APRS traffic study – VK2OMD-3 08/02/2015

APRS traffic study – VK2OMD-3 11/10/2014 reported a study of APRS traffic.

This article is an update after configuration changes at VK2RHR-1 to make it a ‘fill-in digi’, it repeats packets addressed to WIDE1 only.

The study of APRS traffic was conducted by analysing the log of VK2OMD-3, a VHF APRS ‘fill-in’ digi and iGate.

The capture period was during from 0700-1700 local time on Sunday 08/02/2015. Continue reading APRS traffic study – VK2OMD-3 08/02/2015

Last update: 9th February, 2015, 1:14 AM

NEC – current or voltage source

A forum expert recently opined:

Unless there’s a compelling reason NOT to, always use CURRENT sources instead of voltage sources.

Like most online advice, the writer did not give reasons for is advice, and given the strength of it, an explanation is warranted.

Of course, many people giving compelling advice online don’t actually understand the matter and cannot give a rational explanation.
Continue reading NEC – current or voltage source

Last update: 5th February, 2015, 10:08 AM

The laws of physics – common mode currents and coax

I am always suspicious when “the laws of physics” are cited to support some argument. One forum expert recently offered:

The laws of physics require that the current on an ideal coax center conductor and the current on the inside of an ideal coax braid be equal in magnitude and opposite in phase, i.e. nothing but ideal differential currents can flow inside ideal coax. Anything else would violate Maxwell’s equations. All common-mode (non-differential) current must therefore necessarily flow on the outside of the ideal coax braid.

If we consider the end of a coaxial transmission line to have just two terminals, we can define some currents for the purpose of discussion. I1 flowing out of the inner conductor terminal, and I2 flowing into the other terminal (the end of the outer conductor). Continue reading The laws of physics – common mode currents and coax

Last update: 12th July, 2015, 7:38 AM

Loss of Wireman 553 windowed ladder line at MF/HF – does it matter #2

Loss of Wireman 553 windowed ladder line at MF/HF discussed the matched line loss of a windowed ladder line with 19 strand CCS conductors.

Clip 094

Above, the loss curves from the article

You might look at this and ask “does it matter”.

A recent thread on QRZ (Any special consideration for long run of twin-lead?) gives an interesting example. Continue reading Loss of Wireman 553 windowed ladder line at MF/HF – does it matter #2

Last update: 31st January, 2015, 7:04 AM

Measuring balun common mode impedance – #1

At Baluns – show me the numbers I raised the value of quantitative measurement of the complex common mode impedance of current baluns as a figure of merit for certain antenna applications.

This article shows a simple method of measuring the complex common mode impedance of a Guanella 1:1 current balun using a modern Antenna Analyser.

The instrument used for this demonstration is a Rigexpert AA-600, and its PC client Antscope. Continue reading Measuring balun common mode impedance – #1

Last update: 29th July, 2019, 5:59 AM

Diamond X50N measurements – AIMuhf and AA-600

At Diamond X-50N #2 at VK2OMD I reported some details of a new antenna at VK2OMD, including some baseline measurements using a Rigexpert AA-600.

This article expands that information with some measurements made using AIMuhf, and gives some contrasts between the two tools.

The AA-600 measurements were made 26/11/2014, and AIMuhf a couple of months later on 29/01/2015. The system measured should have been identical, the feed line is 9.83m of LDF4-50A with N-type connectors at both ends. Continue reading Diamond X50N measurements – AIMuhf and AA-600

Last update: 25th March, 2015, 11:01 AM

Quiet solar radio flux interpolations calculator

My Quiet solar radio flux interpolations calculator has been broken recently, a result of changes at NOAA where they withdrew the observation data on which the calculator was based.

The data remains available on an FTP server, but my web server cannot access it for security reasons, and it would be unacceptably slow anyway. Continue reading Quiet solar radio flux interpolations calculator

Last update: 27th January, 2015, 10:29 AM