Equivalent Circuits for logarithmic and exponential Potentiometers
The task was simple: For volume control I needed a logarithmic potentiometer. These are not hard to find, but as it turned out, not all "logarithmic" potentiometers actually have a logarithmic characteristic curve. Those with genuine logarithmic curves are expensive, cheaper ones only approximate them. To make matters worse, even many of these cheap models are significantly more expensive than conventional linear potentiometers. I therefore looked for an equivalent circuit so that I would be able to use an ordinary linear potentiometer but still get close to the desired logarithmic response.
Directional couplers with wide-band transformers
There is no amateur radio station without them: Directional couplers are used to measure the standing wave ratio or the power of the transmitter output stage. Accordingly, many circuits of directional couplers have been published in amateur radio magazines and books. Very often a simple and easy to build coupler, comprised of just two transformers, is suggested for short wave and lower bands. The earliest publication of this dates back to 1966, when Carl Sontheimer and Raymond Fredrick had this device patented. A lot of HAM radio operators have implemented this specific directional coupler, but only some of them actually care about how this device really works. This is unfortunate, as sophisticated knowledge of electronics isn't needed at all. Just equipped with the basic knowledge necessary to pass the amateur radio exam, every amateur radio operator can grasp the theory behind it. A decent understanding of the theory behind this directive coupler not only reveals the true elegance of this – at first glance quite primitive – circuit. Moreover, it is then possible for anyone to not just rebuild circuits found in amateur radio literature, but to design a directional coupler from scratch in order to suit their specific needs.
- Sontheimer-Frederick's directional coupler
Attenuators for arbitrary impedances
In many cases, radio and electronics amateurs need attenuators for measurements or to attenuate strong signals. In many cases, however, there is no requirement for industrially manufactured (expensive) attenuators. Using low-inductance resistors and short wiring, self made attenuators can be used even at higher frequencies. This article features a tutorial on attenuator calculation and also provides precalculated resistor values in order to construct several commonly needed attenuators for 50 Ω as well as 75 Ω systems. Unsymmetrical Pi- and T-pads are covered in this article.
- Π-Pad (left) and T-Pad (right)
PA0FBK dualband antenna revisited
In my original version of the article about the PA0FBK dual-band antenna I have provided information on directivity and gain. Several radio amateurs from different countries of the world have expressed doubts about these figures. Their experience was, that in fact there will not be any difficulty in building and matching the antenna structure itself. Also their practical experience showed that my predictions on the 2m band were in line with their measurements. However, when the antenna was mounted relatively high and clear of obstacles, the field strengths observed on 70cm did not agree with my prediction of a gain of about 5 dBi. Normally the signals were all significantly weaker. So, have all these people done something wrong or is it more plausible that I was wrong in my simulations and the conclusions I drew?