DL8KDL, DN1KDL

HAM radio, electronics and computer technology

Since March 2002 I am permitted to participate in the amateur radio service. My call sign is DL8KDL. Due to several HAM radio exam courses which I was allowed to participate in as a tutor, I also hold a call sign for educational purposes, which is DN1KDL.

At the moment I am a supporter of IGFS e.V., an independent regional HAM radio club, operating and maintaining several digital and analog relay stations and HAMNET access points in the area of Bonn and the Cologne Bay. In this club I am the webmaster and try to support the team at the relay location of Ölberg/Siebengebirge in all IT related stuff. My private interest is currently focussed on QRP and self-made transmitters. I am particularly fond of the "BITX", "Minima" and "µBITX", a series of easy to build short wave transceivers developed by Ashar Farhan (VU3ICQ) from India. Furthermore I am interested in Software Defined Radio (SDR); mainly based on the RTL283U chipset and the RTL-SDR project, where amateur radio is concerned. Also, I focus on programming PIC and AVR microcontrollers. For simple programming tasks I regularly stick to the Arduino platform and its free and open source counterpart Freeduino. In addition to that, I prefer programming in Ruby, C++ and sometimes even Pascal.

My computers are run using free and open source software. This starts with the BIOS, which (whenever possible) is based on Libreboot. Normally, I mostly choose GNU/Linux based on the distributions of Trisquel and BunsenLabs for my desktop PCs and Notebooks, CentOS on servers, as well as OpenWRT on routers. For mobile devices, I generally use Android based on Lineage OS. With great interest I also follow (passively) the development of microG as a free and open source alternative to Google's proprietary Android libraries.

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.

Ersatzschaltungen für logarithmische und exponentielle Potentiometer

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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
Sontheimer-Frederick's directional coupler

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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)
Π-Pad (left) and T-Pad (right)

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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?

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