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, an reconfigured topology appeared entitle 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

A generic run on timer using an ATTINY25

At Improved cooling for the MFJ-949E I described a modification to the ATU to improve its cooling using a fan and run on timer.

The run on timer described was based on a Chinese STC15F104E DIP8 8051 like microcontroller.

Because the programming tools for the STC chips work so poorly, and the lack of documentation of their protocol, there is no simple way to update only the calibration data in EEPROM. I have ported the algorithm to an ATTINY25 which doesn’t cost a lot more but had a much better development environment and a range of tools to allow EEPROM update without overwriting the FLASH image, and as well it will run my bootloader, ATB.

This article describes a generic run on timer based on an Atmel AVR chip, a ATTINY25 though the code will also run in ATTINY45 and ATTINY85.


The circuit is very simple, the DC output from the forward power detector is connected to the input pin which turns the BC548C transistor on at input voltage greater than about 0.7V. The high value of base resistor ensures very light loading of the forward power detector.
Continue reading A generic run on timer using an ATTINY25

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6

Sixth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

This article documents measurement of impedance.

Impedance measurement


The antenna system is a G5RV with tuned feeders (9m of home made 450Ω open wire). The tuned feeders terminate on the balun described in this series, and it is located on the outside of the antenna feed entrance panel shown above. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6

Precision GPS experiment #1

This article reports an experiment to evaluate the usefulness of precision GPS for the purpose of location data for automated antenna field strength surveys.

The experiment was conducted with the rover located in a fixed location 13km North of the reference station at Symonston and with very wide view of the sky, about 7:00 am 07/04/2016.

Only GPS satellites were used for the rover.

The software was RTKLIB v2.4.3b8.


The GPS was a UBLOX LEA-6T with a small patch antenna (as sold for small UAVs). The LEA-6T provides binary data as used by RTK for carrier phase measurements. Above is the GPS and a USB-RS232 adapter. Continue reading Precision GPS experiment #1

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

Fifth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Installation / testing


The balun packaged in a non-conductive housing was designed to have minimal stray capacitance to ground to minimise common mode current with asymmetric loads.


Above, the balun is attached to the exterior side of the antenna feed entrance panel using a male to male N adapter, done up very tight. The feed line connections are liberally coated with marine grease to prevent ingress of water and oxygen, a measure to reduce corrosion. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

Improved cooling for the MFJ-949E


At A look at internal losses in a typical ATU I demonstrated that it is quite easy to raise the temperature of the coil in the MFJ-949E to an unsafe level, even with quite modest power.

The most heat sensitive component in this ATU is the coil, specifically the coil supports which are probably polystyrene, and the glass transition temperature of polystyrene is around 100°.

This article documents modification of my MFJ-949E to reduce the risk of damage under some operating conditions. Continue reading Improved cooling for the MFJ-949E

CH340, CH341 USB to RS-232 chip compatibility

One of the several sources of chips for USB to RS-232 converters is WCH, a Chinese firm offering their CH340 and CH341 chipsets.


Information is scarce as their website is mostly in Chinese language.

CH34x chips have become more common, perhaps as consumers have fled the Chinese Prolific product line (for good reason).

In true Chinese style, downgrade knock offs of Arduino boards (eg the Arduino nano) have appeared, the downgrade is achieved by use of CH340 USB to RS-232 converters instead of the FTDI chips on the genuine nano. Continue reading CH340, CH341 USB to RS-232 chip compatibility

Some thoughts on a two turn small transmitting loop

Small transmitting loops (STL) are very popular with hams, and a fashion is developing for N turn loops. This article lays out some thoughts on a 2 turn STL.

Firstly, to the meaning of “small transmitting loop’. There are a range of definitions used, and they mostly centre around the concept of a size sufficiently small that current is approximately uniform. The issue is about the meaning of sufficiently. Accuracy of estimation of radiation resistance of small transmitting loops sets out a rationale for a single turn loop for criteria that perimeter<λ/10.

This article will compare NEC-4.2 models of loops with the following key parameters:

  • 1m diameter (the loop perimeter is 0.07λ);
  • 20mm copper conductor;
  • frequency is nominally 7.1MHz;
  • 16 segments per turn
  • when not specified as in free space, the loop centre is 1m above ‘average’ ground (0.005,13);
  • the loop is directly fed in the middle, opposite to the tuning capacitor position, cap down;
  • pitch is 0.15m.

The model is sensitive to all these parameters. Continue reading Some thoughts on a two turn small transmitting loop

Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’

On review of the Ultrafire XML-T6 torch, I found the mode switching / mode memory so dysfunctional that it rendered the torch useless in my evaluation.

XML-T6This article describes a work around  that makes the thing usable (IMHO). Continue reading Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’

End fed Zepp

The so-called End fed Zepp (EFZ) is often cited as the basis for many more recent antenna designs, and is leveraged to provide and explanation… though few hams understand how the EFZ actually works.

End fed Zepp

Screenshot - 13_03_16 , 08_38_13

Above is a diagram from the ARRL Antenna Handbook  (Silver 2011).

Though a short conductor is shown to the right of the right hand feed line wire, the length is not specified or discussed in the accompanying text. It is popularly held that this is a “counterpoise” that provides a path for current equal to that flowing left into the main horizontal wire.

Let us explore the EFZ using NEC. The models are a reflection left to right of the above diagram, ie the feed is on the left hand end. Continue reading End fed Zepp