The Crawley Amateur Radio Club (CARC) was formed over 40 years ago and moved into its present premises Hut 18 Tilgate Forest Recreational Centre, in the early 90’s. We are an affiliated club of the RSGB. Membership, currently standing at around 55, ranges from a member in their twenties to several octogenarians. The monthly talks and presentations cater for a diversity of technical backgrounds from absolute beginners to well-qualified professional engineers. Members of note include a past President of the RSGB the late John Graham G3TR, and the late Ron Vaughan G3FRV (now VK6RV), General Manager of the RSGB in the late 60’s. Current members include Stewart Bryant, G3YSX (past President of the RSGB, and currently RSGB Chair), Caspar Pierce (Winner of the RSGB Kenwood trophy), and Eugene Sully G0VIO of Big Brother fame who does much to actively promote interest in radio communications and the Sciences in general. Facilities at the spacious Clubhouse include: A contest-grade radio station...
For years inexpensive Digital Volt Meters (DVM) have been available to the amateur radio world. Professional ones are also available, but their prices are sometimes prohibitive to the amateur. In the professional world, four lead ohm-meters are used for measuring resistance. In a basic measuring set up with just two leads, the result is compromised by the resistance of the leads connecting the device under test (DUT), i.e. the resistor in this case to the DVM. It is particularly troublesome when trying to characterise low value resistors. If the leads have say 0.05Ω total resistance, and the DUT is a 0.1Ω resistor, the measurement will be wrong by 50%, as the DVM will show 0.15Ω instead of 1Ω. The way around this is easy to achieve with modern op-amps, and is achieved by using 4-wire Kelvin leads, some circuitry, and a DVM. The circuitry will need to provide a constant known current through the DUT from say a Jfet operating in constant current mode. This known current is connected such...