one is over 2500 pf and it was my original choice.  I'm just not sure if the plate spacing is enough.

It's working for all the class E guys out here that has to handle over 1500 PEP.  Now if you are not going to nominalize to ~ a 50 ohm Z and you are at a voltage max on your VSWR there might be a problem.  But the OWL guys normally use link coupled tuners and the coax guys use "T" matching tuners.  That cap should not be a problem especially using a single 813.

I have not had a single arc with my 1500 PEP AM station.


* CLASS E AMPLIFIER.jpg (556.96 KB, 2560x1920 - viewed 332 times.)
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K1JJ
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« Reply #8 on: April 25, 2015, 07:49:36 PM »

From practical experience, I've found those multi-section 365 caps to be good for about 2500 watts pep (extreme limit) on 75M with a matched 50 ohm antenna load before starting to have occasional arc-overs on voice peaks.  Voltage arc overs will depend on the humidity, antenna impedance and the frequency, with lower ratings as frequency increases and as matched impedance increases.

* This equates to about a 500 watt AM carrier output into 50 ohms.

In general, they are a good all-around 1500 w pep condenser. Some have wider spaced plates than others, so get the widest available. I used to use one on the output of my 8877 linear. But since then I use vacuum variables and turns counters for all KW class stuff.

T
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N2DTS
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« Reply #9 on: April 25, 2015, 08:30:10 PM »

Yes, 1500 or 2000 watts pep is no issue with those loading caps.
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W9ZSL
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« Reply #10 on: April 25, 2015, 08:31:23 PM »

The absolute max input I'll be running is 400 watts...probably closer to 350 AM.
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PD0RTT
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« Reply #11 on: April 26, 2015, 01:45:36 PM »

That's what I mean about 600w carrier is approximately 2400w PEP.
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w1vtp
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« Reply #12 on: April 26, 2015, 02:46:54 PM »

For 400 watts input, assuming 75% less output = 300 watts out.  See attached calculation for PEP output.


* 300 WATTS CARR W 125 PC POS PEAK MOD.jpg (10.56 KB, 600x116 - viewed 263 times.)
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W9ZSL
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« Reply #13 on: April 26, 2015, 03:52:31 PM »

Maybe I'll get one of those Fair Radio jobs.  As long as they are proven and cheap, why not?  As I said in other threads, I'm building this on the KISS principle and the "S"er,the better!
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