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[TowerTalk] Top Hat Switch (mercury)

To: <>
Subject: [TowerTalk] Top Hat Switch (mercury)
From: (
Date: Thu, 19 Jul 2001 15:34:30 EDT
In a message dated 7/18/01 7:55:03 PM Pacific Daylight Time, 

 Many people have responded to my previous post.  All responses were quite
 valid and appreciated--thank you.  The consensus was to simply cut it for 75
 meters and base load it with a variable inductor arrangement (or variations
 on that theme).
 My quest for a top hat switch continues.  Why?  Because the thing models
 like it will play great!
 I'm building an 80/40 open sleeve vertical.  It will tune all of 40 at < 1.6
 or so.  It will tune all of 80 / 75 if I can get this hat on and off the
 tower.  The base loading is the most commonly used and possibly the easiest
 to fabricate.  I live in Minnesota and the base will be 5' or 6' under the
 snow in just 6 months.  Making a tunable roller inductor arrangement that
 will work in this environment is no simple task either...
 How about this option....
 Mount a mercury tip switch at the top of the tower in a box on a cam-like
 rotary arrangement (still designing that) and activate a latching
 arrangement using some 1/8" Stainless cable running up the middle of the
 pipe from the base of the tower.  I have these little 12 v solinoids that
 will pull several pounds about 1/2".  These are details I will work out in
 the shop.  My question is, have I simply created a very complicated
 mercury vapor lamp?
 The mercury switch is a vacuum.  The leads are about 1/4" apart as they
 escape the envelope.  At 21 volts per thousandth of an inch (handbook #), a
 1/4" gap should handle many KV.  (I only run 1KW max here at this time ~~
 3KV).  I know nothing about the conductivity of mercury.  For all I know it
 will explode in this environment!  A buddy of mine said "Hook it up and
 watch it.  If it looks like a strobe light mounted on your tower, you better
 shut it down."
 I think I can master the mechanical stuff.  What about the switch?  Anybody
 venture a guess as to how it will behave to RF as a top hat switch on my 1/4
 wave monopole?

Ford:  A rotary coil is a simple task and can be protected from 6' of snow in 
a wooden box or now days a plastic garbage pails etc.  I've been there and 
done that with both--in the snow.  The technique I described previously 
improves the match also over the whole band.  Verticals are normally a 36 ohm 
Z at best.  In the year 2001 why not step up your design and go for a better 

Any switch at the top of the vertical can go bad and ice loading can inhibit 
any movement or rain get into it unless properly protected.  You either have 
to lower the tower or climb to repair it.  One antenna is not too hard to 
maintain in normal weather.  When you get several antennas you tend to look 
for "fool proof" designs.

There is another "fool proof" way using an open wire 1/4 wave stub adjacent 
to the tower or even inside it.  Use it as the switch to add a top load.  A 
top load has to be electrically insulated and properly mechanically supported 
at the top which is another design problem.  For simplicity use a tower 
resonant at 3.9 MHz and one leg of a guy wire about 10' long to an insulator 
as a "Slanting Load" instead of a Top Hat with more wind load.  Have the 1/4 
wave stub open at the bottom to reflect a short at the top.  Have a knife 
switch or wide spaced relay across it at the bottom to short it out and 
reflect a "Hi-Z Open" at the top.  All the moving parts are at the bottom.  
Since one leg is connected to the top of the tower, the tower or pole can be 
one leg of the 1/4 wave stub.  Just run a single wire adjacent to the tower 
spaced about 6" with good spacers like Delrin or the old stand by porcelain.  
You could vary the resonant frequency on SSB by adding "certain values of Xc 
or XL" in series with the short.  The 1/4 wave stub will invert the reactance 
at the other end.  Strive for reliability and longevity to increase operating 
time in snow or cold country--even with just one antenna.   

Best of All: Resonating the vertical at 3.6 MHz full length or permanently 
connected "Top or Slanting load" and using a series BC 3 gang variable in 
series driven by a selsyn or DC motor in a plastic pail (under the snow) will 
resonate the vertical by tuning out the inductive reactance all the way to 4 
MHz with a "progressively lower SWR"--the Rr (Radiation resistance) goes up 
and the Rloss stays the same.  (That's a mouthful)  Unfortunately increasing 
the Rr is never addressed in literature.  Resonating it at 3.6 MHz and using 
this technique (and other variations) is a concept that some critics just 
can't understand unless perhaps even if they actually tried it--which they 
haven't.  There is some special "RF Matching Magic" there ideal for 36 ohm 
verticals or even yagi's based on the most elementary of Basic Fundamentals 
101 that increases efficiency.  If someone needs a further explanation I can 
do so.  Full data and new concepts are being prepared to be published 
elsewhere. K7GCO

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