|Vertical Antennas for 136 kHz
This page describes the simple vertical antennas used at GW4ALG.
This page starts by summarising my attempts to tune up a pole mounted on the back
wall of our house, through to the use of a 12 m 'experimental' vertical (i.e. not a
permanent structure) located away from the house, and the occasional use of
balloon-supported extensions to the basic 12 m antenna.
Prototype Vertical Antenna
The first vertical antenna at GW4ALG was installed on the gable-end wall at the back of
our (brick-built) house.
A lower loading coil was wound on a plastic bucket and wall-mounted about 3 m high.
The coil, included a 25-turn link winding for connection, via 50 ohm coax, to the
TX/RX. A wire from the top of the lower coil went up the side of the house, and
through a hole in the wall into the loft [attic] to a variometer (using another bucket for
the outer coil, and a short length of plastic pipe for the rotatable inner coil).
The 'hot' side of the variometer was then connected to a good many turns wound on a
plastic laundry basket, and a wire then went back through the hole in the wall, and up to
a 1.2 mH loading coil mounted on a short length of 2" x 2" hardwood. The top of
the 1.2 mH loading coil was then connected to 5 m of aluminium pole mounted on the
Two outrigger wires (spaced about 0.5 m from the aluminium pole), and a small capacity
hat, attempted to increase the top capacitance, but there were no top hat wires (because
there were no suitable supports). The 'cold' end of the lower coil was connected to
the outside water tap. Later, short earth spikes were installed at three corners of
the back garden, and also connected to the water tap.
Successes with prototype
On receive, it was an excellent omni-directional antenna. On transmit, using 40 W
RF, the vertical was used to make the first 2-way GM-GW QSO (to Dave, GM3YXM/P) over a
distance of 395 km.
1) The major problem with the prototype vertical was achieving enough insulation - there's
a lot of volts up there - even when running just 15 W of RF 'barefoot' from the
transverter. So I had to grapple with compromises of mechanical stability
(supporting/guying the 5 m pole) and achieving the required electrical insulation.
Car ignition HT lead (copper conductor) would break down at 100 W RF, causing a fantastic
light display between itself, and where it passed through the brickwork!
2) The proximity of the antenna to the house, and the resulting shunt path to ground,
meant that the RF losses were high. On wet days, the vertical would be detuned, and
the increased losses caused by the shunt path became more significant.
3) The number of turns on the link coupling was difficult to get right. Also, about 30 nF
of series capacitance was required to tune out the reactance of the link winding.
The whole system was time-consuming to adjust - both to maintain resonance, and to obtain
a good match. Even coping with hour-to-hour, and day-to-day variation in the
resonant frequency was difficult due to poor access to the loft space (where the
variometer had been installed).
4) The addition of the three earth spikes made no measureable improvement in antenna
performance, relative to the water tap connection only.
Experimental Vertical Antenna
The general arrangement of the basic 12 m vertical antenna currently used at GW4ALG is
shown below. Unlike the prototype, this antenna is located away from the house,
thereby reducing the dielectric losses caused by proximity to nearby 'earthy'
Also shown is the method used for extending the vertical with a balloon-supported wire:
more about that later . . . .