C1 has capacity 510- pF. I used a mica
capacitor. Inductor L1 has inductance 0.21- uH.
It contains 5.5- turns of bare strand copper wire (18- AWG) air
coiled on a form with diameter 12-mm. Gap between turns is 1-
mm. ATU is tuned by stretching – squeezing of the inductor. Antenna
has SWR 1.0: 1.0 at 14.100- Khz. At the 14.000- kHz SWR was 1.2:1.0. At 14.450- kHz
SWR was 2.0:1.0.
ATU made inside of a clear plastic box (Food Box). With the antenna
the ATU (item 9)
is connected with help of M-M adaptor (item 7). Coaxial cable (item 11) is connected to the ATU.
Copper strip (item
10) was attached to the RF- connector of the coaxial
cable with help of a clamp. The strip was connected to the antenna
grounding system. Antenna is fed through buried coaxial cable (BFLEX,
DAVIS- RF is supplier). The coaxial cable then is going through
a ventilation hole in to the basement where my shack is placed.
Antenna works fine at the 20- meter Band. However,
through some time I decided use to the antenna for other amateur
At first the mostly effective solution was very clear
for me. I need to install ATU made inside a big clear box with
switch that could be match antenna at the used bands. The ATU
circuits for should be similar to the 20- meter ATU. I have measured
the antenna impedance at the other bands. New parts for other
bands were calculated. Old ATU was removed for the modification
and the antenna was fed directly by the coaxial cable. Temporary
fed as I thought… But other deals distracted me from doing of
the universal ATU… Then autumn and winter is coming…
So, the universal ATU still does not made. However,
I discovered that the antenna works good at the 10-, 15-, 20-,
30-, 40- meter Bands with help just ATU between my transceiver
and the coaxial cable going to the antenna. My MFJ VERSA TUNER
II could match the antenna on the bands. Another one tuner, LDG
Z- 11 PRO II also could match the antenna on the bands. Be truth
my door on to backyard is frozen at the cold winter. So I could
not go outside most of the cold days (as I planned to switch the
bands). So, the direct feeding is preferable for me at the situation.
But may be at the coming summer I will do the universal tuner…
73! de va3znw
2 Schematic (A) and
Design (B) of the Helical Antenna ATU
Helical Antenna for the 20- meter