The AM Forum

 I keep burning out the capacitors that are inline with my modified double extended zepp.

To make  a long story short, my antenna needs a pair of 17 pf capacitors, at 49 feet from center on each side. The modified DEX is 306' long overall. The capacitors make the antenna very close to 500 ohms across most of the 80 meter band, plus it has gain and a perfect match across the band using a 9:1 balun.

The capacitors have some *huge* RF voltage across them when the transmitter is running full blast. Bear in mind, the capacitors need to be light since they're hanging on a big wire dipole. So I can't use those big Sangamo broadcast mica caps at 10 pounds each.

I have tried those little 5 KV doorknobs, three 50 pf in series, but they eventually smoked out.

I have had the best luck with homebrew caps made from short sections of RG-213, but they eventually still burn out- I mean carbonized.

I need to repair the thing again for this season.
Smoked again.
I am thinking of using a couple of pieces of RG-393 coax (teflon RG-8 or RG-213) potted with epoxy into pieces of 1" PVC pipe.

Does anyone have any better ideas for bulletproof, light HV capacitors?

Pyrex test tubes with foil on both sides? A piece of window pane?

Try some of that 1" CATV hardline that uses the spaced discs and not foam.

Another option is 3" line from a BC station that uses Teflon spacers.

The biggest problem with those ceramic jobs and even coax is moisture causing flashover.

Carl
KM1H

How about RG393 teflon dielectric RG 214. Then make sure the dielectric sticks out a couple inches beyond the shield on both sides.

That's sort of what I've been using.
RG-213 with the center dielectric sticking out around an inch.

The ends eventually flame across and carbonize.

Bill,

  You sure seem to have an interesting issue. Getting low VSWR across 3.5 to 4.0 Mhz is a challenge that countless antenna articles have tackled. I've looked at at lot of these articles and some approaches achieve low SWR across the band at the expense of poor efficiency at the band edges. This suggests to me that something has to get HOT. Is your antenna like this?

  I do have a few of ideas about making the capacitors.

One is to make the caps as you do from Teflon coax, and then vacuum impregnate them with transformer varnish such that all the micro air pockets are eliminated. These air pockets when there will under high tension stress change chemically from corona. With corona oxygen turns to ozone, and ozone is known to destroy many materials. The idea I'm trying to convey is by eliminating the air pockets between the plates of a capacitor, the dialectic between the plates will be much more robust.

Another idea is to move the location of your capacitors on the antenna while adjusting the capacitance value. I'm no antenna expert, but maybe try moving the cap location towards the center with a higher value, or moving towards the ends with a lower value. A lower value could certainly be lighter in weight.

You mentioned using glass. I'm not sure this is relevant, but I once had a Push Pull 8005 cross neutralized 500 watt RF amp. I converted this over to 805's, and the higher Cgp capacitance of the 805's resulted in the neutralization caps being too close together to standoff the voltage. I got an idea, and took the glass out of two round panel meters and inserted the glass between the round disc neutralizing capacitor plates. Then I neutralized the amp for a nice output null. OK, so far so good. Next I keyed the B+ to see how the amp ran with this modification. The result surprised me. Both pieces of glass shattered immediately, the RF amp flamed out, welded the HV relay contacts such that I couldn't turn it off, and then I had to wait for the garage located 15 amp circuit breaker to pop.  :'(

Regards,
Jim
WD5JKO

Jim, here's what the thing looks like. It's a terrific antenna. No lossiness, it trades *length* for bandwidth. A simple design, I've played with it using modeling software, and it really does have less than a 2:1 VSWR across the entire band. I feed it with a 9:1 3-core balun.

Thanks for the suggestions and comments.

Bill,

  It occurred to me that a short length of open wire line might do the trick so long as the length is very short in terms of wavelength.

Two parallel spaced wires have a certain amount of capacitance per foot. I found this nifty web page:

http://www.pentodepress.com/calculator/wire-capacitance.html

If I did the numbers correctly, two #10 wires spaced 7.62 cm (3"), and 10" long provide 17 pf capacitance.

if this is feasible, then use open wire line ceramic insulators to separate the wires. The antenna would consist of two wires per side, and some insulators to separate the wires where they overlap. The wires could all be horizontal too. What-cha think?

Jim
WD5JKO

There are some small fixed value vacuum caps, glass and copper construction, probably made by Jennings... maybe 3 inches or so long...

I really don't know if they are still manufactured or not. I used one in the omega match of my 20 meter death ray, the voltage rating wasn't needed but reliability was. (the matching network is about 15' out on the boom @ 100') I would guess the breakdown voltage rating to easily exceed coax dielectic.

Might be something to search for.

I know exactly what you're talking about. A pair of 20 pf vacuum jobs might work out fine...IF I could find a pair. I'm pretty sure Jennings stopped making those things a long time ago. Good idea, though.

Bill,

I will have to check my junque box parts when I get home, but I may have some caps.  Those fixed vac caps were used in old ARC-5 ant tuning/sw units along with a vacuum relay.  I don't remember the value of the caps, however.  They may be 15 pf.

73,  Jack, W9GT

                      Jack, those SCR-274N/ARC5 caps are the 50mmf/7.5k units...

I think I'm going to fabricate a pair of teflon coax capacitors, keeping JKO's comments in mind. This time, I'll pot them into epoxy inside a couple pieces of PVC pipe. Glad I saved some chunks of that large Teflon coax over the years, it costs $8 a foot.

1" diameter solid poly RG-17 would be even better, but that stuff is unobtanium. I haven't seen any for 30 years.

Every time the coax caps have failed, it seems to have been due to arcing or corona at the ends of the pieces of coax, especially during wet weather. Hopefully potting will prevent that. Maybe extending the center insulation more than 1" past the shield will help.

Yes, I believe you're right, however, I think I might have some that are the same type of cap that are 15 pf @ 30KV.  I have to look, my memory is not perfect, by any means.

73,  JAck, W9GT

Bill
I may have some 25 pf glass fixed Jennings capacitors, vacuum type. Will check tomorrow.

That's what I'd try...
Just lengthen each of the existing wires by 10+ inches, and run them parallel to each other for that length, seperated with good insulators...

How about these, Bill? They are old Jennings fixed 15 pf caps, rated 17 kV. The glass body is 2 inches long, 1 inch diameter, and the end to end length for the metal contact caps is 3 1/4 inches. Would they work if you put them into a piece of PVC pipe, with caps glued on the ends, having a eyebolt on each? You'd have to use a flexible braid to connect to the caps inside, with a small fuse clip or plate cap to connect to the metal caps on the capacitors.

John, check your PMs.
Thanks

The two vacuum caps went out priority mail to you at KBCO today.

Received them in perfect condition today. Exactly what I needed..Small and light. I plan to put them inside a PVC tube for WX protection. 17 KV ought to hold off what LaBamba delivers!

Many, many thanks, John.

Good deal. Leave a small pinhole as a drainhole for condensation in the PVC. I was looking through the Fair Radio catalog, they list 4 of these caps, one is  25 pf @ 2.6x1.5"... the listing is easy to miss - at the end of vacuum capacitors, bottom right hand corner of page 30.

The one I used in the omega match of my 20 meter deathray has been up there 20 years and no problems. The matching network - in an electrical utility box - is vented for condensation. The resonance and impedance  controls are extended from the box along the boom to the tower permitting tuning the beam in place!

That would make a nice start to a "what was your best flameout" thread.

Just a thought.  Might be a possibility to seal and pressurize the PVC housing by installing a tire valve and pumping it up with a little air or even nitrogen to keep moisture out.  Just a few pounds of pressure would be more than enough.

73,  Jack, W9GT