Improved cooling for the ATR-30

In Improved cooling for the MFJ-949E I described a solution to a problem of demonstrated overheating of the ATU at rated power, indeed at a lot less than rated power.

Though I have never measured the ATR-30 temperature rise, and am probably unlikely to stress the 3kW rated ATU with a 100W transmitter, I have performed a similar cooling modification to the ATR-30.

Continue reading Improved cooling for the ATR-30

On use of enameled wire in transmitting baluns

I have published a number of transmitting balun designs, and none of them use enameled wire. I am sometimes asked why is that so, but more often advised that it is a better solution than the wires that I have used.

enameled wire depends on an insulating coating, and its breakdown voltage depends on the wire diameter, polymer used, the minimum thickness applied, coating cure / bake processes, temperature, humidity etc.

Whilst I have seen specifications promising breakdown voltage of a single round enameled wire in the regions of 5-10kV, and you might hope for nearly double that between a pair of twisted wires, unless you have sourced specific product, new performance may be closer to 2kV. Continue reading On use of enameled wire in transmitting baluns

Exploiting your antenna analyser #15

Measure MLL using the half ReturnLoss method – a spot test with a hand held analyser

At Exploiting your antenna analyser #14 I gave an explanation of the method of approximating MLL of a line section by taking the average half Return Loss with o/c and s/c terminations.

This article demonstrates the technique using the Rigexpert AA-600 analyser in hand held mode.

The task is to assess whether a section of RG58A/U coax has MLL at 3.5MHz similar to specification or not.

The specification loss of 10.13m of RG58A/U has MLL=0.29dB.


Above, the first test with an o/c termination. Return Loss is 0.4dB. Continue reading Exploiting your antenna analyser #15

Assessing the Q of a half wave dipole antenna system – a real world example

Assessing the Q of a half wave dipole antenna system explained that Q can be a valuable indicator of antenna system health.

This article uses a recently published VSWR curve for a 15m half wave dipole antenna system as an example to demonstrate the technique.

The following graph is from a Sark100 style antenna analyser, and it is quite a poor start to diagnostics, but using it draws out what is desired for further analysis.


Above, the captured VSWR(50) sweep. Continue reading Assessing the Q of a half wave dipole antenna system – a real world example

Assessing the Q of a half wave dipole antenna system

Q can be a valuable indicator of antenna system health

The Q of an antenna can be a useful statistic in assessing whether it is operating as it should.

The Q of half wave dipole antenna system on HF depends to some extent on conductor size, its environment (height, type of ground, nearby structures and vegetation, and feed line / matching loss. Nevertheless, it should usually fall in the range of 10 to 13 for good wire dipoles, and if you measure a half wave dipole antenna system to have Q significantly outside that range, it is probably significantly less efficient than it should be. Continue reading Assessing the Q of a half wave dipole antenna system

Field strength survey of an M40-1 short helical vertical on 40m

This article documents a field strength survey of an M40-1 short helical vertical on 40m.

This test is more a feasibility study of the experimental method and apparatus than an absolute measure of the antenna.

The antenna under test is described at AUT – MobileOne M40-1 40m helical.

Field strength was measured using a small square untuned loop and VK3AQZ RF power meter (RFPM1), and data was captured using A prototype data logger for RFPM1.

Power meter

VK3AQZ RF power meter (RFPM1) described my build and calibration  of the RFPM1.

RFPM00Above is the RFPM1, shown with two probes, but only one probe is required for this procedure, the other is disconnected. The RFPM1 directly reads input power in dBm.

Loop antenna

Clip 071

The loop antenna used was described at (Duffy 2007). It is a small square loop (600mm sides) fed in one corner with a 1:1 voltage balun. Continue reading Field strength survey of an M40-1 short helical vertical on 40m


This article describes a build of the PIC Iambic Keyer (PIK).

Screenshot - 18_04_16 , 19_47_22

Above is the generic circuit diagram of the PIK.

This one runs on 4.5V from 3 x AA cells. A 3000mAh battery will run it in ‘sleep’ mode for around 2,000,000 hours or 230 years… the shelf life of the batteries determines their useful life and there is consequently no ON/OFF switch.

So, the variation to the circuit above is that the zener regulator circuit is not required, Z1 is omitted and R5 is replaced by a 50mA Polyfuse. C3 is 0.0068µF to give a range of 6-36WPM on 4.5V.


Above, the internals. The electronics is assembled on a small piece of Veroboard with jacks at the rear for paddle, hand key and output, a pot for speed control and switches for TUNE and AutoSpace.


Above is the external view of the keyer prior to labelling.



Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves

The article describes a current balun with low Insertion VSWR for operation at modest power levels. It is lightweight and well suited to portable operations, and can be made with materials readily available in Australia (LF1260 cores are a little over $1 each in packs of six.)

Balun404 Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves

A tale of three tuners

At HC-500 I showed some VNA plots of the HC-500 matching a 50+j0Ω load at 3.5MHz.

The following commentary is on a single load scenario, a 50+j0Ω load at 3.5MHz, and while the results are not simply extensible to other loads and frequencies, it does provide some interesting insight into the devices.

THP HC-500 (Ultimate Transmatch (McCoy 1970))

Screenshot - 13_04_16 , 08_06_08

Above is the behaviour of the unmodified HC-500 (an Ultimate Transmatch).

Losses at match are 12% of input power. At its rated 500W maximum power, that is 60W (which might seem high but heat tolerant insulation materials are used). On modification to a T match, losses at match were reduced to 8% or 40W at rated maximum power.
Continue reading A tale of three tuners


In the early 1970s I purchased a Tokyo High Power Labs HC-500 ATU based on recommendation of other hams and the seller’s representations (Dick Smith Electronics) that it was a T match with 200pF capacitors.

The circuit configuration is of the so-called Ultimate Transmatch, an invention of (McCoy 1970) that claimed a bunch of advantages over the ordinary T match.

The HC-2500 would appear to use the same circuit.

It wasn’t long before several authors waded into the Ultimate Transmatch over its poorer efficiency. With an ambitious name like Ultimate Transmatch, it had a lot to live up to… but it failed.

Within months, a reconfigured topology appeared entitles the SPC Transmatch, but it also had issues.

The reality is that none of these designs is ‘ultimate’, they all have advantages and disadvantages and are mostly used in ignorance of those.

So, I have had this HC-500 which worked well enough I suppose, but was quite difficult to tune on some loads that ordinary T matches handled with ease. It has always been my intention to reconfigure it to a T match be rewiring the grounded stator of the input cap to parallel it with the other stator… a minimal modification to get rid of the shunt capacitor and use it to help to keep coil voltage down on some loads.

Before performing the modification, I measured transmission loss when matched to a 50+j0Ω load at 3.5MHz using a two port VNA.

Screenshot - 11_04_16 , 20_26_07

Above, transmission loss is 0.54dB, efficiency is 88.3%. Continue reading HC-500