ICE Products - Industrial Communication Engineers

 
TECHNICAL PUBLICATION #42A
Resolving 6 Meter Amateur Interference

Transmitting operations in the 50-54 MHz range offer some unique problems that over the past 50 years have stymied station owners, forcing them to tolerate quiet hours and hostility from family and neighbors attempting to enjoy other electronic services. The 6 Meter band has some very special qualities, and its cases of interference are resolved in a somewhat different manner than radio transmissions at other frequencies. Here are some easy steps to "silent running" and the pursuit of enjoyment of this fine communication band.

First, the station. In all cases try to keep radio equipment on or below the ground where earth ground connections can be made and kept short. This always reduces case and near-field equipment radiation. Elevated stations always suffer more from interference and when a ground lead is a wavelength long (about 18 feet on 6 Meters), it's almost useless. In fact, grounding of an elevated 6 Meter station may make interference worse.

Second, always try to place 6 Meter transmitting antennas as high as possible and away from all other services to increase isolation. Use good quality coaxial line with at least 95% shield. Always crank RF connections down tight with pliers and use anti-oxidants to avoid arcing. Install a lowpass filter with a cutoff frequency right above the 6 Meter band as a matter of good station practice. Although an LPF won't help a great deal with interference to low TV channels it's still a good hedge against higher frequency generations and transmitter stability. It also prevents strong local broadcast signals from entering and interfering with 6 Meter signal reception. And always make sure that coaxial cable shield lines for both 6 Meter antennas and cable television lines are grounded directly to a ground rod using a grounding block, lightning arrestor, or homebrew means. For best results keep the distance from the coaxial connection point to the earth entry point at near zero lead length. It's important for interference, receiver noise, and lightning protection too. And be sure to inspect the transmitter's shield case to insure that the cabinet paint is not preventing good electrical shield contact with the main chassis.

If your interference case is the common disruption of channel 2, 3, 4, etc. of the low VHF channels, then recognize that 50-54 MHz transmissions are not in any way related to the low VHF channels except by proximity. The reason that such interference occurs with regularity is because TV tuners are not filtered in such a way that 6 Meter transmissions are rejected. Thus, near-field transmitting causes huge potential RF voltage to enter the tuner, and destructive interference is easy after that. Run this test. Disconnect the TV antenna or cable lead from the set and see if interference persists during 6 Meter transmission. If it does it indicates that the TV set may be suffering interference from pickup via the

AC line or chassis directly. Install an AC line filter or EMI filter to help decouple the set from the line and run the test again. In all cases try disconnection tests to see what the specific entry point of the interference is. In cases of low VHF channel interference the only practical solution is to stop the introduction of 6 Meter transmission signals to the TV set's tuner, and that can be done with a good highpass filter that has a steep slope of rejection beginning at 55 MHz One or several filters of this type can often introduce enough front-end attenuation to 6 Meter signals that the threat of interference is eliminated. keep repeating the disconnection tests and makes notes as you go. A resolution is possible in nearly all cases if you're persistent and follow good basic troubleshooting practice.