15 m Ground Plane details

15 m Portable Ground Plane

When working portable sometimes a ground plane will work better than a dipole or inverted vee due to a different radiation angle and pattern.
The details here are how I built a 15 m ground plane designed for temporary use hanging from a tree.

The hauling rope must be at least 6.8 metres above the ground. Of course - the higher the better.
I suggest the hauling rope be tied as an endless loop so the rope can be raised and lowered without using the antenna to pull the rope down. This can be very useful if the radiator become detached or breaks in the wind.

Construction

Cut four wires 3.8m long.
Strip about 30 mm off one wire and then solder it to the centre of the SO239 socket. This wire is the radiator.
Next push the remaining 3 wires through 3 holes in the socket, from the threaded side, and loosely tie a small knot.
Strip each wire and crimp an eye lug on the end.
Attach the three lugs to the socket using a nut and bolt through the fourth hole. These three wires are the radials.
Ensure the bolt head is on the threaded side of the socket.


SO239 socket		SO239 socket with wires		Sheet bend

Using a sheet bend tie, the radiator to the hauling rope.
Raise the antenna until the socket is at a convenient working height.
Measure and note the length of your coaxial cable.
Attach the coax to the socket and raise the antenna until the ends of the radials are at a convenient working height.
Attach a rope to end of each radial, again using a sheet bend.
Raise the antenna to its final height and measure how much coax cable is still on the ground.
Subtract the amount of coax on the ground from the total coax length. Note this amount as it is how far out from directly under the antenna each radial rope needs to be pulled before pegging to the ground.
Peg each radial rope out with 120 degrees between each rope. The peg is placed at the distance from under the antenna calculated earlier.
Measure the SWR on 21.200 MHz. If the SWR is higher than 2:1, lower the antenna and trim about 150 mm off the radiator. If the SWR is between 1.5:1 and 2:1 trim about 75 mm off the radiator. If the SWR is between 1.2:1 and 1.5:1 trim 30 mm of the radiator.
Raise the antenna again and check the SWR again.
Repeat these trimming steps until an SWR of 1.2:1 or better is achieved.

The ground plane antenna, as described, should work well as a temporary portable antenna. The angle of the radials is not too critical but does affect the SWR slightly. The height above ground can also change the SWR but, if correctly tuned when first used, the variation of SWR will be minor.

20 m Portable Ground Plane

The same design can be used for 20m starting with 5.1m wires and a minimum hanging height of 8.8 metres.