[AMRadio] Bug catcher questions

[Ken Sands]

The "open end" becomes a "Tesla Coil" very high impedance and subject to
just what you are experiencing......

Ken K8TFD

On Wed, Apr 20, 2011 at 8:39 PM, Geoff Edmonson <w5omr at att.net> wrote:

> 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?
>
> 73 = Best Regards,
> -Geoff/W5OMR (/m5 Houston)
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