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Cantenna Jr. by W4JBM
This is just to show that not all projects have to be complicated to be fun.
And after all, having fun is what the hobby is all about... The
beginning...
I've always wanted a Heathkit Cantenna. In my early teens, I would thumb
through the Heathkit catalog thinking about just what I'd order for my perfect
station if money wasn't an object. And, of course, the Cantenna was always a key
part of the test equipment I'd need in my dream station...
Food for thought...
It all started with getting an Altoids tin from my wife for the FOXX-3. Then
I asked her for the can I used to house the Tuna Tin 2. A while back she was at
the store and found a cute little can of cinnamon that she brought home and
asked, "Can't you build some kind of radio thing in this?"
I wasn't sure what to do with it until I was in the middle of tuning up my
Ten-Tec 1340. I've got a small dummy load, but I wanted something that could
more easily handle 5 watts for minutes at a time (without breaking out the 200
watt dummy load) and with a built voltage divider to feed the o'scope.
So, what about a QRP Cantenna?
Well, I'm now the proud owner of the first Cantenna Jr.! It's fed with an RCA
jack on the top. There is another jack on the back that has the voltage divider
output. The guts are built on a small piece of circuit board. I used a Dremel
tool to seperate into islands.
I used four 10 ohm, 1 watt resistors and four 10 ohm, 1/2 watt resistors. The
half watt resistors were used in pairs to give me the 5 ohms I needed to get the
divide-by-ten. Below is the guts before I attached it to the bottom of the
enclosure using super glue.
The label was a result of my increasing experience in packaging my projects.
It was designed in a computer drafting program (Visio), printed on a laser
printer, glued to the enclosure, and sprayed with a coat of Rust-Oleum Crystal
Clear Enamel.
I learned two lessons in the process. First, I'd put one coat of clear enamel
on prior to gluing the label on if I was doing it over again. It's very easy to
get a bit of glue on the front of the label when you're gluing it on and that
part of the label is a slightly different color than the other parts. Secondly,
make sure you use the minimum amount of glue. If it starts bleeding around the
edges, you've got too much. On my Tuna Tin 2 I felt like I didn't use enough
because you can see when the paper is glued to the tin and where it isn't. The
best solution isn't to use more glue, but instead to use a slightly heavier
cardstock (instead of regular paper) for the label.
Why do you call it a dummy antenna instead of a dummy load?
As several radio books will point out, while it's a dummy antenna it is in
fact a real load. To be honest, I'm as likely to call it one as the other. But I
did want to be as accurate as I could here, so dummy antenna it is..
Why the voltage divider?
My oscilloscope goes up to 5 volts per division and has 8 divisions
vertically. That gives me 40 volts peak-to-peak. At 50 ohms, this would be a
power of 4 watts. I wanted to be able to use the o'scope to make measurements
and look at waveforms for signals in the 5-10 watt range, so I needed some type
of external divider. The Cantenna Jr. is easier (for me) than using a voltage
divider probe.
The Cantenna Jr. project
contained on this page has been reprinted here with permission and is
Copyright © 2001 by Jim McClanahan, W4JBM. All rights reserved.
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