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Having recently moved house, I needed to erect an antenna for general shortwave listening at my new QTH. After bad experiences with a very high noise level at my previous address, I decided to try a loop. My old antenna was an end-fed wire operated against mains earth! If you just stick a random length of wire into your receiver's antenna socket, mains earth becomes part of your antenna system. To get a decent ground, you have to bury a lot of metal. A loop is a balanced antenna which works without reference to ground.
The apex of the delta is supported from a chimney at the back of the house. The other two vertices are guyed off the back fence. At the feed-point, mounted on top of an aluminium tube, with a plastic hood to shelter it from the weather, is a simple home-made balun. 75-ohm coax runs down the inside of the aluminium tube, down to ground level, under the lawn, around to the front of the house, and into my front-bedroom shack. Partially burying the feeder helps to screen it.
Built into an aluminium diecast box, fitted with 4mm banana jacks for the loop, and a satellite-type coax plug for the feeder, the balun houses a toroidal broadband transformer with a 3:1 turns ratio. An impedance step-up is employed from loop to feeder because the radiation resistance is theoretically quite small. Winding inductances of 180µH on the secondary (coax) and 20µH on the primary (loop) were arbitrarily chosen. No attempt was made to achieve an exact impedance match at any frequency.
The coax socket is surrounded by a flanged collar which fits tightly inside the aluminium mast. The hood (and gravity) keep the 4mm banana connectors dry. Liquid water cannot enter the diecast box; however, there is a risk of condensation, and I may drill ventilation holes in the lid.
Acting as a weather-shield, and providing strain-relief for the loop, the hood is a plastic flower vase. The bottom inch was filled with PollyfillaTM exterior-use cement, moulded to the shape of the balun. The hood is easily lifted to remove the balun for maintenance and experimentation. Some fiddling was necessary to adjust the seating of wire in the insulators, and balance the tension. It's best not to cut the wire short too soon - unless you like climbing ladders! A few inches of spare wire can always be coiled inside the hood.
The noise floor is -100 dBm or better. The long and medium wavebands are clearly visible, as are the 31, 41, 49 and 75 meter bands.
|Copyright © Andrew Holme, 2006.|