APRS duplicate removal – trial #2

The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.

Program code has been written to collect posit records direct from APRS-IS and where the record contains a HMS timestamp (as mine do), to apply a timestamp consistent with that timestamp, and then to not write records with duplicate timestamps to the database.

Screenshot - 04_02_16 , 15_57_38

The above map has two journeys, one a loop to the north of home to Mittagong, and another SW to Marulan and back. The Marulan trip extended to 40km from the prominent digi VK2RHR-1, and coverage in the area became poor, evidenced by the poorer APRS mapping in that region.

The map can be browsed interactively at http://owenduffy.net/map/20160204.htm. The red trace is APRS VHF derived using the processing detailed below.


The cyan trace is from stand alone GPS logger above attached to the case of the TinyTrak 3+. The transmitter is 65W output to a quarter wave in the middle of the wagon roof. Continue reading APRS duplicate removal – trial #2

Lithium battery – 1S protection boards

Some of my projects use a single Lithium cell for power, and the ready availability of low-cost battery protection boards offers opportunity for better projects.

IMG_1472Above, a 1S board rated at 4A and which sells for about A$1 posted in lots of 5.


New and good quality
Use BM112 protection chip + AO8810 MOS tube
The protection board is used to protect the battery overcharge, over discharge, so can’t use as a charger,when you want to charge the battery you need to use the dedicated charger,because the protective board has a time to response to the short circuit, can’t to connect too large instant impact current, such as drills and so on

The main performance parameters:
1. PCB Size: 39 * 4 * 2mm
2. Overcharge protection voltage: 4.2750 ± 25MV
3. Over-discharge protection voltage: 2.88 ± 75MV
4. The overcurrent protection: 4-8A
5. Continue working current: > 4A

Note: Only for the equipment which instant start-up current less than 4A,those starting current instant is great, such as high-current motors, drills, etc., are not suitable for use.


Above are protected battery assemblies based on the board and a 1200mAh LiPo (sells for about A$4) and Panasonic 18650 Li-ion 3400mAh (sells for about A$12). The connectors used are 3A rated JST RCY connectors as used in RC battery applications and readily available with tails for way less than A$1 each set.

The 18650 cell has tags spot welded to the battery contact points, the LiPo has contact tags as supplied.

To use these, the power source needs to supply about 4.5V so as to ensure charging when necessary. The power source needs to be current limited unless you choose to depend on the protection boar’s limit.

Exploiting your antenna analyser #13

Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent having read Exploiting your antenna analyser #12 asks whether the measurement provides evidence of loss of the connectors, and referred me to (Arther nd) where he reports some measurements of UHF series adapters and conclusions.


Let’s deal with interpretations of my own measurements first.

Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.

Screenshot - 01_02_16 , 11_40_57

Above is a plot of ρ and ReturnLoss for the DUT. ReturnLoss curiously is plotted ‘upside down’ as ReturnLoss increases downwards… a quirk of AIM software, but remember that ReturnLoss in dB is +ve.
Continue reading Exploiting your antenna analyser #13

Low noise Yagis and 50MHz noise

After reading my post Some thoughts on noise on 6m (50MHz)…, a correspondent asked about the content in the context of comments by G0KSC.

Under the altruistic heading “Low Noise Yagis Explained”, G0KSC takes a competitor to task over the accuracy of statements regarding the importance or not of G/T at 50MHz.

G0KSC says:


Have you ever heard the saying ‘A little knowledge is a dangerous thing’?…

What is G/T? Continue reading Low noise Yagis and 50MHz noise

A low cost home made USB CI-V interface with open collector and solid Windows drivers

This article describes an inexpensive USB adapter for Icom’s CI-V interface.

There are four common options for USB-serial adapters:

  • Prolific;
  • FTDI;
  • WCH; and
  • Silabs.

This article describes an adapter based on an inexpensive FTDI adapter (~$5 on eBay).


You will need the module, a Schottky signal diode (eg 1N5711), wire and a 3.5mm TRS plug or two. I have connected two plugs, one wired for TS (CT-17) and one for RS (OPC-478x). Continue reading A low cost home made USB CI-V interface with open collector and solid Windows drivers

Voltage and current on a transmission line with standing waves

Folk often ask how to calculate the maximum voltage on an antenna feed line with standing waves, often to get a feel for the necessary voltage withstand of baluns, feed line, switches and relays, and ATUs.

Feeding at a current maximum outlines the method described in detail at (Duffy 2011), but the approach is more complex than a lot of hams want.

A simpler method is to treat the transmission line as lossless, and to simply find the worst case voltage and current that can occur… and design for that, or perhaps do the more detailed analysis depending on the outcome.

A new calculator, Calculate Vmax, Vmin, Imax, Imin for lossless line from Zload (or Yload) and Zo, does just that.

Screenshot - 29_01_16 , 19_45_17

Above is the built-in example of a G5RV with tuned feeder on 80m with feed point impedance derived from a modelling package. The voltage and currents calculated are those for a long lossless feed line.
Continue reading Voltage and current on a transmission line with standing waves

Exploiting your antenna analyser #12

Is there a place for UHF series connectors in critical measurement at UHF?

Seeing some recent discussion by a chap who was trying to construct a low power 50Ω termination on a UHF series plug, it bought to mind the futility of using some kinds of connector for critical measurement above perhaps 100MHz.

There is a lot of conjecture about the nature of UHF series connectors, whether they act line a simple transmission line section with fairly uniform Zo, whether they are really just a lumped shunt capacitance, whether it is even important at UHF etc.

To illustrate the issue, I have assembled a simple test jig comprising an N(M)-UHF(F) adapter, UHF(M)-N(F) adapter and a 50Ω N termination (which was also used to calibrate the analyser. This set was assembled and plugged onto a calibrated AIMuhf analyser and swept from 1-500MHz… just into the UHF range (which is 300-3000MHz).


Above, the test jig.

Screenshot - 29_01_16 , 14_15_27

Above is an expanded scale centre of the Smith chart of the sweep. Continue reading Exploiting your antenna analyser #12

APRS duplicate removal

I have written about duplicate position records assigned different timestamps in the APRS – APRS-IS – archiver network, and the corruption of mapping as a result of false timestamps which causes zig zags in the mapping.

Most recently, I demonstrated that at APRS – APRS-IS latency.

I have been challenged that this just does not happen.

As a by product of developing code to circumvent the defects in the design and implementation, I have been running a rover stationary on the driveway during the day to supply test data to the filtering code, and of course these exceptions have occurred.

Here is an extract of the raw packets log from aprs.fi for just one case.

2016-01-26 16:20:25 AEDT: VK2OMD-8>APT314,WIDE1-1,WIDE2-2,qAR,VK2OMD-3:/052024h3429.91S/15027.12E>110/000/A=002263
2016-01-26 16:23:17 AEDT: VK2OMD-8>APT314,VK2RHR-1,WIDE1,VK1RGI-1,VK2RWG-1,WIDE2*,qAS,VK2KAW:/052024h3429.91S/15027.12E>110/000/A=002263

Inspection will reveal that both records have the GPS derived timestamp 052024h, and the message contents are identical, they are both a result of a single posit report at 052024UTC, the relay via VK2KAW has been assigned a timestamp some 173s (almost 3m) later and were the rover moving, this corrupted report would cause a track back and the next valid report would leap forward.

In six hours of operation, there were 36 posits transmitted, and 11 corrupt posits via VK2KAW logged by APRS-IS, that is 31% of valid posits have a false and corrupted posit generated in the wider network.

APRS fails by design, the core infrastructure does not detect these anomalies and discard them. Note that this scenario is ONLY provable because the tracker sends the HMS format timestamp… which almost no-one does.

This is only one source of corruption of posits, vulnerability to this one is designed into APRS, but it is detectable and fixable in core infrastructure.


APRS – APRS-IS latency

A fundamental problem with APRS-IS is that archives assign a timestamp to a position report (posit) when the posit is received from the APRS-IS network. They do this even when the posit contains a timestamp which is almost always derived from the GPS and is quite accurate.

The archives such as findu.com and aprs.fi are not formally part of APRS-IS, but they are the visible interface to APRS-IS for many and are treated as part of the wider APRS-IS network in this article.

The APRS-IS network is designed to permit, encourage even, multiple submissions of the same originating posit via multiple iGates over multiple digipeater radio links.

You might expect that the maximum latency of two or three digipeater links might be a couple of seconds but not only is it ordinarily longer than that due to an overcommitted network, but defective infrastructure using old and misconfigured software can introduce much larger delays.

The effect of delayed duplicates is that they create false back and forth zig zags in a track, and make the mapping hopelessly inaccurate.

For this article, I will use data gained from a recent 100km rural trip using VHF APRS. For all of the trip, the rover was in range of a prominent digipeater which is monitored by many iGates, and for about half of the trip, the rover was in working distance of one of the iGates. About 200 posits were captured by APRS-IS, and they have a GPS derived HHMMSS timestamp.

Lets look at the latency.

Clip 114

Above is a chart of the difference between APRS archive timstamp and the GPS timestamp. A very large number of posits takes more than a couple of seconds to reach the APRS-IS archives. Continue reading APRS – APRS-IS latency