Going Long

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Have you ever wondered why other listeners are hearing a pirate with a very strong signal, while you can’t hear it at all? Or have you been listening to a station with a solid SIO of 555, only to have it fade to nothing, while others on IRC are still reporting solid copy? Chances are, the station was operating in NVIS (Near Vertical Incident Sky Wave) mode, where the radio signals go straight up from the transmitter, and down to the receiving site. NVIS is the mode used for all short distance communications on HF. Think of it as the opposite of “skip”.

In radio-speak, “going long” means a band is no longer able to support short distance communications. The maximum frequency that is reflected (actually refracted, but I’ll use the term reflected as most people are accustomed to that) by the ionosphere is a function of the characteristics of the ionosphere (due to solar activity), and the angle of the radio waves. The maximum frequency gets lower as the angle becomes more steep, reaching a minimum for radio waves directed straight up. This final frequency is called the foF2 or critical F2 layer frequency. Any radio waves directed straight up that are higher than this frequency will pass through the ionosphere into space. There is a real time foF2 map here: http://www.spacew.com/www/foF2.gifhttp://www.spacew.com/www/fof2.gif

For other angles, the maximum frequency that can be propagated is equal to foF2 divided by the sine of the angle. This tells is that as the angle gets smaller (not straight up) the maximum frequency increases. 

Critical Angle

In the above picture, paths A and B are at shallow enough angles that the radio waves get reflected back to the Earth. For path C, the angle is too steep, and the radio waves are not reflected, but pass into space.

During the daytime, the ionosphere is able to support propagation of higher frequencies. As the sun sets and the ionization levels start to decrease, the maximum frequency begins to drop. For a given frequency, shorter transmission path distances will be affected first. The path will “close” very suddenly, sometimes over the span of just a few minutes or even seconds.

Here is a graph plotting the received signal level for WFMT on Dec 10, 2011:

WFMT Signal Strength

The signal strength is shown in dBm. Refer to the previous entry How many watts do you need? for a refresher course in dBm. In general, an S9 signal is -73 dBm, every S unit is theoretically 6 dB, so S8 is -81 dBm, etc.

You can see that the signal was varying between -60 and -70 dBm, so about S9+10 dB. Then quite suddenly, it dropped to about -85 dBm, and then continued to decline to about -90 dBm.

Here is a closeup graph showing one minute of signal strength during the time WFMT went long. You can see that it went long between 20 and 30 seconds. That is, it only took 10 seconds.

one minute of signal strength

Looking carefully, you’ll also observe an increase in signal level just before WFMT went long. I have noticed this many times. My theory is that propagation is best when the incident angle of the radio waves to the ionosphere is very close to the critical angle. In this case, the incident angle is of course fixed, but the critical angle is changing as the ionosphere weakens to nighttime levels.

Note that even though the critical angle was exceeded, some radio waves are still being reflected, as the signal level has not dropped to zero yet (although it does continue to trend down, at some point the station will completely fade out).

The critical angle determines the maximum frequency that can be propagated between two points.

Remember from above that the maximum frequency that can be propagated is equal to foF2 divided by the sine of the angle. We can use some simple math to calculate what frequencies will work, knowing foF2.

The Maximum Usable Frequency (MUF) can be found by:
MUF = foF2 * sqrt( 1+ [D/(2*hmF2)]^2)

Where hmF2 is the height of the F2 layer. There is a map of the F2 height here:
http://www.spacew.com/www/hmf2.gifhttp://www.spacew.com/www/hmf2.gif

For example, if the distance between the two stations is 690 km, and the F2 height is 250 km, and foF2 is 3.5 MHz, then plugging into the above formula gives us a MUF of 5.97 MHz. So we can use frequencies up to that. But, if the stations were closer together, say 300 km, then the MUF is only 4.1 MHz. (Note: It’s for long distances, it is important to remember that the signal probably takes several hops, and you need to use a value of D that is the distance between stations divided by the number of hops)

Note that like foF2, hmF2 is continuously varying. At 1600 UTC on December 14, 2011, the foF2 is about 9 MHz over the eastern USA, and the mmF2 is about 240 km. So 43 meters is potentially open to anywhere on the east coast, even using NVIS. Of course this only takes the MUF into account, there is also the LUF, or Lowest Usable Frequency, which is mostly a function of transmitter power and D layer absorption.

At night, foF2 dramatically drops. Lately it has been going below 7 MHz in around 2300 UTC, turning off NVIS for 43 meter band transmissions. With an foF2 of 6 MHz (observed today at 2330z) the MUF is around 7 MHz for a distance of 200 miles.

For this reason, operators who want to reach their target area (east coast ops reaching east coast listeners) should consider using lower frequencies at nighttime. Years ago, the 3 MHz band was somewhat popular for pirate operations. Even somewhere in 5 MHz would be useful.

NVIS itself is worthy of an entry by itself, which is coming up next.

If you’re interested in getting real time propagation information, take a look at DX ToolBox which is available for both Windows and Mac OS X.

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.

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)

Two More Large Solar Flares

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We’ve had two more large solar flares already this morning, an X at around 0930 UTC and an M7 starting at about 1230 UTC that is still at M6 levels as I type this (1340 UTC).

x ray flare chart

The results are predictable, large fadeouts on HF, especially the lower frequencies. 31 meters is a graveyard, with very few signals, all weak. I am hearing China on 9845, probably because the path to the west of me is mostly in darkness still.

CFRX 6070, which is usually S9+, is about S3, with many deeper fades.

Update: There’s been some more flares today, with an M3.1 just peaking now, at 1730 UTC.

If you want to keep up to date with solar events, including flares and geomagnetic storms, you may want to give DX ToolBox a look. It runs on both Windows and Macintosh systems, and provides real time data and graphs. Plus a zillion other radio related features.

Another intruder into the 43 meter pirate band

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Apparently, now even the French are taking the fun out of pirate radio – literally. A new intruder to the 43 meter band has been tentatively identified as a French OTHR (Over the Horizon Radar) signal nicknamed Nostradamus. The official name is ONERA, and it is transmitted from a site at Dreux, west of Paris.

It is about 30 to 40 kHz wide. You can see two transmissions of it here (click on the image to view full scale):
Over the horizon radar OTHR
The first transmission was using about 6825 to 6845 kHz, then it switched to about 6900 to 6930 kHz, with a stronger signal. These were taken around 2120 UTC.

The signal is quite strong, and capable of obliterating a wide chunk of the 43 meter band. It will switch frequencies every few minutes, so the interference can come and go. When I observed it back on the air at 2225 UTC, the signal was a solid S8.

Thanks to The Hairy Clam for some technical information about Nostradamus.

For completeness, the strong signal you see around 6895, as well as the weaker ones around 6863 and 6938 are PSK utility transmissions. You can also see the carrier on 6860 from Radio Cairo, Egypt.

No word yet on whether Nostradamus sends out real paper QSLs, or worthless eQSLs.

X1.4 Solar Flare

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There was a solar flare this morning, starting at around 1030 UTC and peaking at a level of X1.4 at around 1100 UTC.

Solar flares are sudden bright areas that appear on the Sun. They emit a large burst of energy at various wavelengths, including x-rays, as the graph below shows:
X-Ray Graph of Solar Flare

A movie of the flare is available here: http://sdowww.lmsal.com/sdomedia/h264/2011/09/22/SSW_cutout_20110922T1024-20110922T1159_AIA_211-193-171_N09E89.mov

They also eject a large number of charged particles, such as electrons and protons. The effect of the x-rays is immediate, as soon as they reach the Earth (taking about 9 minutes at the speed of light) they cause a sudden increase in ionization of the ionosphere, but not in a good way. The D layer, which attenuates radio waves, rather than reflecting them, becomes strong enough to block large portions of the HF band. The D layer is what prevents long distance reception of MW signals during the daytime. With a stronger D layer, due to the flare, this attenuation moves up into the HF bands as well.

Depending on the intensity of the flare, frequencies to 10, 15, or 20 MHz, or possibly even the entire HF spectrum, can be completely blocked. The effect is only present on the portion of the ionosphere in daylight, nighttime areas are not affected. As I type this, at 1400 UTC, CHU on 3330 is completely absent. The x-ray levels are down to about the M1 level now, earlier I suspect higher frequencies were being attenuated as well.

Solar flares are categorized by their intensity at x-ray wavelengths. A log type scale is used, much as for earthquakes. An X1 flare is ten times as strong as an M1 flare, which is ten times as strong as a C1 flare, for example. Within each letter, numbers are used to further classify the intensity. An M2 flare is twice a strong as an M1 flare. An X1 flare would be equivalent to an M10 flare, if such a designation existed.

Although strong solar flares can be highly disruptive to HF, they do allow the reception of some signals that would otherwise be difficult to receive, depending on the various locations of the stations and your QTH. A flare could cause the signal from the dominant station on a particular frequency to be heavily attenuated (if the path between your location and that station is in the sunlight portion of the Earth), while allowing another station (if the path to that station is in darkness) to be received.

Realtime solar x-ray flux values, as measured by the GOES weather satellites, are available at this link: http://www.swpc.noaa.gov/rt_plots/xray_1m.html

There is also a graph with a slower (5 minute vs 1 minute) update rate here: http://www.swpc.noaa.gov/rt_plots/xray_5m.html

This particular flare was on the limb of the Sun, so any charged particles ejected by it are not likely to be directed to the Earth. When flares do produce Earth directed CMEs (Coronal Mass Ejections), the charged particles often produce geomagnetic storms when they reach the earth, producing aurora, and degrading MW and HF radio reception, for hours or even days.

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.