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September 2010 I
started working on what I
call my black box vertical antenna. In the airline industry they use
the term (Black Box). A data recording device stored in the tail of
a aircraft to help determine the cause a plane crash.
I built this antenna specially to be used on the 30 meter
band. The coil used is big enough to cover the 40 meter
band as well. The coil is a bit over kill to just run 200 watts
through it, but it was given to me. This coil came from a broadcast
transmitter.
The aluminum used to make this antenna came from another friend that was
scraping his Titan Gap vertical antenna. The antenna is 23 feet long
and
mounted on fence piping about 6 feet long with 3 foot of it in
the ground.
The ground system has about 20 ground radial and is also link to a
chain link fence.
With just 23 feet of aluminum I was able to tune the 30 meter
band with the antenna tuner in my FT-950. The idea was not to use a tuners from within the
shack. So I started working on building a T-Matching Network from
this big broadcasting coil and a single variable capacitor I
had on hand. With a few jumpers and my MFJ 259B antenna analyzer I started doing a little testing. I was able to find the tap for 30 meters
very fast. Since I had this big coil I decided to look for a 40
meter tap. That also was easy to find.
Here's where the (Black Box) comes into play. I needed away to house
this T-Matching Network and protect it from the weather. So a box
was built from some scrap plywood I had around the house.
Using some nuts and bolts and angel iron I mounted the box to the
pipe that supports the antenna mast.
The coil was mounted in the box along with the variable capacitor
and cut up some coax cable, the T-Matching Network was complete.
There's just one big draw back to this whole project, and that is if
I want to change bands, I would have to go outside to the antenna
and change the taps on the coil and re-adjust the tuning capacitor.
I'm working on another way to change bands without going
out to the antenna. The idea is to use some relays and run some
CAT5 cable to the box to control the relay switching. Which means I will need to run a control line under ground
and back into the shack.
This project will continue on later....
Below are a few picture of this project: Click on any image for a larger
view.
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