Topband: NDB's and Fishing Beacons

[Tom Rauch]

This topic comes up from time-to-time, and it is useful to 
understand how these beacons are constructed.

Every transmitter I have seen (several dozen) has NOT had any 
type of low-pass filter in later stages or at the transmitter output. 
They all had solid-state linear amplifier stages driven by two 
oscillators.

One oscillator is the carrier, and the other is the "tone" that 
modulates the oscillator with the identifier. (A few also have 
conventional audio and give airport condition reports, but they are 
rare).

There typically are as many as   **two** distinct types of signals 
from these transmitters. 

1.) We might hear the normal identifier sending regular CW over 
and over again. This is the harmonic of the CW ID oscillator. 

2.) We might hear CW-sounding gibberish with long dashes. It 
might like "di-di-di-dahhh-di-di-dahh-di-di-di-dahhhhhhhhhhhhhhhhh" 
over and over again with no real pauses or gaps. This is the 
harmonic of the actual carrier. Because it is the carrier, dashes 
and dots form the spaces we hear and the normal CW spaces or 
gaps in tone form the dots and dashes! Thus it is a mirror image of 
the real CW.    

These transmitters depend on the transistor stages being perfectly 
linear to not generate excessive harmonics, and a single large 
loading coil provides **all** harmonic filtering. The output generally 
has a zener diode clamp for lightning protection.

The problem with the loading coil "idea" is at some frequency 
above the operating frequency the coil looks like a low reactance. I 
found virtually all FAA certified technical people can't grasp the 
idea that the loading coil is only a high reactance in certain narrow 
frequency ranges.

Worse yet, the transmitter feeds a "T" antenna that is almost a 
perfect 160 meter and AM BCB antenna.

Clearly these things never should have be designed the way they 
are. All it takes is a technician who sets the gain too high, a 
lightning hit that weakens a semiconductor (like the diode clamps), 
or any internal defect that causes non-linearity in amplifiers and 
these things will radiate for thousands of miles. 

This is why they are constant trouble even though power is low.

Fishing buoys are used to mark long fishing lines and illegal drift 
nets. They send number and letter identifiers three times, long 
carriers, and then go QRT for a while. We also are starting to have 
fishing boat communications appear on 160 meters now.

The FCC is aware of fishing boat problems from both domestic and 
foreign fisherman, and has even distributed pre-recorded warning 
messages in several languages to be played back to fishermen 
using illegal frequencies.

73, Tom W8JI
W8JI@contesting.com