Monday, 24 June 2013

Building an inverted “L” antenna

The other day, quite by accident, I realised that my end fed HF wire antenna had some issues. For a while now I have noticed that the bands appeared to be very quiet. Where once the PSK frequencies had been busy they now appeared dead. It so happened that I was tuned in to the 20m band but needed to change the antenna leads about. As I started to unscrew my HF antenna the band burst into life. It appeared that when the shielded side of the PL259 plug touched the rigs socket, the signal strength vanished. If I positioned the connector so that just the centre of the plug was used, I could hear lots of PSK signals. I experimented by connecting my 2m antenna to the HF socket on the rig and I could still hear everything. I therefore concluded that my wire antenna had developed a fault. I had been thinking about changing this antenna for a few months now so here was the opportunity I needed. 

I have decided to build an inverted “L” with the aim of making it resonate at one frequency. Although I use a tuner, I’d like to get maximum performance on at least one band. My initial approach was to get something working with the view to cutting it to length at a later date.

I needed to drive in a copper earthing point so that was my first job. I used a section of copper water pipe and hammered that into the ground near to the antenna feed point. I didn’t want to damage the top of the copper pipe so I used a jubilee clip to give some strength and support and then fitted a wide headed bolt to the top of the tube. I rolled some paper around the thread of the bolt so that it was a tight fit. This hammered into the ground quite easily as this area of ground never gets the sun so is always damp. I then connected the shield of the coax to the earth and the centre to my antenna wire via a terminal block. The “L” shape of the wire has a slightly longer horizontal length over the vertical length. For now this is fine until the second stage which will be to tune the antenna. I plan to screw a box to the wall which will consist of a copper back plate connected directly to my earth rod. I will then attach some sockets to this to enable me to easily connect my inverted “L” and any other experimental antennas. For now I have water proofed the earth and antenna connection as shown in the picture below.

Back at the transmitter, I can once again hear the signals I would expect to hear. At the moment the vertical part of the “L” is next to my house which is far from ideal. If this antenna proves itself I will look to feed it from the other end, reducing possible interference.

The earth rod prepared for being hit with a hammer

Temporary connections made waterproof
A view skywards following the vertical path of the “L”

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