RadCom Basics May 2022, Edition 28

RadCom Basics 19 the inside of the shield I2 are equal and opposite, that is, 180 degrees out of phase. So what can happen? The result can be interference or RFI problems, such as USB ports on your shack computer discon- necting,TV interference or the ra- dio not functioning properly. In ex- treme cases you can get RF burns off the microphone or Morse key. A classic I can remember from 45 years ago while using 100W was breakthrough on other equipment in the shack – a kind of intense hum sometimes after the volume control! These are extreme examples,but they do occur – particularly with higher powers. Common mode currents [3] can also cause problems with your antenna’s radiation, distorting the beam pattern of aYagi for example. Or even altering the apparent SWR of the antenna. As for receive, sig- nals picked up on the coax from switched mode power supplies and other devices can be carried up the outer of the coax to the antenna before being conducted down to your receiver. The result can be interference and noise on receive. What we want is the radiation from both elements cancelling at the feeder because they are equal and opposite. A solution – the balun Let’s start by using a balun at the feed point of the antenna. This will help “choke off ” any common mode currents and may even im- prove reception. You can either use a commercial balun, prices seem to start around £25 (at time of writing) up to a lot of money for high spec and perfor- mance. Or the other option could be to construct one like the simple coiled balun made with a number of turns of the coax feeding the an- tenna. Have a look an extract from Table 1: Coiled coax balun table for HF - extract