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 broadband 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

Last update: 1st November, 2015, 10:03 AM

Zig zags in APRS tracks

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

incorrect positions (ie the tracker was never there); and
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.

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

Last update: 31st October, 2015, 7:24 AM

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.

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

Last update: 29th October, 2015, 3:52 PM

Australian amateur population trends 1998 – 2015

This is a 2015 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 – 2015

Last update: 28th October, 2015, 4:56 PM

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

Last update: 6th November, 2015, 8:20 AM

DK7ZB’s balun

(Steyer nd) describes the DK7ZB balun / match for VHF and UHF Yagis.

To understand how the “DK7ZB-Match” works look at the left picture. Inside the coax cable we have two currents I1 and I2 with the same amount but with a phase shift of 180°.

No. At any point along the coaxial line, a current I on the outer surface of the inner conductor causes an equal current in the opposite direction on the inner surface of the outer conductor.

As the currents are shown with the designated directions, I2=I1, not I2=I1<180.

A consequent simplification is that I4=I2-I3=I1-I3.

There is an issue with the current arrow I3 in the lower right of the diagram. It might imply that the only current in the conductors is I3, but the current between the nearby node and lower end of the shield is I3-I1.

If the structure was much much shorter than the wavelength, there would be negligible phase change in currents along the structure, so I1 would be uniform along the centre conductor, I2 uniform along the inside surface of the outer conductor, and I3 uniform along the outer surface of the outer conductor.

The diagram notation does show that I3 (which is equal to the dipole drive imbalance) is uniform along the structure, and that I3 flows to ground.

It seems that the diagram appears in (Straw 2003).

DK7ZB goes on:

If we connect a dipole or the radiator of a Yagi direct to the coax, a part of I2 is not running to the arm 2 but down the outer part of the coax shield. Therefore I1 and I4 are not in balance and the dipole is fed asymmetric.

But how can we suppress the common-mode current I3? A simple solution is to ground the outer shield in a distance of lambda/4 at the peak of the current.

So, the length of the structure is in fact a quarter wavelength electrically, or close to it to achieve the choking effect. I3 will be in the form of a standing wave with current maximum at the lower (‘grounded’) end, and current minimum at the upper end.

It happens also that his usual configuration of this balun is that there is a standing wave on the inside of the coax, and so I1 and I2 are not uniform along the conductor, and whilst it is relevant to the designed impedance transformation, it is inconsequential to reduction of dipole current imbalance.

DK7ZB continues with the development of his variation of a Pawsey balun:

But now we get a new interesting problem: For the transformation 28/50 Ohm we need a quarterwave piece of coax with an impedance of 37,5 Ohm (2×75 Ohm parallel). The velocity of the wave inside the coax is lower than outside (VF = 0,667 for PE).

The outside of the shield has air (and a litle bit of insulation) in the surrounding and VF = 0,97. For grounding the common mode currents this piece should have a length of 50 cm, with a VF = 0,667 and a length of 34,5 cm this piece of coax is to short. By making a loop of this two cables as shown in the picture down we get an additional inductivity and we come closer to an electrical length of lambda/4. Ideal is coax cable with foam-PE and a VF = 0,82

Above is DK7ZB’s implementation of his balun with the loop and additional inductivity.

I copied the above implementation and measured the common mode impedance Zcm.

Above is the Zcm measurement. There is a quite narrow self resonance where Zcm is quite high for about 10MHz bandwidth centred on 125MHz, but at 144MHz Zcm=83-j260Ω which is too low to qualify as a good common mode choke.

Like all narrowband / tuned common mode chokes, tuning to the desired frequency band is essential to their effective operation.

Like most published balun designs, this one is published without measurements to demonstrate its operation or effectiveness.

Links

Steyer, M. nd. The DK7ZB-Match for Yagis. http://www.mydarc.de/dk7zb/dk7zb-match.htm (accessed 27/10/15).
Straw, Dean, ed. 2003. The ARRL Antenna Book. 20th ed. Newington: ARRL. 26.17.

Last update: 12th October, 2018, 5:45 PM

Trombone capacitors in Small Transmitting Loops

Small Transmitting Loops (STL) usually require a capacitor to tune the loop to resonance for ease of efficient matching.

For an efficient STL that can handle moderate power, the capacitor must withstand extreme voltage, and must have extremely low equivalent series resistance (ESR).

(Straw 2007) describes the so-called ‘trombone’ capacitor which is attributed to Bill Jones, KD7S, originally in Nov 1994 QST.

Above is Jones’ trombone match. Continue reading Trombone capacitors in Small Transmitting Loops

Last update: 13th September, 2016, 6:49 PM

A cheap and cheerful data logger

I had need of a portable serial data logger for proof of concept of a supplementary data logger for an APRS tracker.

The requirement is to capture RS232-TTL data at 4800bps, 8N1 to a data file for later extraction. The logger needs to restart automatically and append new records to the existing file.

A spare Raspberry Pi2 was applied to the job as a headless data logger.

Above is the RPi2 with an inexpensive FTDI USB/RS232-TTL adapter. Only the ground and RD wires attach to the modified TinyTrak. Continue reading A cheap and cheerful data logger

Last update: 31st October, 2015, 8:05 AM

Adjusting KISS TNC AFSK tx level using an improved isochronous test packet

Adjusting KISS TNC AFSK tx level using an isochronous test packet explained 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. The improved packet should be repeated by most digipeaters, allowing observation of their modulation performance.

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 improved isochronous test packet

Last update: 30th October, 2015, 10:10 PM

Steel wire CF dipole on 160m

A correspondent having seen recent discussion and models on eHam regarding steel dipoles, asked about the accuracy of my articles:

Galvanised steel wire CF dipole; and

Galvanised steel wire OCF dipole.

The eHam article gives the gain of a low half wave steel dipole on 160m as 0.5-1dB less than copper depending on steel composition. (The thread was entitled “galvanised steel wire”, but the model was clearly labelled steel. For discussion of the effect of galvanising, see Galvanised steel wire OCF dipole.)

The model used is not fully exposed, but the results are unlikely unless perhaps the permeability of the steel was ignored (NEC-2 does not natively model µr>1).

Above are the gain plots from NEC-4.2 for three different material types, copper, steel, and steel resistivity with µr=1 (-wrong). Continue reading Steel wire CF dipole on 160m

Last update: 19th October, 2015, 7:08 AM