## Accuracy of estimation of radiation resistance of small transmitting loops

A simple formula exists for calculation of radiation resistance of a small transmitting loop in free space. The derivation is in most good antenna text books.

$$R_r=\frac{\mu_0c_0}{6\pi}A^2(\frac{2 \pi}{\lambda})^4\\$$

The expression depends on an assumption that current around the loop is uniform, so the question is what is the acceptable limit for loop size.

NEC might provide some guidance. A series of NEC-4 models of a octagonal loop of thin lossless wire in free space was constructed with varying perimeter. Perimeter shown is that of a circle of the same area.

Above is a comparison of the two methods of estimation of Rr. To the extent that we trust NEC-4, the graph indicates that error in the simple formula grows quickly for loop perimeter greater than 0.1λ. (The results using NEC-2 are visually identical.)

Many authors set the criteria for a small loop to perimeter<0.3λ, but it is clear that current is not sufficiently uniform for perimeter>0.1λ for estimation of Rr as 31149*(A/λ^2)^2 to 0.1pu error or better.

## Small transmitting loop – ground loss relationship to radiation resistance

This article documents a series of NEC-2 models at 7.2MHz of a lossless small transmitting loop near ground for the insight that they might provide about underlying loss mechanisms.

Key model details:

• lossless conductor 25mm diameter;
• octagon of sides 403mm, has same enclosed area as a 1m diameter circle;
• three ground types;
• height varies from 1.5-10m to centre of loop.

Impedance elements discussed in this article are referred to the main loop. Continue reading Small transmitting loop – ground loss relationship to radiation resistance

I bought a USBTiny AVR programmer on eBay for about A$8 posted. Above, the seller’s pic of the package. It is almost always the case that the ISP headers on the programmer use the standard pinout published by Atmel, and in that case the supplied ISP cables need to be pinned pin for pin, ie pin 1 to pin 1 etc. If you look carefully at the pic, the key is towards the top of the pic which means pin 1 on the right hand plug is towards the viewer and pin 1 on the left hand plug is away from the viewer. The cable does not connect pin 1 to pin 1, and as a consequence the package did not work. There is more than one way to connect these plugs, and above is one way that does connect pin 1 to pin 1, and the cable and USBTiny work. Though the seller has been told of this defect, he continues to sell the item with the pic of the defective cable. One wonders how many thousands of these things are and will be sold with this defect. ## A tale of two USB-RS232 adapters There are a number of USB-RS232 adapters available, among them Silabs, FTDI, Prolific and WCH. The Silabs CP series adapters have been reliable but not very widely used. The Prolific adapters are, well, prolific… but given their major compatibility problems and poisoned drivers, they are not a good choice. FTDI was a good choice until they distributed through Microsoft Windows Update their device poisoning trick to disable chips they thought counterfeit… though they seemed to have backed away from that action. The new kid on the block is an adapter by Chinese company WCH, chips that appearing in lots of low cost devices, eg Arduino Nano clones with WCH adapters instead of FTDI selling for around A$3+ on eBay whereas the FTDI equipped Nanos are more like A$12. This article compares the WCH CH340G and FTDI FT232RL. Above are two inexpensive adapters for Arduino boards, the upper one is FTDI based at A$8 each (inc post), and the lower one WCH based at 3 for $3.60 (inc post). Continue reading A tale of two USB-RS232 adapters ## Effect of ground on HF horizontal dipole efficiency – more detail This article expands on Effect of ground on HF horizontal dipole efficiency with some more model detail for the technically minded. See the original article for model details and discussion. Above is the efficiency curve expanded to 80m height, about 2λ. The graph assumes no matching loss (mismatch loss, changed line loss). efficiency may be significantly poorer if not matched efficiently. Continue reading Effect of ground on HF horizontal dipole efficiency – more detail ## Solar power supply for foxflasher2 The Fox flasher MkII is designed to run directly from a 1S LiPo battery at 3.8-4.3V. The battery can be charged by a simple voltage limited charger to 4.1V with a small loss in capacity. This article describes a simple solar charger for such a battery. The regulator uses a TL431 precision programmable reference in a simple shunt regulator. Above, the 6V 1W PV array. Continue reading Solar power supply for foxflasher2 ## Arduino HMC5883 magnetometer – a tutorial This tutorial shows how to explore an inexpensive HMC5883 3 axis magnetometer module with Arduino. The magnetometer module can be purchased on eBay at very low cost (<$3), and is an ideal educational project for the budding Arduino practitioner.

It is an ideal low cost project for a first exploration of explore I2C, and the basis for a digital compass. This particular module breaks out DRDY which is useful for interrupt driven applications.

Fig 1 shows the test setup:

• at the top is a Arduino Pro 328 with Opti bootloader and FTDI adapter for programming;
• middle is the HMC5883L module; and
• bottom is a Logic Shrimp for logic tracing.

## Arduino stepper motor – a tutorial

This tutorial shows how to explore an inexpensive stepper motor and driver board using an Arduino and included stepper library.

The stepper motor and driver board can be purchased on eBay at very low cost, and is an ideal educational project for the budding Arduino practitioner.

Fig 1 shows the test setup:

• at the top is a Arduino Pro 328 with Opti bootloader and FTDI adapter for programming;
• middle is a Logic Shrimp for logic tracing;
• bottom left is a 28BYJ-48 stepper motor; and
• bottom right is a driver module based on UNL2003.

## Arduino thermistor thermometer – a tutorial

This project was designed ad-hoc as a learning exercise for a friend who ‘needed’ to acquaint himself with Arduino.

What better than a practical exercise that demonstrates some key advantages and disadvantages of the Arduino environment.

The project was a simple digital thermometer to display normal environment air temperature, say from -20° to 50° using common and inexpensive Arduino hardware with firmware developed on the free Arduino IDE, all using hardware that was on hand. Continue reading Arduino thermistor thermometer – a tutorial