Where is the best place to measure feed point VSWR

The subject question is often asked, and the usual responses are mindless recitals of Rules of Thumb (RoT).

In the light of the discussion at Feed line length affect on VSWR and The half waves of coax rule for measuring VSWR accurately, lets consider the subject question and develop a rational answer. Continue reading Where is the best place to measure feed point VSWR

The half waves of coax rule for measuring VSWR accurately

Lots of hams recite a rule that accurate measurement of VSWR can only be made at the feed point or an integral number of electrical half waves from the feed point.

It is one of those ‘rules’ that the proponents cannot usually explain… they would regard themselves as experts, but blindly follow folk-lore that they do not understand. Continue reading The half waves of coax rule for measuring VSWR accurately

Headless aprx APRS server – RPi packages

In earlier articles I documented a trial of aprx v2.08r593:

Encouraged by Kenneth Finnegan who merged some fixes by WB4XO (which still didn’t work properly), I set about resolving some problems.

Further fixes and some enhancements have been pushed to the source fork at owenduffy/aprx. This was forked from PhirePhly/aprx which in time may pull the changes.

You can browse owenduffy/aprx for brief descriptions of the changes.

Recent Debian packages for RPi based on my fork have been uploaded to http://owenduffy.net/files/aprx/ .

End fed wires – another matching case study

A reader of my article End fed wires – new hams love ‘em asked about the performance of the matching transformer described at (Osborne 2014).

Screenshot - 25_09_2014 , 08_09_48

(Osborne 2014) gives the transformer design above using a FT82-43 ferrite toroid with three turns on the primary. The concept is to adjust the antenna length and C1 for a ‘perfect match’. Continue reading End fed wires – another matching case study

Headless aprx APRS server

In earlier articles I documented a trial of aprx v2.08r593:

Encouraged by Kenneth Finnegan who merged some fixes by WB4XO (which still didn’t work properly), I set about resolving some problems.

HeadlessJavaprssrvrAbove is the RPi sitting on a TNC-X KISS TNC connected to the TNC’s USB port. In this case, aprx uses the supplied KISS interface, it does not use Linux kernel support for AX.25. Continue reading Headless aprx APRS server

A simple preamp for dynamic capsules to suit radios using electret microphones

A lot of radios are made today to use an electret microphone which has a higher output than common dynamic capsules and may require a preamp to comfortably obtain sufficient audio.

This article describes a microphone + preamp intended as a drop in replacement for an electret capsule.

An electret capsule contains an integral FET preamplifier, and the common two terminal version of the capsule depends on an external load resistor (commonly around 4.7kΩ and source of supply current (commonly from 6-12V via the load resistor.

Screenshot - 10_09_2014 , 11_47_44Above is the preamp schematic from an LTSpice model. A common emitter amplifier operating in this circuit has re of about 20Ω, and so voltage gain from collector to base is very high at 3000/20=150 (assuming total collector load of 3kΩ). Continue reading A simple preamp for dynamic capsules to suit radios using electret microphones

End fed wires – new hams love ’em

Hams impoverished for space to erect a ‘full size’ antenna for HF pursue a range of options and end fed wires figure highly.

New hams who heard the maxim that ‘any antenna is better than no antenna at all’ will try to use bed frames, window frames, balcony railings, anything metallic to get on air. This cry for mediocrity is to prevail over a quest for understanding, ham radio as it has evolved.

The ‘net abounds with conflicting advice:

  • (Yates 2010) recommends a T50-2 powdered iron core;
  •  (VK2AVR 2014) states do not use a powdered iron toroid.. they won’t work.

They cannot both be correct. Continue reading End fed wires – new hams love ’em

MFJ-1270B TNC review

MFJ1270BFront

I have been doing some research on APRS recently, and Brian, VK2AH, kindly donated a MFJ-1270B.

MFJ-1270B Over the years I have heard so many reports that the MFJ-1270B does not accommodate twist (an amplitude / frequency slope usually the result of lack of pre-emphasis, usually the result of audio overdrive of the transmitter, and usual for hams). More detailed discussion is at The extent to which APRS works is often an accident.

There is no shortage of bad signals to test the tolerance of the TNC, with two local overdriven digis, and travellers through the area. Continue reading MFJ-1270B TNC review