VK9GMW ANTENNA, KF

The VK9GMW SpiderPole Antenna
A Simple All-band Antenna for DXpeditions
George Wallner
AA7JV
Apr 2009
Introduction
VK9GMW, operating from Mellish Reef from March 28 to April 13,
2009, put good signals into both NA and Europe on all bands,
including 160 meters. With only a modest solid-state amplifier, the
reason for the good signals was the antenna. (It usually is.)
Background
DXpeditions to small islands, or amateurs living on the water-front,
can take advantage of the surrounding salt-water to create vertical
antennas with impressive performance. The salt water provides an
almost perfect ground and allows for very low take-off angels.
The VK9GMW antenna
was specifically designed
to take advantage of the
excellent conductivity of
sea-water. The design
overcomes the problems
caused by the changing
tides (which change the
feed-point impedance)
and it uses light wires that
are easily held by a pair of
SpiderPoles. The antenna
performs well from 160 to
10 meters. The antenna is
easy to erect and does not
require any adjustments.
Dimensions are not critical
either.
There is no reason why this antenna could not be installed on land.
Adding 32, preferably more, approximately 20 meter long radials to
the base of each pole and tying the two bases together, will provide
an acceptable ground and an antenna that will work well on all bands.
It will not have the low angle radiation that the sea-water based
antenna has, but it will work.
2
Feeding the Antenna
The central component of the antenna is a home built, water-proof,
dual output automatic antenna coupler (the coupler has two antenna
ports with an internal ANT select relay). The coupler solves the
problem of the constantly changing feed-point impedance caused by
the changing tides (1.8 meter tides on Mellish). It allows operation on
any band, and it provides total freedom of antenna geometry. The
same function, or course, could be fulfilled by any weather resistant
commercial automatic antenna coupler and an external relay.
In amateur practice, antenna geometry is often dictated by the desire
for a given feed-point impedance, such as 50 ohms. A lot of sacrifices
are made to obtain this impedance. The use of coupling devices
(such as antenna tuners or couplers) is seen as undesirable because
of losses and complexity (and cost). I believe that for a portable multi-
band antenna the situation is the opposite: a low loss antenna
coupler makes things simple and easy! Simple and easy are good
things on a DXpedition.
Indeed, there is no reason why the above concept could not be
applied to permanent antennas: a low loss, automatic or remotely
controlled antenna coupler will give the user a lot of freedom and will
match the antenna to the coax on all frequencies. The key is low loss!
Especially with short vertical antennas, where the radiation resistance
is low, coupler losses can become a significant percentage of the
overall loss.
Antenna Configuration
The antenna consists of two arrays of wires held aloft by the two 18
meter SpiderPoles. One array, which consists of three wires, is used
on the 160 to 30 meter bands. The other array, which is used on 20
to 10 meters, also uses three wires, which form a triangle.
When standing in salt-water, the antenna does not require radials.
Grounding is directly into the salt-water via the large metal base of
the antenna. Add radials if the antenna is erected on the beach.
3
The following diagram shows the antenna wires and the dual output
antenna coupler unit (ATU). Note that there is nothing sacred about
the wire lengths shown. The vertical wires are 18 meters long
because the SpiderPoles are 18 meters tall. The length of the
horizontal wire connecting the two 18 meter long vertical wires should
be around 12 meters, but anything between 11 to 15 meters will work
almost as well. The dimensions of the triangle forming the high band
radiator are again not important, they are dictated by the desire to
have wires that are around 0.3 wavelengths long on 20 and 15
meters (6 and 4 meter long wires, respectively). The third wire (the 2
meter long wire) is there to provide some capacitive loading to keep
voltages both at the tuner’s output and the wire-ends down. The
angle of the wires is dictated by the desire to keep them away from
the 18 meter vertical wire and the angle of the guy rope, which holds
these wires aloft.
18
12
18
2
6
4
ATU
Guy
Rope
GND
Relay
COAX
GND
GND
Wire lengths in meters
4
The low band wires are derived from an inverted L. For 160 meters a
full size inverted L would consist of an 18 meter vertical wire and an
approximately 24 meter long horizontal wire. The problem with this
configuration is that on 80 meters it will have very high angles of
radiation. This is avoided by adding a second vertical wire (bending
“down” the horizontal wire) – and adding between the lower end of
the “down” wire and the ground a remotely controlled grounding relay.
On 160 meters the relay is open. There will be very little current
flowing in the “down” wire and it will have little effect on the overall
radiation pattern of the antenna. On 80 meters the relay is energized
and it grounds the lower end of the “down” wire. This will result in
greatly reduced high angle radiation. (See plots on Page 7.)
Antenna currents at 3.5 MHz with GND relay closed
5
 

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