An NEC-4.2 model of VK5BR’s 1m square loop for 20m

(Butler 1991) gives a design for a Small Transmitting Loop (STL) for 14MHz and some other bands.

He gives key design data:

Tube Diameter d   0.75 inch
 Circumference S  12.7 feet
 Area A =   10 square feet
 Frequency f =  14.2MHz
 Power P   100 watts
 Radiation Resistance Rr =   0.137 ohm
 Loss Resistance RL =   0.064 ohm
 Efficiency n =  68%
 Inductance L =   3.27 micro-henry
 Q factor =   723
 Inductive reactance XL =   291 ohms
 Bandwidth B =   19.6kHz
 Distributed capacity Cd =   10.4pF
 Capacitor potential Vc =   4587V
 Tuning capacitor Ct =   28pF

The data above appear to ignore some important factors, and estimate some others based on an assumption of uniform current. Continue reading An NEC-4.2 model of VK5BR’s 1m square loop for 20m

Underhill on Small Transmitting Loop efficiency

The meaning of the terms efficiency and radiation resistance are often critical to understanding written work on antennas, yet different authors use them differently, often without declaring their intended meaning.

Mike Underhill (G3LHZ) is an enthusiastic proponent of Small Transmitting Loops and in his slide presentation (Underhill 2006) challenges the proposition that their efficiency is low.

The line taken broadly is to introduce his own interpretation of efficiency and to challenge by experimental evidence other views on expected efficiency. Continue reading Underhill on Small Transmitting Loop efficiency

Return Loss sweep using IC7410, RL bridge, and RFPM1

This article demonstrates an automated Return Loss scan of an antenna using:

  • IC-7410 transceiver with CIV;
  • 40dB power attenuator;
  • Return Loss Bridge (RLB);
  • RFPM1 with USB data logger (A prototype data logger for RFPM1); and
  • a PC orchestrating the test.

Automation facilitates:

  • measurement of a large number of data points;
  • improved accuracy by reducing the risk of recording errors; and
  • reducing the tedium of a measurement task.

Continue reading Return Loss sweep using IC7410, RL bridge, and RFPM1

Mapping trips from APRS archives

I recently created a map from APRS archives of a recent trip by some friends over about eight weeks through central and north west Australia and back by the southern coast.

Google Earth googleearth 01/11/2015 , 08:43:23

Above is a graphic of the created map, but the ‘real’ map is not simply an image, but it is a kml file for Google Earth which you can view / zoom / scroll, for example in Google Maps by clicking on the map above.
Continue reading Mapping trips from APRS archives

The BLHeli sojourn is over

I was an early embracer of BLHeli ESC firmware on a Silabs based ESC, the Hobbywing Skywalker 40A UBEC.


The ESC looked to have quality hardware, and BLHeli was choice in aftermarket firmware.

The Skywalker did perform better with BLHeli than the stock firmware.

When BLHeli was released for the AVR based ESCs, I gave it a trial both on the bench in instrumented tests and in the air on a couple of quadcopters with different ESC / motor / prop / FC combinations.

I cannot say that BLHeli for AVR was any better than SimonK, and generally poorer though not much poorer.

More recently, users have reported some serious issues through BLHeli 14.0 and 14.1, and I removed the Skywalkers from service out of concern there were problems to be fixed. Continue reading The BLHeli sojourn is over

NBN is coming… it seems

The NBN was the Australian government’s response to a dominant carrier that would not engage in government’s attempts to guide industry to develop an equitable access network beyond that delivered off the telephony copper that existed in the street (the 2007 RFP).

That ‘initiative’ sees us return the good old bad old days when government owned most of the street infrastructure used to deliver telecommunications, and of course it has been in their interest as a provider to weaken consumer controls that otherwise gave some measure of protection of service levels.

The above graph shows the performance of my ADSL Internet access which reliably achieved  greater than 7Mb/s when I moved here in 2009 but fell to around 0.5Mb/s. A recent upgrade has seen around 2Mb/s fairly reliably, though not enough for reliable IPTV, a stunning improvement on the previous 3+ years. Continue reading NBN is coming… it seems

Zig zags in APRS tracks

There are many causes of zig zag errors in APRS tracks, and they fall generally into two main categories:

  1. incorrect positions (ie the tracker was never there); and
  2. correct positions with incorrect timestamps.

The first is common and has a number of causes, but principally defects in software used in iGates, most of which is not maintained either by the iGate operator or original developer. APRS is pretty static, but most old software has significant defects.

The second category is again common, and mostly the result of the design of the APRS radio network and its vulnerability to network delays (some of which can be caused by defective equipment).

This article looks at a case in the second category where a vehicle appears to have done a U-turn on the highway, travelled back some distance then another U-turn and caught up their original track speed at the next posit. It is clearly an out of order packet. This article shows how to diagnose the cause from the raw packet log for the tracker.


Screenshot - 30_10_2015 , 08_36_44

Above is a map of the glitch that is not uncommon in APRS for one of several reasons. No, VK2HJ has not done U-turns on the highway, the zig zag track is incorrect. Continue reading Zig zags in APRS tracks

OpenLog for TinyTrak – drive test


A drive test of the OpenLog logger collecting raw NMEA data in parallel with the TinyTrak (VHF) was conducted. To maximise the performance of APRS, a fill-in digi / iGate was run at my home. The tracker used a 65W transmitter with quarter wave vertical in the centre of the car roof.

Google Earth googleearth 29/10/2015 , 08:19:01

Above is an overview of the APRS and OpenLog tracks. Click on the image for a scaleable / zoomable view in Google Maps. Continue reading OpenLog for TinyTrak – drive test

Australian amateur population trends 1998 – 2015

This is a 2015 update of an article written originally in October 2005, earlier editions published on 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 – 2015

OpenLog for TinyTrak

There are a host of factors that contribute to data loss in APRS, to name just some:

  • non-standard / sub-standard / poorly configured digipeaters;
  • defect ridden iGates that lose, duplicate and corrupt packets;
  • poorly configured mobiles;
  • network congestion and interference;
  • unpredictable equipment failures;
  • basic geographical coverage of the network; and
  • dependence on the ionosphere for HF APRS.

This article describes an enhancement to the popular TinyTrak (and its clones) to also capture the GPS stream to an inexpensive local data logger.

The logger does not interfere with normal radio APRS, it coexists with it and creates a properly timestamped fine detail log of positions over a very long time, a log that can be post processed into a range of graphic / map and tabular reports.

Data logger

The datalogger used in an OpenLog. It is a simple logger that writes data to a micro SD card, costs about $A12 (inc post) for the logger and about A$10 (inc post) for a 16GB Class 10 micro SD card. (A slower card could be used, but they aren’t much cheaper.)


Above, the OpenLog data logger.
Continue reading OpenLog for TinyTrak