AM25
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An easy way to describe what happens when you “load” the antenna is that
we electrically increase the “length” of the antenna without increasing the
mechanical length. Our goal is to approximate a functional portion of the full
wavelength (i.e.
λ
/4 or
λ
/8)
Using an 8 foot length of wire, or an old 102” CB radio whip as the antenna, a
series inductance of about 350 µH is required to properly load the circuit. We
can use either a fixed inductor or wind our own loading coil, which is better!
Ideally, we want a high-Q coil - which can be fabricated very easily using a 5
inch long piece of 1 1/2” diameter PVC pipe. Wind 120 turns of wire neatly
spaced on the pipe. Drill a small hole in the pipe at each end and thread the
wire through it a few times to hold it securely. Connect to the center pin of J1,
the RCA output jack and connect the RCA connector shell to a good ground
for maximum transmission range. The 8 foot length of wire should be
extended vertically for maximum range.
Here’s a neat antenna idea, the 1 1/2” PVC loading coil will fit nicely into 2”
diameter PVC pipe. Get a 10 foot length, run the long antenna wire up
through the pipe and over the edge a bit, slip a PVC pipe cap over the end to
hold it in place. Slip the loading coil into the other end of the pipe and you’ve
now got a neat waterproof antenna. And remember use a good ground for
best transmission range.
As long as you’re at it , you can try to “roll your own” loading coil using a
common paper tube from your paper towel dispenser.
Start by shellacking or otherwise clear coating a cardboard tube to “stiffen”
the cardboard. Then, using wire from
an old transformer or other type of
“magnet” wire (available at a local
electronics supply shop or Radio
Shack) wind consecutive “loops”
around the tube.
For a frequency of 1000 KHz (or 1
MHz) you’ll need to wind 190 turns of
wire around the tube for the proper
inductance!
Connect an 8 foot whip antenna to one
end of the coil and the other end to the
center pin of the antenna output jack.
Experiment with different designs to
determine which antenna type works best for you.
8 ' WHIP
PAPER TU
TO TRANSMIT
To center pin of
J1, RF Output
1 1/2” PVC
Summary of Contents for AM25
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