Bug Catcher operation

[w5omr]

With a Henry Allen #680 bug catcher coil, and a (4) 2SC2879 Worthmore amplifier, I've managed (on SSB) to 'zorch' the bug catcher coil, about a half-dozen times, now.  Each time when SWR on the meter showed minimum.

Various things, I'm sure, caused each zorch.  One in particular was curious, as it was a carbon trace that arc'ed from the coil-form support to the top plexiglass 'end-cap', and RF energy poked a hole through the plexi. (I don't know that it's plexi or lexan, either way, it isn't "invisible" to RF.)

I've only had a corona ball off of the top of the whip, when I had an old original bug-catcher coil on my 91 Ranger, and a (2) 2SC2879 amp (~300w).  NEVER had a zorch on the coil.

Until yesterday, I had the coil mounted with the coil being tapped from the bottom up.  Time after time, when I ask if it matters, ham after ham said "it doesn't matter".  I'm not so sure.  The one ham I asked, is one of the most revered and respected (at least to me) in the world of Ham-dom, and is an Anchor of an AM guy.  When I asked John/WA5BXO, he said he tapped his bug-catcher (using an old Original coil by Alex/WA5UHT) from the top down.  Before I could ask why, the phone rang, and he was off again, maintaining his business.  I still don't have the answer, but I have an idea...

When I envision a 'jacobs ladder', the arc starts low and moves up.  In the one instance with the #680 coil, when I created a corona on the whip, the arc started at the top, and was working it's way down towards the top of the coil. (speaks well to the quality and construction of the 600w Amplifier!).  I don't know that that matters.

The way I -think- things happen is, that as the voltage is applied at the bottom of the coil, tapping from the bottom, you basically have a -lot- of inductance on 75m (nearly the entire coil) and by the time it gets up near the whip, the voltage is WAY up there, and looking for a place to "jump"/arc.  If the coil is tapped from the top -down-, there's still just as much inductance, but at the top, where the voltage is looking for a place to 'jump'/arc, you provide a tap and that goes straight to the whip.  What do -you- think?

So far, so good.  No zorches, but that doesn't matter because I could re-work the coil and make it all better again, and run it for a month, or maybe two, without an issue.  Allofasudden, calamity ensues.

Is there a difference in tapping a bug-catcher coil from the top or bottom?

[KA2QFX]

You're intuition is entirely correct. The coil should be tapped so that the unused (shorted turns) are on the bottom of the antenna. The bottom of the coil being only a few feet from the feed end is at low voltage, as it would be a few feet from the feedpoint of full sized antenna.

At the top of the coil, it's a few feet from the voltage and of the antenna, again, as it would be a few feet from the end of full sized antenna.  So the differential across the coil is tremendous!

I also agree it SHOULDN'T matter. Reason would suggest that if the top turns are shorted that the voltage from coil top to the tap below would be zero, but it definitely doesn't seem to work out that way.  I zorched a few things developing my antenna and that was one of the first practical lessons I learned. 

Lexan, and plexi, should have an adequately sufficient dielectric constant to withstand very high voltages. I found that cleaning and polishing with some car wax goes a LONG way in preventing embedded surface contaminants from forming arc tracks.  My antenna has a ceramic based band-switch that I fried and replaced twice until I bathed it in paraffin wax. I cleaned that with an abrasive cleanser like Comet/Ajax and a toothbrush firtst, bake dry and dipped in hot wax. No problems since then and it's been over 20 years now. 

Speaking of dielectrics; I don't know what you're coil form is, but I do not like PVC, at all.  I've seen it cause all sorts of problems around RF.  In my case I opted to have a 'open' coil form as you may have seen on my web site.   http://home.comcast.net/~msed01/ant1.html 

So yes, shorted turns on the 'cold' end of the coil.  Clean dielectric components and fingers crossed.  Hope that helps. 

Mark

[w5omr]

In my case I opted to have a 'open' coil form as you may have seen on my web site.   http://home.comcast.net/~msed01/ant1.html 

I -have- seen your site.  I only wish there were larger pictures to see construction details in a more .. uh.. 'detailed' fashion ;-)

So yes, shorted turns on the 'cold' end of the coil.  Clean dielectric components and fingers crossed.  Hope that helps. 

Does.  Thanks, Mark!

[KA2QFX]

I will take some pix and post them after the kids are in bed.

[KA2QFX]

A few more.

[KA2QFX]

Last one.

[W4AMV]

I was curious about the electrical quality of PVC. There are some excellent papers in the IEEE, but the bottom line from "Complex permittivity measurements of common plastics over variable temperature" is an excellent one. The loss tangent of PVC in the GHz region is .01 vs. .0001 or better for polystyrene and ceramics at room temperature. Warm these fellows up and PVC losses rise on a pretty steep curve. So, if you can use something better than PVC, you should! Even at low frequencies, 60 Hz, PVC is no darling.

 

Alan

[KA2QFX]

Good stuff to know Alan.  My experiences with PVC have always met with distress. In the case of my antenna (above) the PVC structural member is 2" and the coil is about 5 1/4", so the spacing alleviates any grief.  At 17" long I suspect the voltage end to end is sufficiently distributed that I needn't worry. It's been that way for a long time but I still have a 2" fiberglass replacement in the wings. You know how some temporary fixes become permanent?... 
 It's amazing how many "Bug Catchers" and dipole traps I've seen wound right on the stuff. The loss of Q is immediately noticeable even if they don't fry.

Mark

[w5omr]

Last one.

Thanks for the pics.  It gives me ideas. 
Such as... I've got the 40m version of that coil, but missing now the top cap.  Removing the  coil from the center insulator is now desired.  My thought is to mount (to the insulating shaft of what's left of the 40m coil) a 6 position non-shorting coax switch.  Coil completely bypassed for 10m, probably completely enabled for 75m, and somewhere inbetween for 40, 17 and 15m.  I don't -like- 20m.  17 has all the propagation characteristics of 20m, without the 20m attitude!

73

[n1ps]

We work with PVC at work.  The RF properties are not bad actually (dielectric constant something like 3.3 or so depending on the chemistry).  But the real problem with using this product for a coil form is it has no structural strength and it can be thermoformed.  This latter characteristic is what actually happens when a coil gets warm, even hot. Then you have a coil form similar to cooked pasta

Bug catchers probably work better in dry areas.  Not so much up here in the great north where we have a winter (and salted roads). This is why I like to use single band coils.  Its all sealed.  I use fiberglass (FRP actually) shapes and wind with enameled wire potted with epoxy.  BUT, I have not run any serious power yet in the mobile and not sure how these coils will hold up.  I'll bring a fire extinguisher along when I do.

It seems like the motorized commercial products handle the coils pretty well by sealing or gasketing the coils. Even then you need to keep them clean.

Peter - frequently pi$$weak mobile