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Posts Tagged ‘Electronics’

DIY Magnetic Loop Antenna – Part 1

February 17th, 2012 |

Author: Owen KF5CZO

Do you live in a neighborhood with a restrictive antenna policy and despair of having a useful HF antenna?

Can you solder or know someone who can?

A magnetic loop antenna may be the answer and they are not as difficult to build as you might think. Like getting on the air for the first time or taking your license exam there is a certain amount of uncertainty when you first approach magnetic loop antennas, there are a few new ideas to grasp. However, thanks to other hams like Steve AA5TB there are tried and tested designs, calculators & building methods that are known to work and that you can follow.

At the heart of every radio and MLA (Magnetic Loop Antenna) is the resonant circuit. The combination of an inductor (a wire has inductance, but a coil of wire has more) and a capacitor (two conductors separated by an insulator) in a circuit will resonate or ‘ring’ at a certain frequency. Sound vibrations at a certain frequency can cause a piano string to vibrate in sympathy and a vibration of the correct radio frequency will cause a resonant circuit to electrically vibrate in sympathy.
Since there is no such thing as a free lunch, the sacrifice you make with a MLA is that it needs to be re-tuned whenever you change frequency on your transceiver. The frequency range over which it is resonant is very small, typically only a few hundred kilohertz at the most.

The materials you can get your hands on is going to decide the capabilities of your MLA. Ideally you’ll have a loop made from a conductor with very low resistance (usually copper) and a capacitor that can handle high voltages. A variable capacitor is required if you want to use your antenna on multiple frequencies but you can use or make a fixed capacitor if you operate on one frequency, for Eg PSK31.

A MLA calculator like the Excel spreadsheet from Steve AA5TB or this web page from 66pacific.com will help you to decide what size components you’ll need to make your antenna.

The four pieces of information required are:

What frequency or frequencies do you wish to transmit on?
How large do you want the loop to be (It should have a circumference less than 10% of the design frequency wavelength, both calculators help you figure this out)
The diameter of your conductor (Three quarter inch (0.75 inch) copper pipe is a good start)
How much power you want to use (The voltage across the capacitor is proportional to the input power to the MLA)

A MLA of a certain circumference will be more or less efficient based on the frequency you transmit at. It is worth changing the loop size in the calculator to get the best efficiency possible in your favorite band.

For example, my 30-10 Meter loop is 8.5 ft in circumference and 23% efficient at 10.1Mhz but 91% efficient at 28Mhz. This represents a difference of about 6dB or 1 S-Unit.

The diameter of the conductor determines its resistance and this becomes important due to the large current flowing through the loop. Large diameter copper pipe is better since the radio frequency current flows primarily on the outside of the conductor rather than the core. Typical 3/4 inch plumbing pipe is a good balance between low resistance and weight, Heliax coax is good for portable antennas where the loop has to be rolled up into a compact space.

Probably the most problematic part of the MLA is the tuning capacitor. While everything else can be found in your junk box or on a shelf at the local hardware store the capacitor takes a little more digging. Ideally it should be variable from a minimum to a fairly high capacitance (0-300 pF would be good) it should also be able to withstand high voltages. If you are planning to run 100W then you could expect at least 4000 volts across the capacitor. This is not as bad as it sounds as it takes 3300 volts to jump just 1mm or .04 of an inch.

Ideally you could purchase a vacuum variable capacitor, the Rolls Royce of high voltage variable capacitors. You would expect to pay anywhere from $100 up when buying a 10 – 500 pF unit on Ebay and a LOT more if you buy new. If that is the way you want to go then great, you’re all set. If not then read on …

There are several alternatives to the vacuum variable capacitor that will work almost as well. The primary concern now becomes resistance and plate separation. For QRP power levels you can use the tuning capacitors from old vacuum tube radios, the type that have a set of fixed metal plates and a set of moving plates that mesh into them.

For higher power there are two other types of capacitor that are suitable.

One is the split-stator capacitor, so called because each terminal of the capacitor is connected to a stator that is electrically isolated from the other and the frame. The rotors and shaft form the rest of the circuit so there is very low resistive loss and no sliding contacts.

The other type is called a butterfly capacitor due to the shape of the rotor plates. In this type the stator plates are placed opposite each other with the butterfly rotor in between. When each of the butterfly “wings” are fully meshed between the stator plates the capacitor is providing maximum capacitance, when rotated 90 degrees the wings are completely un-meshed and the capacitor is providing minimum capacitance. Because of the construction of butterfly capacitor there is a higher minimum capacitance that should be noted when planning the frequencies your loop will cover.

If all else fails then you can construct your own variable capacitor. The idea is to have two conductors separated by an insulator, the larger the area of the conductors and the better the insulator the higher the capacitance. Sliding metal plates, trombones of copper tube and even Coke cans have been used as variable capacitors.

A fixed capacitor can be created out of coaxial cable if you intend to operate on one frequency and a table of approximate capacitances for different types of coax is included here, click to enlarge the table below.

You can cut a length of coax a bit longer than the length suggested by the table above and trim it to frequency once it is attached to the loop. Make sure there are no stray pieces of braid between the shield and the center conductor as the voltage rating is determined by the spacing of the two closest conductors.

Part two of this post will continue with calculating the antenna dimensions and performance.

One important thing to remember … If you don’t have access to the best materials then use what you have and improvise. If you don’t have copper pipe then use coax or heavy wire. If you don’t have a high end variable capacitor then use what you can find and keep the power levels QRP. The only antenna that is a complete failure is the antenna you never get around to building.

Now I understand – Phase Locked Loops

February 15th, 2012 |

Author: Owen KF5CZO

Every now and then I come across great books or videos that explain a concept in such a way that it becomes immediately obvious what is going on. I’m a great believer in learning by demonstration or even better, learning by doing.
I came across another explanatory video recently that I thought was too good to keep to myself. It covers a topic that was a complete mystery to me: Phase Locked Loops. We utilize them in almost every modern transmitter and receiver yet most people I have talked to view them as a black box that, fortunately, does its job well and usually without interruption.
The video below does a good job on opening the black box and showing just what makes phase locked loops … well, lock.

Hellschreiber and microprocessors – Bridging more than 80 years

January 17th, 2012 |

Author: Owen KF5CZO

ZL1HIT (Bryan Rentoul) has bridged a gap of more than 80 years by combining the text transmission system developed by Rudolf Hell in the late 1920’s with current microprocessor technology.

A sample of received Hellschreiber test from Bryan’s beacon

Hellschreiber sends a line of text as a series of vertical columns. Each column is broken down vertically into a series of pixels, normally using a 7 by 7 pixel grid to represent characters. The data for a line is then sent as a series of on-off signals to the receiver, using a variety of formats depending on the medium, but normally at a rate of 112.5 baud.

This process was historically accomplished with mechanical equipment but there are very few examples of this equipment still in operation and it is now sent and received by computer. Hellschreiber is very tolerant of noise and interference and requires only simple transmitters and receivers to work effectively.

German Hellschreiber unit in operation

With a microprocessor generating the digital on-off signals a simple crystal oscillator transmitter can be used to form a beacon station, one that transmits a call sign and perhaps some other information over and over. Changing the transmitted message is as simple as reprogramming the microprocessor or having it respond to a connected input, for Eg. A thermometer, light sensor, switch, etc.

Receiving the signal and decoding requires a radio receiver capable of CW reception and a computer running any of several free software packages like FLdigi or Digital Master 780.

The ZL1HIT beacon using a PIC microprocessor and a simple crystal oscillator transmitter.

For more information and the PIC microprocessor source code please visit the web page of Bryan Rentoul here : ZL1HIT Hellschreiber / PIC Beacon

AmateurLogic.TV 33: One Jam Packed Show

September 22nd, 2011 |

Author: George W5JDX

George visits the Twit.tv studios and interviews Randy Hall, K7AGE. Tommy visits the Huntsville Hamfest. Jim builds an Audio Isolation Interface. Peter shows us the DATV QSO Party.

State of Electronics documentary

March 26th, 2011 |

Author: Alan VA3STL

Karl Von Moller has been putting together a documentary called ‘State of Electronics – A discussion on the electronics industry in Australia’. This looks like a fascinating and thorough look at the electronics industry in one country. Although focussed on Australia much of the content and discussion will be recognizable and similar to other countries. The final version has not been released yet but it looks like it will be great viewing when it is. Seems it is growing to three parts. I’m looking forward to seeing the finished documentary.

Interesting point to note is the documentary has been filmed using a Canon D5 MarkII digital SLR.

Below is the trailer followed by the ‘roll call’ of people who appear in the documentary.

Learning electronics with the aid of the Internet

February 3rd, 2011 |

Author: Alan VA3STL

You have to learn electronics by building circuits. However, it can be very intimidating to get started and once you have some familiarity you may need further guidance, especially in the art of circuit design. Last week I watched three excellent tutorial videos which show there are some who are putting excellent material on the Web to help others further their learning of electronics. Here are the three.

First video covers some basic, but necessary understanding of multimeters and their use. Essential knowledge for beginners. This is by Colin Cunningham of Makezine.com and part of his ongoing electronics video posts at Make magazine’s website.

The next one steps it up quite a lot in detail but it is Dave Jones of the EEVblog showing how a component can be selected by a designer for a circuit, in this case for a DC-DC booster. It a long video, but shows how to use online catalogues, conduct parametric searches as well as the essential examination of component datasheets. Well worth watching carefully.

FInally, what looks like a new video podcast series from component supplier Element 14 and Jeri Ellsworth. In this video Jeri designs a circuit for some music playing installation art, that is triggered by motion detected by a PIR. This is a great tutorial video and I look forward to seeing more of these. The link to the video is here.

The latest good news seems to be that today it was announced that Jeri is joining Element14′s “Ask an Expert” panel.

Showing Brownies simple electronics

January 30th, 2011 |

Author: Alan VA3STL

(Copyright V Steele)

Ginny, my wife, is a one of a group of leaders for a Brownie pack in a nearby village. Last week we showed the Brownies some simple electronics and they had some fun with a really simple circuit.

We had them put a number of LEDs onto a 3V button cell, rather like a LED throwies but without the magnet. Once they had an arrangement of three or four LEDs on a cell Ginny took photographs of them in a darkened room as they waved the LEDs around. Taking a photograph on a slow file speed setting (ISO 80) and over a relatively long period (15 s) the Brownies created some great art by ‘painting with light’. As well as the example above, a small selection is shown below.

(All photographs copyright V Steele)

There was about 22 or so Brownies and we had them work on an LED ‘circuit’ in pairs. They really enjoyed the fun of arranging the LEDs and working out which way around to place them. However, I think they had the most fun with the ‘drawing with light’.

While some were involved with the photography I showed others the fun that could be had with electronics and how circuits could make things work. To do that I showed them how I could not light up an ultra-bright LED with a used 1.5V battery on its own. But, when I used a few components, connected into a Joule thief circuit, then the LED would light up brightly with the 1.5V battery. I also had some plastic optical fibre too and they had fun watching light bending around corners.

We kept the exercise simple and very visual and they all seemed to enjoy it.

If you want to make your own Joule thief you may find the Make magazine podcast below interesting and useful to watch.