Australian amateur population trends 1998 – 2014

This is a 2014 update of an article written originally in October 2005, earlier editions published on VK1OD.net which is now offline.

Over recent years to 2002, the number of issued amateur licences was declining, the trend was about 2.8% pa decline over the five years to 2002.

This has concerned some people, who took the view that the decline was a harbinger of the impending demise of Amateur Radio. Continue reading Australian amateur population trends 1998 – 2014

Exploring VK2XSO’s transmission line example

VK2XSO posted a sweep of “Return Loss (SWR) (the lower plot) from 500 to 2500MHz of a 50Ω load through ~5m of RG59” apparently to demonstrate his knowledge of transmission line basics. As he says “here are also many other things we can deduce from looking at these two lines.”

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For students of transmission lines, some deductions… Continue reading Exploring VK2XSO’s transmission line example

Feed line length affect on VSWR

This article explores the way in which VSWR varies along a feed line.

Actual VSWR

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The graph above shows R and X, and VSWR along a feed line with a 100+j0Ω load at 3.6MHz. The feed line is Belden 8262 50Ω coax, manufacturing tolerances are taken to be zero, and the displacement is relative to the feed point, ie -ve distance is distance before the feed point, the feed point then is at the right hand side of the graph.
Continue reading Feed line length affect on VSWR

Why the preference for Guanella 1:1 current baluns for HF wire antennas

I am often asked why I recommend a Guanella 1:1 balun with high choking impedance for most HF wire antennas over voltage baluns and 4:1 current baluns. This article explores the topic using SPICE models for a voltage balun, 1:1 current balun, and 4:1 current balun. Continue reading Why the preference for Guanella 1:1 current baluns for HF wire antennas

Adjusting KISS TNC AFSK tx level using an isochronous test packet

This post explains a technique to drive a KISS TNC with a specially constructed packet that contains an ISOCHRONOUS test packet, a packet that will produce equal high and low tone alternation in the transmitted AFSK signal.

F0000TEK

Above is the waveform recovered from a receiver without de-emphasis (a Motorola R2009D communications analyser in this case).
Continue reading Adjusting KISS TNC AFSK tx level using an isochronous test packet

Cron configuration improvements for Raspberry Pi

One of the shortcomings or RPi for (m)any serious applications is that it lacks a hardware clock so when it starts, the clock is not set correctly.

This deficiency can be overcome to some extent for some applications with the fake-hwclock module and SNTP where the RPi is always connected to a network.

The standard scheduling daemon cron does not run tasks that were missed because the system was not running at the time they were scheduled. It is a significant shortcoming that has been overcome by adding another element, anacron, which wraps the cron tasks and is scheduled by cron and at startup. All of this is very dependent on correct clock time, and for RPi systems that lack a hardware clock (the out of the box hardware), the system time can be quite wrong at the time anacron is scheduled as part of the startup procedure.
Continue reading Cron configuration improvements for Raspberry Pi