VK9GMW_ANTENNA.pdf

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.

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.

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.

ATU

Guy

Rope

GND

Relay

COAX

GND

Wire lengths in meters

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

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