Topband: T - vertical improvement

[Robert Carroll]

I have often done this, but N4GG educated me on another aspect of the old
micas.  While they usually carry a voltage rating, they normally do not
carry a current rating.  I never worried about this until just after Hal
mentioned the issue to me--and soon after I popped my first mica.  I believe
Hal told me that you are often pressing your luck when get up to around two
amps rms through one of the typical transmitting micas you find at a
hamfest.  But as Hal says, at the price you pay why worry?  Actually he
recommended paralleling a few chosen to give the correct cap value if you
get into the 2 amp total current region.  That cured the problem in my case.
I have lived too close to the edge with several matching networks since then
and have always cured the problem through paralleling.  Interestingly
enough, none of the "popped" caps were either open or shorted, but whatever
had happened inside had whacked off maybe 25% of their cap value.

I have found some small plastic electrical boxes at Home Depot are excellent
for housing such matching units.  I can't recall the brand, but they have
seals which keep water out, and if you drill a weep hole in the bottom, you
can use them in exposed areas with no issue. The boxes come in small to
large sizes, and I have often gotten away on 160m with using an inductor
made from #12 or #14 THN wire wound on a short length of PVC.  The losses
aren't bad on 160m if you want to be quick and dirty.

Hal has an environmental test going.  He has quite of few micas used for
legal-limit matching in use at his place and has left them exposed to the
elements.  I believe he is going on four years now with no failures.

After having our local antenna guru, K4DLI, nudge me and K4UEE for years to
go from an L to a T on 160m, we both made the move a little while back.  The
L's have always served me well, but the T seems to have  an edge on it--and
K4UEE feels the same.

73
Bob W2WG

-----Original Message-----
From: topband-bounces at contesting.com [mailto:topband-bounces at contesting.com]
On Behalf Of Herb Schoenbohm
Sent: Friday, July 24, 2009 8:52 AM
To: K4SAV
Cc: topband at contesting.com
Subject: Re: Topband: T - vertical improvement



K4SAV wrote:
> WX5G wrote: At 1.5 kW the 370 pF cap will see 10 kV RMS. You are looking 
> at a vacuum variable capacitor.
>
> Mike wrote: Great discussion, is there a good source of vacuum variables 
> (at reasonable prices)
> for this application.  Everything I've found has been in the $300+ 
> dollar range?
>
>   

There is a lower cost approach to tune out inductive reactance then the 
more expensive vacuum variable especially in the 15KV range. Also bread 
slicers can be a problem with corrosion, pitting, and breakdown,  over 
time when mounted outside even in good enclosures.  What I recommend is 
an old broadcast ATU trick of making a fixed mica capacitor variable.  
Heres the trick..... if you need 370 pf as in the example quoted, put a 
fixed mica G2 of a higher value, lets say 500 pf.  then add an inductor 
in series.  (Preferably a silver pated flat wound) Move the taps until 
the combination gives the inductive reactance (you want to tune out) of 
the antenna equal to the C plus L that you have placed in series. 
Compared to what the originally cost AeroVox mica high current 
capacitors are very low cost on eBay.  Also if you want to start with 
500pf and can't find that value you can always buy two 1000pf units and 
bolt them in series for half the total capacitance..  In the later case 
two 5000 volt units will give you 10,000 volt rating. 


Good luck

Herb Schoenbohm, KV4FZ
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