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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.

Above 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

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

Last update: 22nd April, 2016, 4:06 AM

USB-A to DC 5.5/2.1mm power cable – current carrying capacity

The growing popularity of 5V plug packs and Li-ion power banks with USB A connectors provides a convenient source of power for some projects, and a USB-A to 5.5/2.1mm DC cable is a possible connection option.

Scouring eBay turned up some sources, but one can never assess the quality of the things because usually there are no meaningful specification offered, and lets face it, they are Chinese.

Above are two sample 1m cables that I purchased, the left one for about A$1, and the right for about A$3.50 (posted).

Loop resistance of the cables was measured with Kelvin probes to assess their current carrying capacity from a voltage drop perspective.

DC loop resistance of the one on the right was 0.16Ω, so the maximum current for a 5% voltage drop is 5*0.05/0.16=1.6A… not quite a 2A rating.

DC loop resistance of the one on the left was 3.3Ω, so the maximum current for a 5% voltage drop is 5*0.05/3.3=0.075A… not even a 100mA rating.

This is not surprising, experience with USB-A to USB-micro cables has revealed similar variation, and an explanation why so many of these cables are hopeless in battery charging applications.

Last update: 20th April, 2016, 7:52 AM

PIK

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

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.

Last update: 22nd April, 2016, 12:58 PM

Counterfeit FTDI

I purchased a USB-RS232 adapter which claimed to use an FTDI chipset.

Above is a pic of the device, branded Hexin model HXSP-2108D.

The device delivered pretends to be FTDI to the extent it ships with a FTDI driver disk, uses FTDI’s VID and PID to identify to USB, and works to some extent with separately sourced FTDI drivers, but it does not use FTDI chips. Attempts to read the chip with FTPROG return an empty EEPROM that cannot be programmed… in fact it causes errors in FTPROG.

eBay effectively supports these sellers of counterfeit goods as they require return of the goods to the seller for possible refund, and in this case that would cost about the same as was paid for the goods.

The seller insisted that they would test it and return it or a replacement, carefully avoiding the question of whether it used a genuine FTDI chipset, further reason to not return it as it would just cost good money to get the same counterfeit product back..

eBay harbours counterfeiters, and whilst I have bought plenty of FTDI based devices that appeared genuine (eg using FTPROG), this Hexin product is an incomplete knock off.

Last update: 17th April, 2016, 2:13 PM

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.)

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

Last update: 7th May, 2017, 10:45 AM

Making sense of LED output figures

Browsing eBay for some high power LEDs for a current project created frustration in trying to wade through the stated performance figures (to they extent that they can be relied upon).

LEDs are often headlined as having some luminous intensity in candelas, but while that might seem to be a good measure of the ‘brightness’ of the LED viewed on-axis, it gives no information about the spatial distribution off-axis and the total luminous flux output or flux density.

I wrote a little online calculator that can be of assistance in finding the total luminous flux and flux density give luminous intensity and apex angle, Calculate luminous flux (lm) from luminous intensity (cd) and apex angle (°). (Note that specified luminous intensity is usually on axis and should be discounted by perhaps 20% to provide an average luminous intensity over the cone angle.)

Example 1

An example, an eBay seller advertises:

Specifications:
Source Material: InGaN !
Emitting Colour: 0.5W 10MM HI POWER White 0.5W LED
LENS Type: Water clear
Luminous Intensity-MCD: Typ: 290,000 mcd
Reverse Voltage: 5.0 V
DC Forward Voltage: 3.2 ~ 3.4V
DC Forward Current: 100mA
Viewing Angle: 40 degree
Lead Soldering Temp: 260¡ãC for 5 seconds
Power Dissipation: 500mW

Does it appear rational? Lets calculate average luminous intensity at 80% of 290cd, 232cd. Lets assume the viewing angle is the half power beamwidth.

Above is a calculation from the specifications. Of concern is the calculated luminous efficiency of 266lm/W, it is perhaps three times or more the expected value, so it questions the accuracy of the claims. Even at 0.5W input, the luminous efficiency is unrealistically high. Continue reading Making sense of LED output figures

Last update: 15th April, 2016, 2:20 PM

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))

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

Last update: 7th January, 2018, 7:27 AM

HC-500

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.

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

Last update: 7th October, 2018, 9:02 AM

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

Last update: 14th April, 2016, 7:32 AM

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

Last update: 13th February, 2017, 4:34 AM