In a message dated 7/18/01 7:55:03 PM Pacific Daylight Time, ford@cmgate.com
writes:
<<
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-N0FP
ford@cmgate.com
>>
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
match?
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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