Oct 15, 20224 min read

40M center loaded vertical antenna

Inspired by Peter VK3YE who recently created the Wadetenna, a 5 metre center loaded vertical for 40m, I built a similar antenna combined with a 4 wire ground plane. Thanks Peter for the inspiration. Not only will the antenna serve me well at Pigeon House, it will make a good low-profile antenna option for Mt Kosciuszko.

Antenna parts:

light duty 5 metre squid pole (ebay)
10.2 metres of light duty wire. Jaycar wire WH3013 13 x 0.12 on a 100 metre roll.
plastic pipe 43 mm dia, 110 mm long
1 small alligator clip
5 banana plugs
5 banana sockets
1 SO239 socket or BNC socket (your preference)
4 ground plane elements each 6.8 metres long non resonant length. I used heavier wire on hand in the shack.

Draft Construction Notes:

Cut a length of wire to 10.2 metres. The antenna element is 10.1 metres, you will use 100 mm for the loading coil taps.
From one end measure 2.5 metres and mark the plastic insulation with a permanent marker. This will be the section below the loading coil
From the other end measure 2.5 metres and mark the insulation. This will be the top section above the loading coil. The wire between the markings will be the coil.
Drill a small hole in the plastic pipe 25 mm from one end. The hole diameter should match the wire insulation diameter.
Feed the bottom section through the hole to the permanent maker point.
Start winding the wire, at four turns strip back the insulation, twist the copper wire and solder
Wind another four turns and repeat the process to strip the insulation and solder.
Repeat this process until the top section permanent marker is on the plastic tube. For a 43 mm plastic pipe the number of turns is 35.5. The loading coil will contain 5.1 metres of wire.
Where the last turn meets the plastic pipe drill a hole and feed the wire through the tube.
Below the first turn and above the last turn drill a second hole and feed the bottom and top sections of wire out through the tube. (see picture of the loading coil the wire is now on the outside of the plastic wall)
Make a jumper lead.
Cut the bottom section to expose the copper wire, solder the jumper lead in place. You can place the jumper lead at the bottom or top of the coil to suit your preference.
With the coil in place you should have 2.5 metres of wire below and above the coil.
Drill another pair of holes in the plastic pipe to add strain relief for the wire and the jumper solder joint. (see picture)
Make a common ground plate (I used perspex and four banana sockets. The sockets are coupled on the reverse side using copper wire.)
Install a coax connector (your choice) and the last banana socket. On the reverse side join the center pin to the banana socket.
Solder one banana plug to the bottom section of the antenna.
Cut four lengths of wire each 6.8 metres long (non resonant) and solder a banana plug to one end.

Putting it all together:

Feed the loading coil over the squid pole and tape the wire to the top of the pole. The squid pole is very flexible at the top two sections. Add additional tape to keep the wire against the pole. This will prevent the pole from excessive bending.

Connect the antenna banana plug to the base plate socket. Plug the ground plane elements to the common ground banana sockets.

Adjust the jumper lead tap position to obtain a low SWR on 40m. With 27 turns in circuit the SWR is 1.5:1. For multi-band operation select a different tap position or bypass the loading coil and use a QRP tuner, a variable L or Z match at the antenna base.

For operation on 20m bypass the loading coil and adjust the length by folding back the top section. Don’t cut the wire.

How did the antenna perform on air from my front garden using a FT817ND at 5 watts output?

Andrew VK1MBE on SOTA summit VK1/AC-043, 7 km west on a ground wave, signal report was S58 R59.
Matt VK1MA, 15 km to my north on a ground wave reported my signal as 5-4 to 5-5. There are two ridge lines between our stations.
Andrew VK2UH, 65 km north-west first on a ground wave reported 5-5 then later in the QSO the signal improved to 5-9. Happy so far, what would a distant SOTA station show?
Andrew VK3BQ on SOTA summit VK3/VC-002 Mt Donna Buang, 400 km south-west, signal report was S58 R56. Very happy with 5-6 considering antenna polarisation differences
Mark VK3ASC on SOTA summit VK3/VE-129 Mt Lawson, 165 km south-west, signal report was S56 R55.

So overall a good result for a short antenna. Clearly it doesn’t have the same performance as a half wave dipole. I am comfortable it will perform well from Pigeon House Mountain. Four chaser contacts is all I need.