NASWA Journal Columns · Technical Topics, March 1998

Technical Topics, March 1998

Isolating Your Antenna from Noise and Interference

If you are bothered by noise and interference from sources in your home or apartment building, this month’s column may have the solution to your problem.

I live in a multiple dwelling townhouse. My own TV and computer and my neighbor’s TV sets were disturbing reception on the short-wave bands. By implementing the techniques contained in this article, I eliminated any interference above 3 MHz.

The first thing you must do is to move your antenna as far away from the noise source as possible. Noise coupling, caused by electrical or magnetic fields, falls off rapidly as the separation distance is increased. Remember that noise currents can be coupled via power lines in the walls and ceilings of your dwelling. Get the antenna away from your house as far as possible.

Second, you should feed your antenna with a good quality coaxial cable. Satellite TV cable features a solid aluminum foil outer conductor providing excellent shielding and are designed for direct burial. The loss is low even in the UHF frequency range which are used for the intermediate frequency between the antenna-mounted converter and the receiver in the house.

My antenna had to be hidden because of deed restrictions which prohibit outside antennas for TV and radio. I have a salt-water swamp about 30 feet from the townhouse separated by a grass lawn. One dark night I carefully slit the grass with a spade and pressed the coax cable into the slit. I tamped the slit closed with my foot. By dawn the slit was barely visible. After a week the grass had grown sufficiently to make the slit invisible.

One end of the cable is in the undergrowth at the edge of the swamp. Here the shield connects to a four-foot ground stake sunk into the salty mush. The center conductor connects to my random wire antenna. The cable runs under the lawn, behind a gutter drain pipe up to the roof where it enters the attic through a soffit ventilation hole and then runs across the attic floor dropping down inside a closet to my radio room. Here it connects into the antenna multicoupler which drives three receivers. The multicoupler is grounded to the house power ground through the three prong power cable. All receivers are also grounded to the multicoupler and the house power ground.

Here is the problem. Interference currents from noisy appliances can find their way to ground via two paths. One path is via the power ground system. The other path is via the outer shield of the coaxial feedline. At the far end of the feedline these currents can reflect back and enter my feedline. More on this later.

My antenna consists of 300 feet of wire strung through the bushes about 1 foot above ground. The wire connects to the coax feedline through a ferrite isolator. This is the device that magically eliminates most of the man-made noise that previously limited my listening pleasure.

What is an isolator? Some people refer to these devices as current baluns. Here is how they work.

Normally we think of a coax cable as a two conductor device. There is an inner conductor and an outer conductor that shields the inner conductor. In reality at short-wave frequencies a coaxial cable is really a three conductor device. The outer conductor is really two conductors because short-wave frequency currents will propagate independently on both the inner surface and the outer surface at the same time. Engineers refer to this property as “skin effect”. The actual circuit looks like Figure 1.

Figure 1. The outer conductor of a coaxial cable can carry high frequency currents flowing in opposite directions at the same time.

I1 is the current flowing at any instant on the surface of the center conductor. I2 is the reverse current flowing on the inside surface of the coax shield back to your receiver. In the absence of any other currents and capacitive coupling to interference sources, these are the only currents that will be present. If current I3 is present because of coupling to some noise source, it ideally flows to ground never to be heard from again. But if the ground connection has some resistance, and all real grounds do, some of the current from I3 will loop around the end of the coax and add to the current I2 flowing back to your receiver. What you need is an isolator to prevent the current I3 from reaching the end of the coax where it can turn around and flow back. That is where the line isolator comes in.

A typical line isolator is shown in Figure 2.

Figure 2 Ferrite rings placed over the outer conductor of a coaxial cable effectively prevent current flow on the outside of the cable while internal currents flow freely.

Many ferrite rings are placed over the outer conductor causing a high impedance impeding the flow of short-wave frequencies on the outside. The currents inside the cable are not affected by the presence of the ferrite rings. The more rings that are used, the greater the attenuation. The ferrite type is specifically selected to be effective in the frequency range of interest.

You can buy appropriate rings from companies like Amidon or Palomar. Most SWL’s will be happy to know there are ready-made line isolators available for reasonable cost from Radio Works in Portsmouth VA. Their model number T4-G features a ground strap, presents more than 75,000 ohms impedance to 3.5 MHz outer shield currents and is useful from 2 to 30 MHz. Best of all the cost is only $33.95 plus shipping. Radio Works has a fine catalog with lots of good antenna designs. You can contact them at 757-484-0140. Their free order line is 800-280-8327. You can e-mail them at jim@radioworks.com.

The line isolator made a big difference in my ability to enjoy this hobby. If you have local interference, it might work for you too. I hope to see you at the Kulpsville SWL Winterfest where I will conduct a comprehensive forum on different types of antennas and how they work. Until next time, stay tuned.

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