Category Archives: RF

Spying on your neighbor’s grill thermometer – Monitoring the 433.92 MHz ISM Band with an RTL Dongle

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Remote weather stations, some car key fobs (although many in the US use 315 MHz), wireless grill thermometers, and many other devices use the 433.92 MHz ISM (Industrial, Scientific and Medical) band. Chances are good that if it is a wireless sensor, it uses this band.

Here is a waterfall showing transmissions observed here, using one of the inexpensive USB RTL DVB-TV Dongles:

The entire waterfall occupies 139 seconds.

You can observe several periodic transmissions. I have a remote weather station and a remote thermometer, so that accounts for two of them.

If you have an RTL tuner dongle, take a look and see what 433 MHz transmissions are occurring near you.

Solar Storms (plus Solar Hurricanes, Typhoons, and other ways the Sun will vaporize us or even worse, possibly cause your iPhone to not work)

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Over the past few years, there has been dramatically increased media coverage of solar flares, and the effects they can have on the Earth, primarily on our electrical distribution and communications systems. The emphasis has been on the ability of the flares to cause geomagnetic storms on the Earth, which then can induce currents in our electrical power grids, causing them to go offline from damage. There has been a tremendous amount of fear instilled in the public by the hundreds, if not thousands, of news articles that appear every time there is a solar flare.

Those of us who are shortwave listeners or amateur (ham) radio operators are typically familiar with solar flares, and some of the effects they can cause. To summarize:

A solar flare is a sudden brightening of a portion of the Sun’s surface. A solar flare releases a large amount of energy in the form of ultraviolet light, x-rays and various charged particles, which blast away from the solar surface. The x-rays can have an almost immediate effect on the Earth’s ionosphere, the layer of charged particles above the atmosphere, which allows for distant reception of shortwave radio signals. I discuss the effects of x-rays on the ionosphere this earlier article. Energetic solar flares can cause what are known as radio blackouts, where all of the shortwave spectrum appears to be dead, with few or no stations audible. Other communications bands, such as AM / medium wave (MW between 526-1705 kHz), FM, TV, and cellular phones are not affected. Just shortwave. Also, the portion of the Sun producing the flare must be roughly facing the Earth to have an effect, and only the sunlit portion of the Earth is affected.

The solar flare can also cause a Coronal Mass Ejection (CME), which is a burst of energy, plasma, particles, and magnetic fields. The CME typically takes one to three days to reach the Earth. When it does, it can cause a geomagnetic storm, which is a disruption of the Earth’s magnetosphere. The magnetosphere is a region of space surrounding the Earth, where the magnetic field of the earth interacts with charged particles from the sun and interplanetary space.

The CME can produce very high radiation levels, but only in outer space. If you’re an astronaut on the International Space Station, this could be a concern. If not, you don’t really have much to worry about. High altitude airline flights can result in somewhat higher than usual radiation doses, but high airline flights always result in higher than usual radiation doses, due to less atmosphere protecting you from cosmic radiation. You might just get a little more especially if you fly over the North Pole. Or over the South Pole, but I don’t think there are too many of us who do that.

The effects of a geomagnetic storm:

The Earth’s magnetic fields are disturbed. This can cause compass needles to deviate from their correct direction towards the poles, and has been frequently mentioned in medieval texts.

Communications systems can be impacted. As with solar flares, shortwave radio is most affected. AM can also be affected to some degree. FM TV, and cell phones are not affected.

Back in 1859, there was a large geomagnetic storm, often called the Carrington Event because the solar flare that caused it was seen by British astronomer Richard Carrington. The effects were dramatic. Aurora were seen as far south as the Caribbean. Telegraph lines failed, and some threw sparks that shocked operators. Storms of this magnitude are estimated to occur about every 500 years. Other very large geomagnetic storms occurred in 1921 and 1960, although neither was the magnitude of the 1859 storm. The term “Carrington Event” has now come to mean an extremely large geomagnetic storm that could cause devastating damage to the communications and electrical systems around the world. But these forecasts are often based on the notion that, with more communications and electrical systems in place, we are much more reliant on these systems and vulnerable to disruption, meanwhile ignoring the fact that we better understand how geomagnetic storms cause damage, and what can be done to prevent it. Remember, this was 1859, and very little was even known about how electricity worked, let alone the effects of geomagnetic storms. This was in fact the first time that the relationship between solar flares and geomagnetic storms was established.

Communications satellites can be affected due to the higher radiation levels and unequal currents induced in various parts of the satellites. This could cause the satellites to temporarily malfunction, or even be damaged (which could affect FM, TV, and cell phone calls, which would otherwise be unaffected). As satellites are always in a high radiation environment, they are protected, and it would take very severe conditions to cause extensive damage.

Between 19 October and 5 November 2003 there were 17 solar flares, including the most intense flare ever measured on the GOES x-ray sensor, a huge X28 flare that occurred on November 4. These flares produced what is referred to as the Halloween Storm, causing the loss of one satellite, the Japanese ADEOS-2. Bear in mind that there are almost a thousand active satellites in orbit.

GPS navigation can also be affected, due to variations in the density of different layers of the ionosphere. This can cause navigation errors.

But the effect that, thanks to media hype, everyone is most concerned about is the possibility of a solar flare causing a geomagnetic storm that destroys the entire power grid, leaving virtually the entire United States without power for weeks or even months. The good news is that this is highly unlikely to happen.

Here’s the scenario: The geomagnetic storm causes currents to be induced in the wires that make up the long distance transmission lines that connect the various electrical power plants and users across the United States, aka the power grid. If these currents become large, they can damage equipment such as transformers, leading to widespread power outages.

And indeed this happened, on a much smaller scale, on March 13, 1989. A geomagnetic storm caused ground induced currents that severely damaged seven static compensators (devices that are used to regulate the grid voltage) on the La Grande network in the Hydro-Quebec, Canada power grid, causing them to trip or shut down automatically. The loss of the compensators allowed fluctuations in the grid voltage, causing damaging to other devices. The net result was that over 6 million people in Quebec and the Northeastern United States were without power for about 9 hours. Another million were without power after the 9 hours. Parts of Sweden also had electrical power disruptions.

While being without power for 9 hours, or even several days, sounds dreadful, especially in this age of constant communications, it does happen routinely. Hurricanes and tropical storms often cause millions to lose power outage each year, as do snowstorms, ice storms, and thunderstorms. Even heat waves have caused massive blackouts. I was without power for a week after Hurricane Isabel in 2003. The concern with an extreme geomagnetic storm, such as a repeat of the Carrington Event, is that critical components such as large transformers could be damaged, which can take time to repair or replace. And there’s the fear that widespread damage to the electrical grid could result in more components being damaged than spare parts exist, causing even longer delays until they can be replaced.

In the two decades since the 1989 event, more protective devices have been installed, and electrical transmission line operators are more aware of the damage caused by induced currents from geomagnetic storms. Preventative measures, such as temporary blackouts for several hours until conditions stabilize, can prevent much of the damage from a large geomagnetic storm. The advanced warning of geomagnetic storms now possible due to the satellites that are continuously monitoring the Sun and the Earth’s geomagnetic field can give electrical transmission line operators the advanced warning they need to take preventative measures.

Also, the 1989 event occurred in Quebec, which is at a very northern latitude. Geomagnetic storms tend to be stronger near the poles, and less severe as you move towards the equator (much like how the aurora is commonly seen near the poles, but not elsewhere).

It’s also worth noting that there are actually three electrical grids in the United States: an Easter, Western, and Texas grid. They are not currently connected to each other, although there are discussions to do so.

Finally, while a repeat of the Carrington Event is possible, it is extremely unlikely (remember, they are thought to occur about once every 500 years). There are far more important things to plan for, such as blizzards, hurricanes, tornadoes, and even severe thunderstorms, which routinely do occur. It is certainly more prudent to prepare for events like these, by keeping batteries, portable radios, canned food, and jugs of water on hand, than to worry about an event that probably won’t happen again for several hundred years.

So, why all the media frenzy and public concern over solar storms?

First, the Sun operates on a roughly 11 year solar cycle. Solar activity, including the appearance of sunspots and solar flares, peaks about every 11 years, and then fades until the next solar peak. There’s a solar peak occurring right about now. The last one was in 2001. This was before Facebook, Twitter, and everyone spending several hours a day on the internet, obsessing about the crisis du jour. Or crisis of the year in this case. Back in 2001, very few people even knew there was such a thing as a solar flare, other than space scientists and ham radio operators.

Those of us who have been involved with radio related hobbies for some time are used to the 11 year cycle. As an SWL since 1978, I’ve witnessed several solar cycles. During a solar peak we get many more flares which disrupt reception, although the overall higher level of solar activity is actually beneficial to shortwave propagation. Plus, it’s more likely that we’ll get to see the aurora. Then things calm down for many years, until the next solar peak.

There’s also been a substantial increase in advocacy by scientists and other public officials for increased spending on solar flare / geomagnetic storm research and related programs. Obviously this is justified to some extent, as we are much more reliant upon technology, and even just electricity, than we were decades ago. Still, I wonder if things are being exaggerated just a wee bit. Government officials and those involved in research have a vested interest in increasing their budgets and staffs – it’s job security for them. I’m not suggesting any malice, pretty much everyone thinks their job is important, especially those in the scientific field. It’s human nature.

This increased advocacy has resulted in increased media coverage as well. I’m far less sympathetic here. The motto of many news organizations seems to be “If it bleeds, it leads”. Pretty much every time there’s a solar flare, there’s a flurry of news articles announcing impending doom. The titles are amusing, not only are there SOLAR STORMS, but also SOLAR HURRICANES, SOLAR TYPHOONS, and SOLAR TSUNAMIS. I haven’t heard of any SOLAR TORNADOES, but maybe next month. Invariably the articles describe how a solar flare can wipe out the entire power grid, sending us all back to the stone age. And this might be the one that does it! Of course, a day or two later, when the CME arrives and little happens other than poor shortwave radio listening and some enhanced Northern Lights, there’s no followup article. Although if there was, I’m sure it would state that while we dodged the bullet this time, the next flare will surely fry us all. And our iPhones.

Some examples:

Nasa scientists braced for ‘solar tsunami’ to hit earth

The Daily Telegraph disclosed in June that senior space agency scientists believed the Earth will be hit with unprecedented levels of magnetic energy from solar flares after the Sun wakes “from a deep slumber” sometime around 2013.

Cities blacked out for up to a year, $2 TRILLION of damage – with a 1 in 8 chance of solar ‘megastorm’ by 2014

Imagine large cities without power for a week, a month, or a year. The losses could be $1 to $2 trillion, and the effects could be felt for years.

‘A longer-term outage would likely include, for example, disruption of the transportation, communication, banking, and finance systems, and government services,’ the NRC report said, it was reported on Gizmodo.

‘It could also cause the breakdown of the distribution of water owing to pump failure; and the loss of perishable foods and medications because of lack of refrigeration.’

Solar Flare: What If Biggest Known Sun Storm Hit Today?

Repeat of 1859 space-weather event could paralyze modern life, experts say.

A powerful sun storm—associated with the second biggest solar flare of the current 11-year sun cycle—is now hitting Earth, so far with few consequences. But the potentially “severe geomagnetic storm,” in NASA’s words, could disrupt power grids, radio communications, and GPS as well as spark dazzling auroras.

The storm expected Thursday, though, won’t hold a candle to an 1859 space-weather event, scientists say—and it’s a good thing too.

If a similar sun storm were to occur in the current day—as it well could—modern life could come to a standstill, they add.

The news articles are bad enough, but I suspect the fact that 11 years ago no one saw articles like this, or even knew solar flares existed, has convinced a lot of the public that solar flares (of this magnitude and frequency of occurrence) are a new phenomena. It probably doesn’t help that this is the year 2012, and we’ve had the Mayan 2012 END OF THE WORLD nonsense to deal with for the last decade or so. I wonder if anyone has retconned Mayan history into them having a solar observatory and been aware of the 11 year solar cycle, and how it would peak in 2012, destroying the Earth. Maybe they even had an x-ray satellite in orbit. I bet the aliens that helped them build their pyramids left them one. The grays are helpful, like that.

Perhaps the most ironic part of this entire saga is that the 2012 solar cycle peak is forecast to be extremely low. Here’s the latest forecast and current progress through the cycle, click to enlarge it:

The peak smoothed sunspot number (SSN) is forecast to be about 60, vs the 120 for the previous cycle. The lower peak SSN means lower overall solar activity. That means fewer flares, and they should (overall) be less severe. The peak is also forecast to be in 2013, so I’m not sure how that works out for all the 2012 Doomsayers.

To put this further into context, here’s a graph showing all the previous solar cycles:

The red arrow points to the cycle peaking in 1928, the forecast at the time (2009) was that the cycle we’re in now would be similar to that one, it’s since turned out that activity is even lower.

The largest peak is Cycle 19, from the 1950s. Many older ham radio operators have fond memories of Cycle 19, when radio propagation conditions were excellent. They were hoping for a repeat with Cycle 24, but that is clearly not the case. And Cycle 25 is currently being forecast by some to be even lower than Cycle 24, although it’s not worth putting much, if any, stock into long range solar cycle predictions. Predictions for our current cycle (24) from just a few years ago had it being as strong as, or even stronger than, the previous cycle, which is clearly not the case.

The period marked as the Maunder Minimum on the above graph was a period of extremely low solar activity around the late 17th century. Very few sunspots were noted during this time period.

While we are indeed entering the peak of a solar cycle, which means more solar flares (and more powerful flare), which can have impacts on the Earth, I believe the historical evidence shows that the doomsday scenarios proposed by many alarmists are not warranted. I would suggest checking with various websites such as http://www.spaceweather.com/ to keep track of when a solar flare has occurred. Not to panic that the end is near, but to know when to go outside and look at the Northern Lights. They can be quite beautiful.

Beating Carriers

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You’ve probably heard the low frequency beat that occurs when two closely spaced carriers are present, like in this recording.

Here’s what it looks like in a waterfall, taken with a netSDR:
beating carriers

The two bright greenish lines are the carriers, one at about 1620.0076 kHz and the other around 1620.0095 kHz (and wandering around). The result of the two carriers mixing is the difference frequency, 1620.0095-1620.0076=0.0019 kHz or 1.9 Hz. The higher and wandering carrier is the local college station (it’s actually about 12 miles away), the other station is probably WDND from South Bend, IN.

In case you’re wondering, the netSDR settings were a 200 kHz bandwidth (250 kHz output rate), and a 2,097,152 FFT with a resolution of 0.12 Hz.

An Interesting Example of a Station Going Long

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A fairly active pirate station the past week or so has been the “Fruitcake” station, which plays songs and sound clips related to, well, fruitcake. Hence the name. On December 20, 2011 at 2300 UTC I recorded a transmission of this station with my netSDR. What I ended up capturing was a very interesting and educational example of a station going long.

Here is a graph of the received signal strength:
Signal Strength in dBm

An S9 signal is -73 dBm, right about the received signal level at the beginning of the broadcast. There is some fading up and down, typical with shortwave radio. What’s interesting is that the change in signal strength seems to have a definite period, rising and falling every few seconds. After a few minutes, the period starts to become longer, and the amplitude of the variation also increases. About half way through the transmission, the amplitude becomes quite large. There is then one deep fade, one large increase in signal strength, and then the signal almost fades out, going down to about -95 dBm (about S4). Notice that 10 minutes ago it was S9.

Next, here is a waterfall of the recorded transmission:
Waterfall

A waterfall is a color coded representation of the signal strength of a band of frequencies over time. In this case, it shows us the signal strength from about 2300 to 2310 UTC, over a frequency range of 6900 to 6950 kHz. The blue background represents the weak background noise that is always present, in this case about -97 dBm. The brighter colors towards green represent stronger signals. We can see the station’s carrier at 6924 kHz, and the sidebands containing the audio modulation (this is an AM signal).

The change in bandwidth of the received signal about a minute and a half into the transmission is due to the audio that was transmitted, one song ended, and another sound clip, with wider audio, began.

This is an extremely educational image. We can see several things happening here:

1. The short choppy fades at the beginning of the transmission are evident.

2. As time goes on, the fades become more prominent, and we can see the increase in their period.

3. We can see the background noise levels increasing in amplitude. Look just outside the passband of the station itself, and you can see waves of increasing and decreasing background noise.

4. The fades all start at a higher frequency, and drift down to lower frequencies over time. This is a type of phenomena called selective fading, which you may have read about.

So, what is the cause of the selective fading? There are several possibilities.

One is when both ground wave and sky wave signals are being received. If there are phase differences between the two signals, they cancel out, reducing the received signal strength. Likewise, if they are in phase, they support each other, and add together, increasing the signal strength. One common example of this is with medium wave (AM broadcast) stations. When you are close to the station, the ground wave signal is extremely strong, and the sky wave is relatively weak, resulting in excellent reception with no fading. At a long distance away from the station, the ground wave is extremely weak or nonexistent, resulting in only a sky wave. Reception is weaker than the first example, but often reliable for stronger stations. This is why you can pick up AM stations over long distances at night. However, if you are at an intermediate distance, you can receive both the sky wave and ground wave. As the relative phase between them changes, you get fades. I’ve noticed this with a semi-local AM station. It has excellent reception in the daytime, but once evening approaches, reception gets very choppy. This is even before other stations begin to roll in.

I don’t think this is the cause in this case, as there should be little or no ground wave. And if there was, I would still be able to pick up the station after the band went long, since the ground wave was present. (Being HF instead of MW, the ground wave does not travel very far anyway)

Another possibility is due to propagation via both the E and F layers. In this case, it is again relative phase differences that cause the fading. I’m not sold on this scenario either, because I don’t believe the E layer would support propagation of 7 MHz signals. (E layer propagation should not be confused with sporadic E layer propagation that often causes VHF skip)

Next up, and the idea I am presently sold on, is propagation via both the F1 and F2 layers. During the daytime, when ionization is strongest, the F layer splits into two layers, the F1 at about 150-220 km and the F2 at 220-800 km. At night, the F1 layer merges with the F2 layer.

Perhaps, during the daytime, only one layer is responsible for NVIS propagation. My thought is that the F1 layer is providing the propagation, as it is the lower layer, and the first one the radio waves would interact with. Then, in the evening, when the band is going long and the F1 layer starts to dissipate allowing some radio waves to reach the F2 layer, propagation is occurring via both layers. Relative phase differences between the signals propagated by each layer cause the selective fading effects. Once the F1 layer completely dissipates, only the F2 layer is left, but it is unable to support NVIS propagation at 7 MHz.

Comments welcome and appreciated!

A comparison of three low power AM shortwave pirate transmitters

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Recently shortwave free radio station Channel Z Radio conducted test broadcasts using three different transmitters, all on the same frequency with the same antenna, a half-wave horizontal dipole cut for 6925 kHz, mounted about 40 feet high. As described in a recent article, this setup should be ideal for NVIS or regional operation.

It was interesting to see how closely theory predicted real world performance for signal intelligibility and propagation. For background information, see the September 2011 articles “Signal to Noise Ratios” for which simulations were run, and the related article “How many watts do you need?”

These recordings were made with a netSDR receiver, and a 635 ft sky loop antenna. The I/Q data was recorded to disk, and later demodulated with my own SDR software, which is based on the cuteSDR code. If you hear any glitches in the audio, that’s my fault, the code is still under development.

In all cases, I used a 4 kHz wide filter on the demodulated signal. I chose 4 kHz because examining the waterfall of the received signal, that seemed to encompass the entire transmitted audio.

First up, he used a Corsette transmitter, putting out 1.1 watts:
Corsette transmitter
The average received signal strength was -90.9 dBm. This is about an S6.
This recording was made starting at 1949 UTC

Next he used a Grenade transmitter, putting out 14 watts:
Grenade transmitter
The average received signal strength was -77.0 dBm. This is about an S8 signal.
This recording was made starting at 2010 UTC

Finally he used a Commando transmitter, putting out 25 watts:
Commando Transmitter
The average received signal strength was -73.4 dBm. This is almost exactly an “official” S9 signal.
This recording was made starting at 2028 UTC

The playlists for the three transmissions included several of the same songs, so I recorded the same song for these comparisons, to be as fair as possible. Listen for yourself to decide what the differences are.

It’s also interesting to compare the received signal levels to theory. A 10 dB increase in the received signal level is expected for a 10x increase in transmitter power. In the case of the 1.1 watt Corsette and 14 watt Grenade, we have a power ratio of 14 / 1.1 = 12.7, which is 11 dB. So we expect an 11 dB difference in received signal strength. We actually had a 90.9 – 77.0 = 13.9 dB.

In the case of the Grenade vs Commando, we had a power ratio of 25 / 14 = 1.79, or 2.5 dB. We had a received power difference of 77.0 – 73.4 = 3.6 dB, very close.

Comparing the Commando and Corsette, we had a power ratio of 25 / 1.1 = 22.7, or 13.6 dB. We had a received power difference of 90.9 – 73.4 = 17.5 dB.

I went back and measured the background noise levels during each transmission, on an adjacent (unoccupied) frequency, with the same 4 kHz bandwidth. During the Corsette transmission it was -98.1 dBm. During the Grenade transmission, it was -97.8 dBm. And during the Commando transmission, it was -95.9 dBm.

So it seems the background noise levels went up as time went on, possibly due to changes (for the better) in propagation. This might explain why the measured power differences were larger than we expected from theory – propagation was getting better.

Still, it’s nice to see how close our results are to theory.

Speaking of theory, I am ran some predictions of the expected signal levels using DX ToolBox. Obviously I have no idea where Channel Z is located, nor do I want to speculate. But since this is NVIS operation, selecting any location in a several hundred mile radius produces about the same results (I played around with various locations). So I selected Buffalo, because I like chicken wings. Here are the results:

1 Watt Corsette Prediction:
1 watt calculated signal level

14 Watt Grenade Prediction:
14 watt calculated signal level

25 Watt Commando Prediction:
25 watt calculated signal level

Ignore the box drawn around the 1700z prediction, that was the time today that I ran the software. You can see that for the 1 watt case, it predicts S5, for 14 watts between S6 and S7, and for 25 watts about S7. Numbers lower but in line with what I experienced. Note that my setup uses a 635 ft sky loop antenna, which likely produces stronger received signals than estimated.

You also see that the signal strength curves upwards as time goes on, showing an increasing signal. This is also what I experienced with the increasing background noise levels, and suspected increase in received signal from Channel Z from the first to last transmission. As it got later, the signal increased. This is something I have experienced with NVIS – the signal improves, until the band suddenly closes, and the signal level suddenly drops.

My thanks to Channel Z for running these tests on three of his transmitters, I believe the results are very interesting, and shed some light on how well signals with different transmitter power levels get out, under the same conditions.

Comments welcome and appreciated!

An AD8307 Based RF Meter

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The AD8307 IC is advertised as being a “Low Cost, DC to 500 MHz, 92 dB Logarithmic Amplifier”. This is a block diagram:
AD8307 Block Diagram

From the AD8307 datasheet: The essential purpose
of a log amp is not to amplify, though amplification is utilized to
achieve the function. Rather, it is to compress a signal of wide
dynamic range to its decibel equivalent. It is thus a measurement
device. A better term may be logarithmic converter, because its
basic function is the conversion of a signal from one domain of
representation to another via a precise nonlinear transformation.

And here’s a basic AD8307 circuit, mine is similar:
AD8307 Circuit

In my case, I have an LC filter on the incoming DC power, as well as the outgoing DC signal level, to reduce noise pickup. My meter is built into a small paint can, on the underside of the lid, which works as an excellent ground plane:
RF Meter

And here is the top of the lid, mounted on the can:
RF Meter
The toggle switch isn’t being used. I was going to power the meter off of a 9 volt battery to further reduce noise, but comparison tests between the battery and DC power supply showed no difference.

The output of the meter is a voltage proportional to the input power, measured in dBm. Zero volts is output for −84 dBm,
corresponding to a sine amplitude of 20 μV. There is a noise floor, and the specified range of the AD8307 is −74 dBm to +16 dBm. The output voltage increases by 25 mV for each dBm increase in RF input.

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.

Signal to Noise Ratios

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In a previous entry, How many watts do you need?, I discussed how transmitter power affects the received signal, and touched on the concept of the SNR, Signal to Noise Ratio. Seeing numbers expressed in dB is one thing, but actually hearing the difference between a station with an SNR of 10 dB and one of 20 dB is far more enlightening.

I created some simulated Signal to Noise Radio recordings. They were produced by mixing a relatively constant noise signal (actual static RF from a Software Defined Radio connected to an antenna) with a software generated AM modulated signal. One difference between these recordings and an actual station is that there is no fading, so real world conditions are likely to be somewhat worse, depending on the amount of fading the station is experiencing.

I’ve produced five recordings, with SNR’s of 0, 6, 10, 20 and 40 dB. A SNR of 0 dB means that the signal and noise levels are exactly the same. This is essentially the weakest signal that you could possibly receive. On the other hand, an SNR of 40 dB represents excellent reception conditions, say that of a local high powered MW station. The others obviously fall in between.

Remember that every 6 dB (voltage) of SNR is equivalent to 6 dB more signal (with the noise level held constant), in other words, doubling the transmitter power. Conversely, a drop of 6 dB is the same as cutting the transmitter power in half.

Let’s make up a crude example. A very strong pirate signal may have an SNR of 30 dB, somewhat weaker than a local station. Going from 30 dB to 10 dB, or 20 dB, is a change in transmitter power of a factor of 10 times. Going, for example, from 200 watt transmitter to a 20 watt transmitter. A 10 watt transmitter, half the power, would be 6 dB lower, or around 4 dB. It would be slightly weaker than the 6 dB simulated recording below.

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)

DDS-60 Direct digital synthesizer

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Recently I put together a DDS-60. DDS stands for Direct Digital Synthesizer. It is a way to generate arbitrary frequencies. Samples are fed to a D/A (Digital to Analog Converter) at a fixed clock rate (in this case 180 MHz derived from a 30 MHz oscillator). These samples are generated by a NCO (Numerically Controlled Oscillator). Think of it as a sine wave being generated point by point, at a fixed (depending on the ratio of the output frequency to the 180 MHz clock) number of degrees per sample. The output frequency can instantly be changed by just altering this degrees per sample value.

In the case of the DDS-60, any output frequency from 0 to 60 MHz can be generated. AD9851 DDS chip is used. This chip, along with a buffer/amplifier, low pass filter, and voltage regulator is all contained on a small (about one by two inch) board. The output amplitude is set by a small trimmer pot, with a maximum of about 4 volts peak-peak.

Three TTL level digital control lines are used to select the frequency. In my case, I have them connected to the parallel port of a PC.

I mounted the DDS-60 on the underside of the lid of a one quart paint can. The output goes to a BNC connector, there is also a 2.5mm barrel jack for 12V DC power, and a 9 pin D-SUB connector for the digital lines to the PC:

DDS-60 Board
There is a small LC filter (about 3 mH and 1000 uF) on the incoming DC power line.

DDS-60 board

Here is the resulting unit. Ugly, but it works!
DDS-60 in tin can

And here is the output on a scope:
DDS-60 output on an oscilloscope

So what can you do with a DDS?

First, it’s a very handy piece of gear for the RF test bench. You have a stable and precise source of RF that can cover the entire LF, MF, and HF bands. One of my next goals is to write some software to do automated testing and sweeps of RF, using an RF voltmeter as the input. I hope to blog about that shortly.

Second, you can use it as an exciter to drive an RF amplifier.

How many watts do you need?

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Let’s say you’re a ham radio operator, or even a (gasp!) pirate radio broadcaster. How many watts of transmitter power do you need to reach your target(s)? Well, if you’re the typical ham, the answer is easy – just crank up the transmitter RF output knob to max. If you’re the typical pirate, you may do the same, although you’re a little more cognizant of the risks involved. Higher power is more likely to cause RFI issues with the neighbors’ TV, and possibly get you some unwanted attention from the FCC.

The alternative is to run low power. In ham lingo, this is called QRP. Most transmitters let you adjust your power level, so you can just dial it down. But to what level? How low can you go? What you’re trying to accomplish is to be heard by your listener(s). That is, the received signal is large enough to overcome noise levels, both from other signals and static, as well as receiver noise. The latter is a concern at VHF/UHF frequencies, but essentially a non-issue for HF, where atmospheric noise always dominates.

The signal to noise ratio (SNR) is defined as the ratio between the signal and noise levels, and is usually expressed in decibels (dB). 0 dB means the ratio is 1, the signal and noise power levels are the same. a 10 dB SNR means the signal power is 10 times the noise power, 20 dB means the signal is 100 times (it is a log based scale). These are for power values, for voltage ratios the SNR is twice the power value. A SNR of 0 dB would just be barely detectable, in practice you need a few dBs for even a weak signal, and a SNR of 30 or 40 dB is considered an excellent quality signal.

Noise levels vary tremendously, of course. Atmospheric noise varies with the frequency (higher at lower frequencies) and time of day (higher at night, when static from distant thunderstorms is more easily propagated). Then there are the potential man made sources of noise, such as other stations, as well as unintentional noise from the multitude of TVs, computers, switching power supplies, and so on, which have all contributed to a rise in the noise floor over the years.

There are many software tools to estimate received signal levels, based on transmitter power levels and propagation conditions, such as DX Toolbox. Plug in the numbers, and you can get an estimate of the received signal level. It might even be close – there are a lot of factors to consider, and many of them are unknowns, or at least estimates, such as solar effects on propagation.

Another way is to actually measure the received signal level. The good news is that most shortwave receivers have an s-meter, to tell you how strong a signal is. The bad news is that most of the time, the s-meter is wrong.

First, there is no concrete definition of how an s-meter should work. The ARRL suggestion is that an S9 signal is 50 microvolts at the antenna input, and that each S unit represents a 6 dB change in input voltage and power (that is, the voltage doubles, meaning the power level is 4 times higher). Tests on common receivers and transceivers show about a 1 to 5 dB per S unit change. That is, each increase in indicated signal level on the s-meter actually represents a smaller change in received power level, as compared to the theoretical 6 dB/S unit standard.

All other things equal, a change in transmitter power level causes a corresponding change in received power level. So if you double your transmitter power, the received signal will also double. According to the ARRL standard, increasing the transmitter power by a factor of 4 would add one S unit, in practice with most receivers it would add several S units, depending on what the original received signal level was. Again, the s-meter is just an indicator, the actual received signal level is what is important. Doubling the transmitter power will increase the signal, and SNR, by 3 dB. Likewise, cutting it in half will reduce the SNR by 3 dB.

So, let’s assume we have a transmitter running at 150 W (a pretty reasonable value for a good old fashioned tube rig like a Johnson Viking II). And let’s assume that the received s-meter reading is S9 dB, a very good signal, and it’s nighttime with a noise level, as indicated on the s-meter, is S4.

Here’s the Icom IC-730 S-meter sensitivity values from the previous link I gave:
S1 - 2 1.4 dB
S2 - 3 1.3 dB
S3 - 4 1.6 dB
S4 - 5 2.3 dB
S5 - 6 1.8 dB
S6 - 7 3.2 dB
S7 - 8 3.1 dB
S8 - 9 4.0 dB
S9 - S9+10dB 5.6 dB
S9+10dB - S9+20dB 7.3 dB
S9+20dB - S9+30dB 6.6 dB
S9+30dB - S9+40dB 10.5 dB
S9+40dB - S9+50dB 11.3 dB
S9+50dB - S9+60dB 13.5 dB

Ok, so let’s see what happens as we reduce the transmitter power. Each time we cut it in half, we reduce the received signal by 3 dB. Reducing it to a quarter would be 6 dB, an eight would be 9 dB, and a tenth would be 10 dB. Got it?

Looking at the chart, going from S8 to S9 is a 4 dB change. That would correspond with reducing the transmitter power by a factor of 0.40, or down to 60 watts. Going from S8 to S7 is 3.1 dB, a power reduction to 0.49, or 29.4 watts. S7 to S6 is 3.2 dB, a factor of 0.48, or down to 14.1 watts. S6 to S5 is a factor of 1.8 dB, 0.66, or 9.3 watts. And finally S5 to S4 is 2.3 dB, a ratio of 0.59, or down to 5.5 watts.

So what does all this mean? Well, if we dropped our transmitter power from 150 watts to 5.5 watts, the received signal would drop from S9 to S4. We stopped there because the noise levels were S4. At this point, the signal is barely audible. At 9.3 watts, pretty close to the magic 10 watts that most grenade type transmitters put out, the received signal is S5, one S unit above the S4 noise, 2.3 dB above, so an SNR of 2.3 dB. Something you could listen to, but it would really be down in the noise.

What about the original 150 watts that produced an S9 signal? Well, let’s just add up our dBs. 2.3 + 1.8 + 3.2 + 3.1 + 4.0 = 14.4 dB. So in this case, the SNR is 14.4 dB. Not the 20 or 30 dB you’d expect from say the BBC, but certainly pleasant enough to listen to.

Obviously this is just one example. With different assumptions, especially noise levels, the results will be different. Much lower noise levels would allow weaker transmissions to be heard. If the noise was S1 instead of S4, that’s 4.3 dB of SNR right there. Likewise, higher noise levels intuitively imply more transmitter power is necessary. But I think these are reasonable assumptions for nighttime noise levels on 43 meters, and typical pirate transmitter power levels.

The numbers speak for themselves. The difference between the received SNR for a 150 watt and 10 watt transmitter is huge. Of course, as the difference between getting the knock and not is also huge. Assuming transmitter power levels have an influence on FCC enforcement activity…

A 30 mW unlicensed CW beacon was busted last year:
Last summer the F.C.C. DFed the Echo beacon that was on 11002 Khz. It had been running 30mW.
The FCC agent was kind and considerate and dropped further investigation. In fact he was respectful of the fact that it was
completely Homebrew and will under 100mW. There was no complaint
and the only one who cared about the beacon was the DF site in Maryland. The little beacon was disconnected and dismantled.
The operators pirate beacon days are over.

20 to 30 mW is extremely lower power, even for CW. This tells us two things: first, the FCC has very good ears, and can pick up weak signals. If they can pick up a 20 mW beacon, they can easily pick up your 10 watt grenade. Second, and more importantly, they are probably more concerned with the frequency the unlicensed station is using, than the number of watts. If you look through recent busts of pirates, you will see that they are mostly due to choice of frequency, as well as the unusual bust of Weather Radio, which appears to have been motivated by their use of the National Weather Service’s DECtalk speech synthesizer voice.

This is not to say that transmitter power plays no role. But it may not be the FCC’s primary enforcement trigger. The FCC is Complaint Driven. A scan through their Enforcement Bureau confirms this. Busts are mostly for FM pirates, likely based on complaints from licensed stations, as well as for other offending transmissions, such as those that interfere with cellular phone service.

Busts of HF pirates would also likely be due to complaints from licensed services, especially the military, which does use parts of 43 meters. Those BLEEP BLEEP BLAPPP sounds you hear are the TADIL-A/Link 11 system. I can imagine that 70s pop music or cut and paste audio loops interfering with them don’t go over well with the men in uniform. One call is all it takes for the offending pirate to suddenly be #1 on the FCC’s enforcement list.

Back in the 1990s, The infamous pirate Voice of the Night, operated by Lad, was QRMing a Havana/Moscow CW net on 7415 kHz. The operators could often be heard sending strings if FUFUFU in CW in response. Apparently it also annoyed certain radio monitors employed by No Such Agency, as Lad was quickly busted.

So worry about how many watts you’re sending into the ether, but also worry about your choice of frequency.