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”

Building a new matching transformer

Having a recent antenna failure I decided to rebuild the matching transformer that feeds my end fed wire. This design comes from the “Emergency Amateur Radio Club” based in Honolulu (http://www.earchi.org). This project creates a trifilar wound 9:1 UNUN (unbalanced to unbalanced) toroid matching transformer. This will match the high input impedance of an end fed wire into a range that most antenna tuners can handle. The designer says it should handle 100 watts of power.

The key parts are: 

• A small plastic box (something that can be made waterproof)
• A powdered iron toroid (EG T106-2)
• Three 20 inch pieces of 22 gauge solid insulated copper wire (in three different colours if possible)
• An RF panel mounted connector (BNC or S0-239)
• You will also need the relevant screws, nuts and solder tags etc, to fit the package together.

I first drilled the box and fitted the connectors to ensure that everything would fit correctly. 

 

Next job was to wind the toroid. As I didn’t have access to different coloured copper wire, I instead fitted coloured sleeves to the ends of each cut length.

 

 

It is important to keep the wires in the correct order and not cross them as they are wound. They should be appear in the following order (left to right), Green, Black and Red. Wrap these round the toroid 9 times as shown in the picture below. I found that the first few windings were a little loose so I added a couple at the end and unwound the loose ones from the start. That way I still had 9 turns but a much better spread and even tightness. After winding, ensure that the wires on the last turn are in the correct order. If they aren’t then they must have twisted at some point.

 

It is important to get the next sections correct else you will have to start again with new wire. Holding the toroid as in the above picture, solder the “Left Black” wire to the “Right Red” wire. I found the best way was to first scrape off the insulating coating from both wires where they would touch. Then twist both wires together with a single turn. This will pull both together and make a sound mechanical joint. Solder the two wires and remove the trailing ends. 

Next you need twist the “Left Green” wire and “Right Black” wire together. First clean off the insulation before twisting together. This joint needs to be attached to the centre connection of the RF connector fitted to the box. Before cutting off the excess wire. Place the toroid into the box to see where the cut should be made.

 

The (Left Red) wire should be cleaned and soldered to the connector on the box that the wire antenna will be connected. The remaining (Right Green) wire should be cleaned and soldered to the outer ground connection of the RF connector.

 

Now inspect the solder joints for any dry joints and ensure the box is free of any solder splashes etc. I used a silicon grease spread around the lip of the box and lid before screwing the two together. In the past I have used a silicon sealant, the type used in the bathroom. If you use this type and it has the vinegar type smell, keep this away from and metal as it tends to promote corrosion.

In the next few days I hope to get up the ladder and fit the unit to my mast. I will take additional pictures and post these in the next week or so. Please ask any related question and I’ll try to assist. Further details including a kit of parts are available from the EARC website: http://www.earchi.org