Amateur Radio – Page 69

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

Last update: 21st February, 2020, 3:20 AM

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

Last update: 26th December, 2015, 9:03 AM

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.

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

Last update: 3rd November, 2015, 2:43 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

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

Does VSWR damage HF ham transmitters

This Jan 2011 article has been copied from my VK1OD.net web site which is no longer online. It is for reference in further articles discussing the popular reflections explanations. The article may contain links to articles on that site and which are no longer available.

The statement is often made to the effect that:

VSWR will damage a HF ham transmitter, and the mechanism is that the ‘reflected power’ in a standing wave will be absorbed by the Power Amplifier (PA), increasing heat dissipation and damaging the PA.

There are two problems with this statement: Continue reading Does VSWR damage HF ham transmitters

Last update: 13th October, 2015, 10:05 AM

Transmission line loss under mismatch explanations – the missing TWLLC model

At Transmission line loss under mismatch explanations I wrote that there is a lot of woolly thinking amongst hams about transmission line loss under mismatch and worked a simple example that could be done ‘by hand’ to show that formulas that some authors have produced as implementations of their explanations don’t stack up.

I also gave a solution to the Zo*3 scenario using TWLLC, but not the Zo/3 scenario which a few eagle hounds have pounced on as evidence that the solution would not support the article.

Not at all, the Zo/3 TWLLC solution was not given so as to keep the article short and within the attention span of modern hams though it was eventually a quite long article, and for that reason I will address it separately, here. Continue reading Transmission line loss under mismatch explanations – the missing TWLLC model

Last update: 25th May, 2016, 8:31 AM

Transmission line loss under mismatch explanations

There is a lot of woolly thinking amongst hams about transmission line loss under mismatch, perhaps exemplified by Walt Maxwell (Maxwell 2001):

The power lost in a given line is least when the line is terminated in a resistance equal to its characteristic impedance, and as stated previously, that is called the matched-line loss. There is however an additional loss that increases with an increase in the SWR.

This article probes the folk lore with an example scenario designed to expose the failure of such thinking. Continue reading Transmission line loss under mismatch explanations

Last update: 10th October, 2015, 6:36 PM

Some thoughts on the popular Carolina Windom antenna

The Carolina Windom is very popular with modern hams, and at the same time is commonly the discussion of problems in online fora.

The question is whether it is its popularity that is the reason for cries for help, or whether there is something inherently high risk in the ‘design’.

The original Windom

The first type is the classic ‘original’ Windom with single wire feed which folk lore explains as a horizontal wire being tapped at a point where Z matches the vertical ‘single wire’ feeder, that there is not a standing wave on the feed line, and that it does not radiate. Traditional characterisation as a single-feeder Hertz denies the existence of the vertical radiating element.

It is a folly to designate the vertical wire as a non-radiating feeder, it carries an RF current that contributes to radiation just like current on the horizontal wire does. Continue reading Some thoughts on the popular Carolina Windom antenna

Last update: 2nd October, 2015, 9:49 AM