Antennas

Link Dipole

A link dipole is useful in that by using openable links set at the resonant length for the required frequency you can have an antenna for as many bands as you have links in the antenna all in one length of wire. The antenna is resonant so you do not need a tuner so losses are not introduced through that. This is important when operating portable as you are normally operating at a much lower power than in your home station.

My first antenna I built for portable operation was a link dipole made from 1.5mm electrical wire & it was rather heavy. When one of the SOTA guys offered some lightweight but strong wire I bought  some off him and made a lighter version that would not weigh the squid pole down so much. I used a twin banana plug to BNC adaptor as a centre insulator and a couple of egg insulators with sailing cord that is strong, thin and weather resistant to tie the ends down. For the links for the various bands I used automotive bullet connectors with plastic insulators to take the strain off the connectors.

 

The End Fed Half Wave Antenna ( EFHWA)

I had read a lot about the End Fed Half Wave Antenna and many portable operators use them so I thought I would give it a try as a change from my normal link dipole.

The link dipole works fine but is quite heavy at the centre point where the coax attaches & can cause the squid pole to bend.

To use an EFHWA with a 50 ohm radio requires a matching unit to match the high impedance at the end of a half wave antenna to the radio, so I bought a 40/17m unit from SOTA beams. I cut a piece of my thin wire to the length suggested for 40m in the matching unit instructions, added an insulator or two and soldered on a banana plug to connect to the matching unit. I made a lightweight winder to store/transport the antenna on, from a piece of corflute from a old real estate sign. A 1m length of the wire and another banana plug is used as the counterpoise.

In use I set the EFHWA up as an inverted vee with a tiny insulator slipped over the top of the squidpole that sits about 500mm from the tip. Cord attached to insulators secure the ends to suitable tie down spots (trees, stumps, fenceposts, logs or rocks). Then I plug the wires to the matcher & a short coax from the matcher to the radio. Adjust the pot on the matcher for maximum signal & look for some contacts. I was amazed at the loudness of the signals received when I first tried this antenna. I am not sure if it was the antenna or just excellent conditions, maybe the short length of coax helps as the operating point is virtually right at the end of the wire. A normal dipole has about 9-10 metres of coax running down from the top of the squid pole. Most signals from VK5, VK3 were almost all 5/9, so very pleased with the first trial.

 

20m Delta Loop

I searched the web for another antenna for 14mHz that would be suitable for portable operation, ie light anad utilising the squid pole for support and also a little directional. I decided on a delta loop. The 3 sides are 7.2 meters each and when hooked on the top of the pole the bottom horizontal leg is about 1.5m above the ground. It is fed on one side a quarter wavelength down from the top apex, through a quarter wave 75 ohm coax stub into 50 ohm coax to the radio. Set up temporarily at home I worked a SOTA station (VK2TWR) with 57 reports each way. Compared to the OCF dipole the noise level is slightly less, but that may be because one end of the dipole is 30m closer to the power lines than the loop is.

 

2m Moxon

I read about the moxon antenna & thought that it would make an ideal project to try to build a 2 metre version. The moxon consists of a driven element and a reflector but the ends are bent around in a rectangular shape with the ends a few cm apart. It is directional off the end of the driven element.

Many are built with wire or tubing and while at a hardware store I spotted some aluminium flat bar 3mm x 12mm and my mind began designing the antenna. With wire you need supports to hold it straight, the thin bar would support itself, so with only a centre PVC conduit as support I began measuring, cutting, drilling and building.

It did not take too long and the antenna was ready. I set it up in the backyard at Williamstown & found it could receive the Barossa repeater, Elizabeth and several near Adelaide at Houghton,  Crafers etc and at only about 3m above the ground.

 

2m Slim Jim

I love experimenting with antennas so I soon was looking for something omnidirectional so I did not have to turn the antenna. I found a design for a slim jim that uses a PVC pipe to support the wire antenna with slots in each end to hold the wire. Then the whole thing is encased in a larger diameter PVC pipe to keep out the weather. I just have put it on a mast and I can use it.

10m Moxon

I wanted to try some DX on 10m so I thought a moxon might be fairly small as the ends are folded in. Well!! even with the folded end it is still fairly wide at nearly 4 meters but only 1.4 meters deep. It is still under construction but it will need some support of the ends via  cords run from the mast out to each corner to help support the wide span.

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