The G0CWT Small Tuned Loop.
I will first describe a 40-meter
loop that can be placed in almost any location; I have one mounted
on the roof of my mobile home. It can be pulled up into position from
inside the vehicle in less than 3 seconds.
I use this loop on 40, 80, and 160.meters.
First,
I will describe the 40-meter version,
The element is one-quarter wavelength
of wire or coax minus about 5% and is approximately 32 foot long.
To
support this loop I use a fiberglass fishing rod for the vertical
support and a six-foot spreader, which is made from fishing rod tops.
The tuning and matching unit is mounted inside the roof of the
van where it is easily acsessable. This removes the need for remote
control tuning it also allows me to make many tests and experiments.
On 80 meters, the loop becomes a 1/8 wave and the input impedance
is now only 2.249 ohm and the transformer turns ratio is approximately
(4.5 to 1) this is 9 turns to 2.
Because of the extremely low input
impedance at the 160 mtr or 1/16, wave loop, the input
impedance is only (0.2278 ohms) the number of turns on the primary
is 20 turns.
Therefore the primary of the transformer consists of
20 turns tapped at three turns and again at nine turns which means
that the input to the transformer has four wires.- 1,= start. 2,=
three turn tap. 3, = nine turn tap. and no4,= twenty turns.
This transformer
will then match the 50 ohms of the transceiver to the input impedance
of the loop at 1/4, 1/8, and 1/16 wave.
This means that you can switch
through 40, 80, and 160, by using a suitable switch to the primary
of the transformer.
As can be seen in the pictures the loop support
is a fishing pole with a cross member at the top. The pole is pivoted
on a four-legged support. There is an extension at the bottom of the
pole that overhangs the pivot, to this is attached a cord that runs
through to the inside of the van where it can be used to pull the
loop into position when required. The loop element (coax or flexible
wire) is attached to the top spreader and the ends bought down to
pass through into the roof of the van and there connected to the Tuning
and matching unit.
