Pirate Radio – How To Avoid Causing Interference

During my Jr. High and High School years I ran a number of bootleg radio stations ranging up to as much as 1Kw, but mostly in the 25-100 watt range on the AM broadcast bands.

I had a number of friends who also ran AM and FM pirate radio stations, one ran a pirate SW station.

I just ran across this article, “The Story of Bootleg Radio 1610“, and for anyone considering operating a pirate radio station, it’s an excellent example of what not to do.

In all of the years I ran a bootleg radio station, I only ran afoul of the FCC once, and that’s when my antenna broke and resonated in the 80 meter ham band and caused interference.

Prior to that occurrence I had run a pirate radio station in a major metropolitan area for more than half a decade. And I’m not talking occasional operation, I’m talking daily multi-hour operation, during the summer months 24 hour operation, with an average power level of around 100 watts.

The lesson here is simple, while any operation without a license that exceeds part 15 is illegal; the laws were written with a purpose of preventing interference and when you cause interference you’re much more likely to invoke enforcement than if you avoid causing any interference. Even if you are operating completely legally within the constraints of part 15, you still may not cause interference to licensed operations.

Avoiding causing interference is a function of being knowledgeable and exercising careful engineering to create a clean signal that does not interfere with any legitimate licensed service. The higher the power level you operate at, the cleaner your signal must be in order to avoid causing interference.

Avoiding interference to licensed service involves first doing a careful survey to find a frequency upon which you will not cause interference to legitimate services. The AM broadcasting band has become so crowded that this is a very difficult if not impossible undertaking in many locations and at night when ionospheric reflection transmits signals over the horizon by reflection off of the ionosphere.

You want to find a frequency that not only is it vacant, but where there are no nearby stations on the adjacent channels that might receive interference from your sidebands or in cases where the receivers selectivity is garbage.

A good example of what not to do was set by a friend of mine who operated a pirate AM station on 1570 Khz AM when a local broadcast station was operating on 1590 Khz. In addition to being close in frequency; there was no limiter employed so that overmodulation would create additional sideband components by clipping the audio waveform. Interference to the commercial broadcast station brought the FCC field engineer to his house.

Most of the precautions you would take to prevent interference also happen to be good for your signals strength and quality. RF power that is outside of the intended frequency spectrum is RF power not going towards your intended transmission.

With AM stations, most of the commercial kits are garbage having either entirely untuned and unmatched outputs that are inefficient and have high harmonic content, or at best very low-Q output circuits that are only broadly tuned and do not provide good harmonic suppression.

I would suggest either building your own gear or using commercial broadcast quality gear. If you build your own, take the time to design your transmitter, matching system, and antenna. Be sure your oscillator is stable and within 20 Hz of the selected frequency. I would really suggest aiming to get it spot on. Most commercial stations will be very close and if you can do likewise you’ll avoid generating an audible beat note.

Take care to put together a high quality low distortion audio chain that includes a compressor and limiter or optimize your modulation and avoid any negative modulation peaks of 100% or greater. While theoretically, your audio power has to be half the DC final input power for 100% modulation, in practice I like to have about twice the audio power as DC input power to insure that modulation is highly linear up to 100% and also to allow for unsymmetrical modulation where the positive peaks may be allowed to exceed 100% while limiting the negative peaks to less than 100%.

Design your transmitter to meet all the requirements that a commercial station would have to meet save for the power levels. Pay careful attention to suppressing harmonics and any other out-of-band radiation. Design the output section, antenna, any matching or antenna tuning, to use tuned circuits of as high-Q as possible within the constraints of providing the necessary bandwidth for the desired audio frequency response.

Call around to local AM stations and you’ll often find that they have surplus equipment, old compressor / limiters, mixing consoles, cart machines, turn tables, etc, that you can acquire free or very inexpensively if you get to know the engineers.

Once you’ve designed and built your equipment, take extensive measurements and monitor it closely to be sure that it’s operating as expected. Some things I would suggest, first, be absolutely sure you have a decent modulation limiter. Anytime you exceed 100% modulation on the negative portion of the waveform, it is clipped because you can’t have less than zero carrier signal; that clipping generates high order harmonics that cause sidebands far outside of your intended channel and will interfere with other stations above or below your operating frequency.

A simple and yet highly effective modulation monitor for AM signals is an oscilloscope setup as a trapezoid modulation monitor. To do this connect a sample of the transmitters RF signal to the normal vertical input, and put an audio signal on the horizontal input.

Here are some example images. Forgive my art work, I know it isn’t the best, but hopefully it will serve to demonstrate the concepts.

0% modulation trapezoidTo the right is an example of a display without modulation. Since there is no audio input you see only the RF carrier signal deflecting the beam vertically.

Only your sidebands actually carry information, the carrier signal serves only as a reference for their decoding. An unmodulated carrier is a complete waste of power.

You should strive to design your program content so that this (dead air) never happens. High average modulation will mask noise where your signal is weak providing a higher perceived signal strength.

50% modulation trapezoidTo the left is a trapezoid modulation pattern with approximately 50% modulation.

The vertical lines indicate the minimum and maximum transmitter power points of the modulation envelope.

The slanted lines should be straight. Any curvature represents non-linearity in your modulation system and audio distortion results.

Non-linearity reduces audio quality and produces high order harmonics that will cause adjacent channel interference.

100% modulation trapezoidTo the right is a trapezoid modulation pattern with approximately 100% modulation.

In practice, you’ll want to set your limiter so that your negative modulation never quite reaches 100%. The trapezoid never comes to a perfect triangle or goes beyond that point.

No limiter will limit completely precisely owing to variations in the audio waveform so you need to allow a small margin of safety.

Imperfect frequency response, linearity, or phase shift in the transmitters modulation system may cause peaks to overshoot slightly.

125% modulation trapezoidTo left is an example of approximately 125% modulation.

Notice the line to the left of the trapezoid pattern. This represents the negative portion of the modulation envelope where the carrier has reached zero.

Also notice that at the other side of the trapezoid, the lines are no longer straight but taper off. This represents non-linearity in your transmitters output as it’s modulation capabilities are exceeded. This will also result in harmonics and adjacent channel interference but not as bad as going over 100% negative.

Note that if your transmitter is capable of exceeding 100% modulation on positive peaks cleanly then there is no harm in allowing positive modulation to exceed 100% and there is substantial advantage in doing so. Commercial broadcast stations are allowed to go up to 120% modulation on positive peaks.

This is why building your modulator to be capable of more than the theoretically required 50%, personal experience has taught me that it’s preferable to have about twice the audio power capability as DC input to the final, not that you will utilize all of this but it will insure your audio section is distortion free throughout the full range of modulation levels you might choose to utilize.

Commercial stations will often use clippers after the limiter followed by a low pass filter to remove any harmonics they generate but this really chews up the audio and makes it sound like crap so I don’t recommend it.

If you want to obtain good audio density I would make a number of recommendations, first a small amount of reverb injected into the audio chain, at a level where it is just barely audible, will provide additional density and cover noise.

Follow this with a good triband or multiband compressor. These break the audio into “bands” and compress each individually. This prevents things like a kick-drum or vocal cybalance from modulating the middle frequencies which are most noticable.

If you have the dollars, you can get a device called a peak inverter. Since you can’t exceed 100% negative modulation without extreme distortion and interference, but positive modulation can go arbitrarily high within the capabilities of your transmitter, a peak inverter will detect natural asymmetry in the audio waveform and switch the polarity such that the natural asymmetry is such that the peak is always greater in the positive direction. This allows you to modulate in excess of 100% without distorting the waveform of the audio source by taking advantage of any natural asymmetry in the waveform.

Lastly follow up by a fast asymmetrical limiter. I recommend setting the attack speed as fast as it goes, and the release speed as fast as you can get away with without the bass modulating the midrange. If you hear voices take on a chewed sound with bass notes, then the release time is too fast and the gain is following the bass waveform.

Set the limiter to limit the negative peaks a few percentage points shy of 100%, set the positive just shy of what your transmitter is capable of cleanly. If you see any rounding on the upper end of the trapezoid modulation pattern, you’re exceeding your transmitters capability. If you design your transmitter with a 2:1 audio to DC input you’ll never see this problem.

If you do these things you’ll have a loud, but clean, signal with good density that will mask noise to a large degree making the most of your coverage area.

Now assuming your transmitter, antenna tuning, and antenna are all designed well, your oscillator is stable and on frequency, you did a good survey before hand and found a truly unused channel with adjacent channels locally unused, your audio chain is clean, your modulation never exceeds 100% negative and positive doesn’t exceed your transmitters capability, you should have a professional sounding signal that won’t cause distortion and won’t sound amateurish (your program material may still be an issue).

Having done all of this work, I still recommend careful monitoring. You should have a good portable communications receiver capable of covering your operating frequeny and up to 30Mhz or so contiguously. I do not recommend getting the receiver too close to the transmitting antenna or you may fry your receivers front end.

Take your receiver about a block from your transmitting antenna while your transmitter is operating and modulated normally. Tune to the second adjancent channels to your transmitted channel. For example, if you are transmitting at 1000 Khz, then tune to 980 Khz and 1020 Khz and listen for any splatter from your station. If you hear splatter then something in your audio chain or the transmitter itelf is distorting your audio signal.

Then tune to harmonics of your transmitted signal, in the case of a 1Mhz transmission, tune to 2 Mhzs, 3 Mhz, 4 Mhz, etc. If you hear your signal, then your transmission isn’t clean and you either need to increase the quality of the tuned circuits in the transmitters output, antenna tuner, or the antenna itself, or possibly install a low pass filter between the transmitter and antenna.

If you’ve done everything to make sure your signal is clean, possibly even to the point of using a spectrum analyzer, and you still hear harmonics, there may be something non-linear object connected to something that receives your signal, rectifies it, and retransmits it. A good example might be metal gutters with a corroded joint. The gutter might play antenna and the corroded joint may act non-linearly distorting the signal and creating harmonics.

Not doing regular monitoring is the thing that allowed my broken antenna to resonate at 80 meters radiating a third harmonic of our transmitting frequency and drawing the attention of the FCC after six years of uneventful transmitting.

Leave a Reply