OBSERVATIONS    GM1SXX          22 October 2010

A simple and easy effective field or home aerial.

Some time ago... a couple of years I think, I was asked to come up with a simple transmitting aerial for field / gala use. In a gala situation, you have a lot of people around, ones who don't understand about radio aerials and RF fields and safety matters, so I sought to put together a simple transmitting aerial that would work well on 14 and 18Mhz where we mostly like to work. Of great importance in a gala situation is the business of radials. For safety reasons (trip hazard) you pretty much can't have any. That rather limits one's choices.

My solution was an old one, and one that has been rehashed many times over and 'reinvented' by more people than I have spokes on both of my bicycle wheels!  I'm talking about the end-fed wire.  I built a simple 9:1 ratio UN-UN and used it to feed a 65 foot length of wire arranged as an inverted L.   For a ground, I used nothing more than a length of steel rod hammered into the ground.  Bigger is obviously better here, but even a 1M length will work.

Our club is lucky enough to have a rather nice 40 foot fibreglass mast, which I used to support the vertical part of the wire, with 25 feet running horizontally over to a convenient nearby tree.  We have worked many stations using this very simple aerial system at galas and the organisers like and appreciate the lack of radials. We usually cordon off a small area of ground behind the operating tent to provide a safe area around our petrol generator and aerial.  It definitely helps to use a second ground stake to properly ground the generator and we use clip-on ferrites on the power cable to our operating tent.

Details of the UN-UN we use. This can be wound from any suitable thin wire on a T130-2 or T200-2 powdered Iron core.   The type 2 material is red and grey.   SPRATREADER on EBay is a source of these cores and they are probably also available elsewhere. Just Google T130-2    A T200-2 core can also be used and will handle more power.  If you use a T200-2 instead, add 2 or 3 more turns of wire.   Take three lengths of wire, mark the individual cores at each end so you can identify wires A,B & C (I use splodges of old nail polish). Twist the wires together into a single bundle and wind 9 turns of the wire onto the core.  Each time the wire passes through the centre of the core counts as ONE TURN.  Mount the core in a suitable small plastic box. I fitted  my UN-UN box with an SO239 connector for the co-ax and a pair of red and black connectors (from maplin) for the aerial and ground connections.   At this point, test the aerial and UN-UN. 

This aerial doesn't need much of a ground so is suitable for portable use. We normally carry a hammer, a pair of large Mole grips  and a 1M steel spike to use as a ground. The Mole grips are needed to pull the spike back out!  If you plan to leave the UN-UN outdoors as a permanent installation, it should be made waterproof. I usually just melt down a cheap wax candle and use the wax as a potting compound. It's a LOT cheaper than Epoxy potting compound but take care with it.  Hot wax is *very* nasty stuff! Once the potting compound is set, you are ready to go.

This is how we usually lay out our field radio system.   Not shown are the ground stakes. We use a radio tent with folding tables and chairs and directly behind we have a cordoned-off area with the aerial mast mounted centrally and the generator off to one side.  The generator is grounded by a steel spike driven into the Earth and the aerial also has it's own ground spike. The UN-UN in it's plastic housing is sticky-taped to the base of the fibreglass mast and the co-ax, aerial and ground spike are connected to it. We use striped chevron tape to cordon off the area.  The 65 foot aerial wire is just cheap plastic coated stranded hookup wire. Almost any old bit of wire will do.    The same sort of setup will also work well at home if you have a restricted QTH although you'd probably want to place the feedpoint at the far end of your garden rather than close to the house. This should help a bit to avoid local noise sources (in your own home).  I don't use an insulator at the far end. I'm Scottish... remember!  I just tie on some nylon or Kevlar cord.

This aerial design is as old as the hills and offers quite a good performance in a restricted area without the need for a sizeable radial system.  Try it. It may surprise you.   Total cost should be under £10 if you shop carefully.

 73 Al