Why is Donald Duck on the radio?

AM, or amplitude modulation, was the earliest way of sending voice over radio waves. That makes sense because it is easy to modulate a signal and easy to demodulate it, as well. A carbon microphone is sufficient to crudely modulate an AM signal and diode — even a piece of natural crystal — will suffice to demodulate it. Outside of broadcast radio, most AM users migrated to single side band or SSB. On an AM receiver that sounds like Donald Duck, but with a little work, it will sound almost as good as AM, and in many cases better. If you want a better understanding of how SSB carries audio, have a look at [Radio Physics and Electronics] video on the subject.

The video covers the math of what you probably already know: AM has a carrier and two identical side bands. SSB suppresses the carrier and one redundant side band. But the math behind it is elegant, although you probably ought to know some trigonometry. Don’t worry though. At the end of the video, there’s a practical demonstration that will help even if you are math challenged.

Dealing with signals as math equations are a staple if you seriously study electrical engineering. For example, you can model a pure sine wave with the equation: S(t) = A sin(2*pi*F*t) where A is amplitude, F is the frequency in Hertz, and t is time. Try it. In that example, the frequency is 1 kHz and the amplitude is 20, but you can tweak the values and see what happens. If you add something to the argument to the sin function, you’ll change the phase of the wave. If you followed that, you should have no problem with the video.

This math isn’t just good for working homework problems. It is the same math you need to do synthesis with a computer or digital signal processor. If you want to dig in, we talked about phase angle mathematics, earlier.

But wait, what’s with that screwy title? Have you ever heard an SSB signal on an AM radio? Most people describe it as sounding like Donald Duck. Listen at about the 20 second mark of this mp3 file and just after that, too where it is Upper Side Band (the other kind of USB) but off frequency and you’ll hear that famous voice. If all of this is unfamiliar, you need to explore the speech transmission origins of AM.

Measuring HF signal speeds in a DIY coaxial collinear antenna

Air Traffic Controllers use Automatic Dependent Surveillance-Broadcast (ADS-B) as an alternative to secondary radar to track aircraft. The ADS-B is transmitted by the aircraft and contains information such as GPS position, pressure, altitude, and callsign among other things at a 1090 MHz frequency, which can be decoded using any of a number of software tools.

[Mike Field] lives near an airport, and decided he wanted to peek into the tracking signals for fun. He turned to an RTL-based TV Dongle. Since the stock antenna was not cutting it, he decided to make one specifically for the 1090 MHz signal. His design is based on Coaxial Collinear Antenna for ADS-B Receiver by [Dusan Balara] which uses pieces of the coaxial cable cut to the right length. There are a number of calculations involved in determining the size of the cable, however, the hack in this design is the way he uses a USB based oscilloscope to measure the speed of RF waves inside the line in question.

We reached out to [Mike], and this is what he had to say. The idea is to use a cable of half the size of the wavelength which is calculated as

lambda = c/f

For the best reception, the sections of coax need to be half a wavelength long – but the wavelength of the signal inside the coax, which is shorter than the wavelength in free space. As this was a generic cable he had no idea of the dielectric that separates the core from the shield, so the ‘velocity factor’ could be anything depending on the exact composition.

To determine the speed of the signal in the cable, his approach omits the more expensive equipment. A length of coax acts as a stub – any energy that is sent into the cable reaches the far end of the transmission line and is then reflected back to the source. When the cable is 1/4th of the wavelength long, the reflected signal arrives back at the start of the signal 180 degrees out of phase – in a perfect world it would completely null out the input signal.

[Mike] starts his experiment with a 10m cable as he needs a test signal with a wavelength of 40m. In order to get the test signal into the cable, just two resistors at the back of a connector are all that was needed. The diagram shows the 330 ohms and 100 ohms in series with the center point at around 75 ohm which is a match for the cable.

Using the Digilent Analog Discovery 2’s Network Analyzer the connector is swept from 1kHz to 10Mhz without the cable attached, and then with the cable attached. The dip at 5.666 MHz, caused by the reflected signal down the coax is very clearly seen. From there it is simple math – 40m/cycle * 5,666,000 cycles per second = 226,640,000 meters per second or 75.6% of the speed of light.

So the wavelength of the ADS-B signal is (226,640,000 m/s) / (1090,000,000 Hz) = 0.208m, and the desired length to cut is 104mm 1/2 wave elements and 52mm 1/4 wave elements and get soldering!

This is a great example of how a little bit of math and human ingenuity can be better than expensive test equipment and if you looking to get into software defined radios from scratch, start with Scratch.

HAM sale VRZA Zuid-Limburg 18 november 2017

Op zaterdag 18 november a.s. van 10.00 tot 14.00 uur organiseert de VRZA de jaarlijkse HAM sale in het clubgebouw aan de Floribertusstraat te Brunssum.

Zendamateurs uit de hele regio kunnen kosteloos een tafel reserveren voor het verkopen van hun spullen en iedereen is welkom als koper natuurlijk. De VRZA zorgt voor koffie, fris en vlaai tegen amateurprijzen. Het wordt vast en zeker, net als de vorige edities, een gezellige boel waar de amateurs elkaar weer eens treffen, ook niet-leden van de VRZA.

Tafel reserveren kan door een mail te sturen aan pa4ul@vrza.nl . Let op: de markt is alleen bedoeld voor het verkopen van spullen die zijn gerelateerd aan de radio-hobby dus niet zelfgemaakte kerstkaarten of moonshine.

Tot zaterdag 18 november!

Cursus zendamateur inventarisatie

In de afgelopen jaren hebben zich mensen gemeld die interesse hebben in een cursus voor radiozendamateur. Als verenigingen hebben we je laten weten dat we hiervoor een minimum aan cursisten nodig hebben om met een cursus te starten. Inmiddels lijkt het er op dat we voldoende belangstellenden hebben. Net zo belangrijk is dat Frans van Venrooij PA0VRO binnenkort een langer lopende klus kan afronden en daarmee weer tijd vrij krijgt om de cursus te geven. We verwachten dat er nog dit jaar kan worden gestart met een cursus; eind november/ begin december. Deze zal zich richten op de Full-vergunning. Frans wil, net als bij eerdere Novice cursussen, de lesstof beperkt houden tot datgene wat nodig is voor het examen. Er zal hiervoor een set bestanden met de beknopte lesstof worden vervaardigd. Het VERON cursusboek zou daarmee niet strikt nodig zijn. Mensen die voorlopig alleen voor de novice willen gaan kunnen ook deelnemen. Er zal duidelijk worden verteld welke onderdelen niet tot het Novice examen behoren. Als locatie is gekozen voor ’t Weverke in Schimmert. De cursus  worden gehouden op woensdag- of vrijdagavonden.

Op dit moment willen we het volgende van je weten:

  • heb je nog steeds belangstelling?
  • kan je de woensdagavond, respectievelijk vrijdagavond vrijmaken voor de cursus?

Mail je reactie naar Tom PC5D email pc5d@home.nl of 046-4527 847

Graag uiterlijk begin november je reactie zodat we e.e.a. concreter kunnen gaan plannen.