Feed line length affect on VSWR

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

Actual VSWR

Clip 023

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.
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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).
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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.
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Real time clock 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, but this is not a solution if the RPi is not always connected to a network time reference.

One solution is a real time clock module such as the DS1307, or better still the DS3231. The latter has an internal crystal with 2ppm (5s/month) accuracy from 0-40° which is much better than typically achieved with a DS1307 (which is dependent on the external crystal). Continue reading Real time clock for Raspberry Pi

APRS: how many digi hops to make it to APRS-IS

APRS is almost exclusively source routed, and depends greatly on sensible path configuration by each and every user.

This is one of those cases where individuals acting independently and rationally for their own personal behave contrary to the whole group’s long-term best interests by depleting some common resource (the tragedy of the commons).

Increasing the available WIDE path length does expose a station’s packets to more iGates, but the average effect of everyone following that strategy is to literally exponentially increase channel busy as the available hops is increased. Continue reading APRS: how many digi hops to make it to APRS-IS