Monthly Archives: October 2011

Admiral Wombat

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I’ve been running my netSDR almost continuously lately, recording the
6900-7000 kHz 48 meter band. While going through some recordings, I found these two very short transmissions:

6925 kHz USB 2125z October 15, 2011
6930 kHz USB 0004z October 16, 2011

The first transmission begins with “Attention, all bunnies and monkeys, attention, all bunnies and monkeys, this is Admiral Wombat, this is Admiral Wombat.

It is difficult to make out the entire text, this is what I was able to pick out, with a lot of help from Guise Faux:

“Apparently you have been … from your followers. You are weak. The wombat species is strong. We will become the dominant species and have control over the drooling monkey population.”

“Monkeys, come to my side. If you … I will free you from Commander Bunny and I will supply you with all the banana pies you can eat.”

“Submit to me, Commander Bunny, or else.”

“No more rabbit domination. Wombats make their own rules. Monkeys send reception reports to admiralwombat@gmail.com, admiralwombat@gmail.com
Admiral Wombat will control all the monkeys…”

The second transmission is much longer and starts with music, The Wombats circa mid 1980s, and then repeats the spoken text from the first transmission.

I dutifully followed Admiral Wombat’s instructions, and was pleased to receive the following QR Code by return email”

Admiral Wombat QR Code

When decoded, it contained the following text message:

This verifies reception by the famous Chris by the even more famous Admiral Wombat about 1 minute into 10-16-2011 on 6930.

I for one welcome our new wombat overlords.

Analyzing Half Wave Dipole Antennas

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There are two characteristics that we’re particularly interested in:

First, the radiation pattern. This describes how well the antenna receives (or transmits) a signal in various directions. Below is the radiation pattern for the standard half wave dipole in “free space”, that is, without a ground below it. You can imagine it is in outer space, or so far above the Earth’s surface that there are no effects from the ground.

free space dipole radiation pattern

The antenna wire is oriented east/west. The image on the left is the horizontal pattern. Imagine you’re above the antenna, looking down. This is the pattern around the antenna, all 360 degrees of the compass. There are two main lobes, one to the north, and one to the south. This means that the antenna is particularly sensitive to signals to the north and south, and less so to signals to the east and west. For a transmitting antenna, most of the radiated signal is directed the same way. One rule for antennas is that the radiation patterns are the same for both transmitting and receiving.

The image on the right is the vertical pattern. Imagine you’re at the same height as the antenna, looking at it. The top of the graph represents the signal strength going up, the bottom going down, etc. In this case, there are two sharp nulls directly to the left and right of the antenna. These are in line with the antenna. What this is telling us is that most of the RF energy is directed around the line containing the antenna wire. Here is what it looks like in 3D:
free space dipole radiation pattern 3D

Now let’s make the antenna more realistic by putting it above the ground. In this case, we’re going to put a dipole cut for the 6.9 MHz pirate band 30 feet above the ground, which is probably a typical case for many listeners (and operators). Here’s the resulting radiation pattern:
free space dipole radiation pattern

Here is what it looks like in 3D:
free space dipole radiation pattern 3D

We can think about what happened. The ground obviously blocks reception of radio waves from that direction. Likewise, it absorbs most of the RF energy directed to the ground (some of it is reflected, especially at shallow angles). The resulting antenna pattern is directed upwards.

There’s actually a term for such an antenna – the NVIS – Near Vertical Incident Skywave antenna. Most of the RF energy is directed upwards, where it is then reflected downwards by the ionosphere. Good reception coverage is obtained for a distance of several hundred miles around the antenna, providing the frequency is low enough. If it is too high, the radio waves will pass through the ionosphere without being reflected. NVIS is commonly used below 10 MHz, although higher frequencies are possible with active solar conditions.

Similarly, such an antenna is more sensitive to radio waves coming almost straight down from the ionosphere, that is, from transmitting stations that are several hundred miles away. It’s basic geometry, the more distant the transmitting station is, the lower the angle of radiation.

On the other hand, if you want to reach distant listeners, you need to get more of your radio waves to be directed at a lower angle. If we double the height to 60 feet, here’s what we get:
e dipole radiation pattern

It’s a significant improvement, but the maximum radiation angle is still pretty high. If we triple the height to 90 feet, here’s what we get:
e dipole radiation pattern

That may actually be worse! The radiation pattern changes dramatically with height, often in difficult to predict ways.

A horizontal half wave dipole is still a very useful antenna for shortwave radio, especially for transmitting distances of several hundred miles. Further reception is certainly possible, when conditions are good. In the next entry, I’ll take a look at another type of antenna, the vertical.

SSB vs AM

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Previously, in Signal to Noise Ratios, I compared how the SNR affects the quality of the received signal, with some simulated recordings at various Signal to Noise Ratios.

I thought it would be interesting to also compare AM (Amplitude Modulation) vs SSB (Single Side Band) transmissions. While I’ve never been a huge fan of SSB (also referred to as Satan Side Band) for transmissions involving music, there’s no doubt that it does get out much better than AM.

Let’s take a look at the spectrum of an AM signal (click on it to enlarge it):

AM Spectrum

You can see the carrier on 9980 kHz, which consumes most of the transmitter power. Indeed, for a 100% modulated AM transmission, the carrier consumes half of the transmitted power. The carrier power is constant, so for less than 100% modulation (which is typical) the carrier is using more than half of the power. The carrier is necessary for demodulation of the sidebands at the receiver, but conveys no useful information.

To the left and right of the carrier are the lower and upper sidebands. They are symmetrical about the carrier, and convey identical information. Each has the same amount of transmitted power. For the case of 100% modulation, each has one quarter of the total transmitted power. For the typical case of less than 100% modulation, each has less than a quarter.

Next is the spectrum of an SSB signal, USB (Upper Side Band) in this case (click on it to enlarge it):

USB Spectrum

This station is transmitting on 13270 kHz. There is no carrier, and only one sideband is transmitted. Remember that the carrier consumes at least half of the transmitted power, and each sideband uses half of the remaining power, or one quarter for 100% modulation. So in the case of SSB, for 100% modulation, four times the power is available for the sideband as compared to AM, for a given total transmitter power. Four times is equivalent to 12 dB, or two S units.

As you can imagine, this is significant. I’ve created some simulated recordings of USB signals. For these simulations, I assumed that the typical modulation would be about 50%. A 100% modulated signal would sound louder (less noise, better SNR).

Listen to the simulated recordings below to see the effects of various Signal to Noise Ratios:
0 dB Signal to Noise Radio (SNR)
6 dB Signal to Noise Radio (SNR)
10 dB Signal to Noise Radio (SNR)
20 dB Signal to Noise Radio (SNR)
40 dB Signal to Noise Radio (SNR)

It might also be useful to compare them to the previously generated AM signals:
0 dB Signal to Noise Radio (SNR)
6 dB Signal to Noise Radio (SNR)
10 dB Signal to Noise Radio (SNR)
20 dB Signal to Noise Radio (SNR)
40 dB Signal to Noise Radio (SNR)

An AM signal with a SNR of 0 dB is almost impossible to listen to, while an SSB signal, while difficult, is intelligible.

These results suggest that homebrew 10 watt SSB transmitters would produce signals that could quite easily be received by listeners, in cases where an AM transmitter of the same power level would produce a weak signal with an SNR too low to be readily received. The problem, of course, is that SSB transmitters are much more difficult to construct. Ham transceivers are of course quite easy to obtain, and used ones are often relatively inexpensive (although not as cheap as the $30 or so it costs to build a grenade type transmitter).

Many operators run their SSB transmitters at full power, but it is possible that they would reach many of their listeners with lower power, possibly reducing the risk of FCC enforcement actions, if they are indeed related to power levels.

On a related note – why refer to SSB as “Satan Side Band”? While SSB is a far more efficient transmission method than AM, it does have one drawback. With an AM signal, being “on frequency” is not important. The transmitter and receiver frequencies can be off by hundreds of hertz, with virtually no impact on the received signal. The carrier is used in the demodulation (reconstruction of audio) of the signal by the receiver. As long as the carrier and sidebands fit within the receiver’s passband, the signal will be correctly demodulated.

This is not true with SSB. With SSB, there is no transmitted carrer. The receiver must produce it’s own carrier (often referred to as the BFO or Beat Frequency Oscillator in older radios). Ideally, the BFO frequency is exactly on the frequency of the missing carrier from the transmitted signal. In practice, there will always be an offset, due to neither radio being exactly on frequency. This offset is directly translated into an offset for all demodulated audio.

For example, if the radios are off frequency by 100 Hz, then all of the demodulated audio will be shifted by 100 Hz. For voice communications, this is not a serious problem. The speech can still be understood, and it is usually quite easy for the listener to adjust the received frequency until the audio “sounds right”.

The problem is with music. Here, even small tuning errors of ten Hz can cause the audio to “not sound right”. If you know the song in question well enough, you can adjust the received frequency until this error is reduced enough. There are two potential remaining problems, however:

First, many digitally tuned radios cannot tune with infinite resolution, or even in 1 Hz steps. Rather, they may be limited to a 10 Hz tuning step. 10 Hz is still too much of an error for listening to music. Some radios get around this by having a knob that can be turned to adjust the BFO in an analog fashion (often called fine tuning, etc).

The second problem is drift. If the transmitter is drifting around (or the receiver, or both), then the tuning knob will need to be continuously adjusted to bring the station back on frequency.

While I’ve always preferred AM over SSB due to the audio quality, there’s no doubt that watt for watt, SSB results in a much better SNR for the listener.