Identifying interference can be difficult. Things don't always behave like we first assume.

NDB Signals

Non-Directional Beacon or NDB 's are generally low to medium power transmitters. They are often in 25-100 watt power range, although in certain applications some can run kilowatts. With good antennas and low noise, NDB's can be heard over very long distances. Here's a recording of "OS" from Ohio to Georgia on 515kHz!

All of the airport or aviation NBD transmitters I'm aware of operate below the USA Standard AM Broadcast Band of 530-1700 kHz. NDB's are found near but not necessarily at airports of all sizes; ranging from grass strips to large International Airports. NDB transmitters can be used for other applications, so they are not necessarily airport related or on an aviation NDB frequency.

The primary low frequency aviation Radio Location Band in the US and Canada extends from 190-435 kHz, although there are licensed NDB's on other frequencies. For example 510.525kHz is a valid FCC issued RLB channel falling under Part 87 Aviation Service rules, and Canada (link) has 510-535 kHz listed as a RLB band in both Canada and the USA. 

NDB transmitters may or may not have an identifier or callsign that represents the associated airport. Often there will be no obvious association with an identifier and the airport or area of the beacon. For example the dirty poorly adjusted NDB transmitter at Peachtree City, GA repeats the identifier "FF".  If we look up "FF" at this link Beacon Search we see no obvious name or callsign association with Peachtree City Airport. This is true for many NDB systems. There also are duplicate identifiers or calls listed, such as CO.

The Peachtree NDB has all the signs of typical misaligned or poorly designed and improperly maintained beacon. FF at this time of monitoring (Feb 11, 2007) has:

1.) Negative carrier level shift on 316 kHz when the ID tone comes on. This can also be interpreted as image or negative keying.

2.) Spurious modulation products caused by overdriving the system including desired modulation tones at 315 and 317, with undesired illegal level spurious tones at 319, 318, 314, and 313 kHz.

3.) Excessive harmonic levels on 632kHz and other frequencies

I'll explain why and how problems like this commonly occur. Transmitters like these are an interference problem waiting to happen, and this is why people have logged dozens of transmitters at great distances on harmonics as high as the tenth harmonic and higher! I've personally logged 25 watt NDB's on the 11th harmonic at distances over 2000 miles! 

Identifying Frequency and Modulation

When we tune a modern amateur transceiver in the CW mode, a carrier or an additional  tone from amplitude modulating the carrier will be audible and match the CW pitch setting of the transceiver when the dial display indicates the approximate  frequency of the signal.

In other words in the CW setting with normal narrow CW filters, if I adjust my receiver to 7008, I will hear a 450Hz tone because I have my receiver setup to use with a 450Hz CW pitch. I will hear another 450 Hz tone that represents the Morse amplitude modulation tone of the transmitter when I tune to 7008.8 (this is the upper sideband) or 7007.2 (this is the lower sideband) or multiples of .8kHz up and down from 7008. The sidebands will have the desired keying information. The carrier may or may not have the negative or image of the desired modulation.

If I use the SSB mode and tune 7008 I will not hear any carrier! The only  thing I might hear is the modulation, since it is offset from the carrier.

When we deal with distortion products from an overdriven stage or a device that is clipping the waveform, the sidebands will NOT all be the same level. The characteristics of modulation will often vary considerably on all harmonics. You might find some harmonics that have a predominant upper sideband and some with a predominant lower sideband even if the transmitter is perfectly symmetrical in base modulation.

Most important properly identifying signal characteristics demands you have a very low noise environment compared to the signal's level. At my location the NDB carrier is up to 40dB above background noise. This means I can identify spurious products that are approaching 40dB below the carrier level. I can hear the USB voice transmission on every frequency so long as PUN carrier is 15-20dB out of background noise. 

We really must have good signal to noise to identify modulation characteristics properly. Poor or moderate S/N ratio often causes problematic transmitters to get "clean" bills of health. If we are giving a critical or exacting report of spurious signals or modulation we must have a very low background noise level and know how, what, and where to listen! 

I am fortunate enough to live in a very rural location without locally generated noise. The noise floor is limited by noise propagated in from the equatorial regions when I listen south.  When PUN is very strong compared to my noise floor I can hear several modulation harmonics that are multiples of approximately 800 Hz. If the noise was only 10dB stronger I would not hear many of the modulation products, and I would have an entirely different opinion of the modulation characteristics of this beacon. 

Because of wide variations in propagation and local noise and difficulty in identifying products, modulation characteristics are generally not important beyond the first few observations. With poor signal-to-noise it is easy to get the impression there is no modulation or a false impression of modulation characteristics.

Another important point is harmonics can actually appear to skip certain multiples. Say we have a transmitter on 400 kHz. There is no rule that we would hear all harmonics of 400 kHz at any location. We might only hear multiples of 800 kHz or 2400kHz from the 400kHz transmitter. We might hear multiples of any multiple of 400 kHz. This is true for any transmitter frequency.

We  also have to know how to use our receivers to measure frequency. In the CW mode we must tune for the proper pitch, in the SSB mode we zero beat to read frequency.   

NDB Transmitters

NDB transmitters are mostly modulated by keyed Morse code audio tones, but a few are also voice or digital telemetry modulated. Keying is at a slow speed and generally repeats a two or three letter group endlessly at slow speed(mp3 file) with very even spacing and no excessively long pauses. A larger airport NDB might include weather or other airport information like altitude or barometer readings in standard amplitude modulated voice, often this appears as a single voice sideband with carrier. 

Transmitter Construction

While initially high quality plate modulated AM transmitters with excellent filtering, the recent trend towards "cheap is good" by manufacturers has resulted in many problems with NDB transmitters. I've seen and worked on a few NDB transmitters over the years, and even parted out a few retired units.

The big design flaw in most NDB transmitters is they now almost exclusively use low level modulation. A system like that avoids expensive high level modulation circuits, but depends on multiple stages to be perfectly linear. Worse yet most transmitters use little- to-no harmonic suppression throughout the stages. They are built nearly like a broadband audio amplifier, depending almost exclusively on an external ATU (antenna tuning unit) for suppression of harmonics.

Transmitters almost always employ a MOV or zener diode clamp for lightning suppression. Even if the transmitter had some provisions for harmonic suppression, it would only take a high SWR or a bad component to generate harmonics in an otherwise perfect transmitter. 

Even with a properly functioning ATU and clamp, all it takes is a bit too much gain or a defective component in earlier stages to generate excessive harmonic levels. The FAA and manufacturers nearly always require use of a simple unreliable oscilloscope to determine proper adjustment. This is something any real engineer would laugh at, since excessive harmonics can occur without a noticeable or obvious flaw in the sinewave displayed on a conventional oscilloscope. This is why other services use spectrum analyzers, or better yet calibrated field strength meters, to verify harmonic performance.

Negative Image ID or CW

Anyone familiar with multiple tone testing of linear devices will immediately grasp the reasons behind the negative CW image that sometimes occurs from overdriven linear stages processing a tone modulated signal. It is a common effect when dealing with distortion products or non-linear devices processing complex waveforms. It is more obscure unless you have seen it in real life.

An NDB transmitter is like any low level amplitude modulated transmitter. When the drive is excessive, subsequent stages become non-linear. Later stages go into gain compression or clipping. The effect of gain compression is the carrier level is reduced when modulation appears. Some of the power normally available for the carrier goes into the sidebands. Because the stage simply hasn't enough available power for the modulation and the carrier, the carrier level decreases when modulation appears. 

Another way to look at this is with a Fourier or "Harmonic" Analysis of the output waveform. Any waveform other than dc or a pure sine wave is really comprised of multiple sine waves of varying level and frequency. When modulation is applied the clipped or saturated stage or stages changes the RF waveform, and this changes the spectral distribution. The fundamental or carrier is reduced as energy in harmonics and distortion products increases, and this is why we hear a negative carrier level shift.

You might hear either the modulation product on a harmonic or the carrier, or both. The carrier might be stable or it might shift amplitude with modulation, and it can change level any amount.

Drift Net or Fishing Buoy Transmitters

These transmitters send periodic identifiers that repeat along with a carrier and long pauses. They generally have numbers in the identifier, repeat the identifier in groups of three, and then have long silent periods. They are battery powered and low power, but they can be heard for hundreds or thousands of miles. They are most frequently found between 1.8 and 3.6 MHz, and are very common on the 160-meter band.

The identifier has nothing to do with the location, it is something programmed by the manufacturer, installer, or his technical support. They often contain a receiver and can be turned off and on remotely by the owner.

The most effective way to get rid of one is to operate on or very close to the beacon frequency. It takes some period of time, but if the owner can't hear the beacon reliably he will program a new frequency. Several nights of heavy activity near a beacon often results in a channel switch.

The same is true for illegal fishing vessel transmitters. They often coordinate operation on or near a beacon's frequency on USB. They often take the hint they are operating illegally when they hear legal activity on or near their frequency. Illegal marine operations often occur on 160 and 80 meters. It isn't always the Japanese or Korean boats that are problem, there are fishing boats based out of Canada and the United States operating on illegal frequencies.

since 11 Feb 2007