Low Noise Antenna Connection

From: jpd@space.mit.edu (John Doty)
Newsgroups: rec.radio.shortwave
Subject: Low Noise Antenna Connection
Date: 26 Nov 1993 16:55:24 GMT


It doesn't take very much wire to pick up an adequate signal for anything but the crudest shortwave receiver. The difference between a mediocre antenna system and a great antenna system isn't the antenna itself: it's the way you feed signals from the antenna to the receiver. The real trick with a shortwave receiving antenna system is to keep your receiver from picking up noise from all the electrical and electronic gadgets you and your neighbors have.

The Problem:

Any unshielded conductor in your antenna/ground system is capable of picking up noise: the antenna, the "lead-in" wire, the ground wire, etc. Even the widely recommended cold water pipe ground can pick up noise if it runs a significant distance before it goes underground

Symptoms of this problem include buzzing noises, especially at lower frequencies, clicks as appliances are turned on or off, and whines from motorized devices. Sometimes the problem can be reduced by running the radio from batteries.

The Solution:

The solution is to keep the antenna as far as possible from houses, power lines, and telephone lines, and to use a shielded (coaxial) transmission line to connect it to the receiver. To get this to work well, two problems must be avoided: noise currents on the shield must be kept away from the antenna, and, if you want to listen to a wide range of frequencies, the cable must be coupled to the antenna in a non-resonant way.

You can keep noise currents away from the antenna by giving them a path to ground near the house, giving antenna currents a path to ground away from the house, and burying the the coaxial cable from the house to the antenna. Resonance can be avoided by coupling the antenna to the coaxial cable with a transformer.

Construction:

My antenna and feed system are built with television antenna system components and other common hardware. These parts are inexpensive and easily obtainable in most places.

The transformer is built around a toroid extracted from a TV "matching transformer". If you're a pack rat like me, you have a few in your basement: you typically get one with every TV or VCR (or you can buy one). Pop the plastic case off and snip the wires from the toroid (it looks either like a tiny donut, or a pair of tiny donuts stuck together). The transformer windings should be made with thin wire: I use #32 magnet wire. The primary is 30 turns while the secondary is 10 turns. For a one-hole toroid, count each passage of the wire down through the hole as one turn. For a two-holer, each turn is a passage of the wire down through the right hole and up through the left.

Mount the transformer in an aluminum "minibox" with a "chassis mount" F connector for the coax cable and a "binding post" or other insulated terminal for the antenna. Ground one end of each winding to the aluminum box. Solder the ungrounded end of the primary to the antenna terminal, and solder the ungrounded end of the secondary to the center conductor of the coax connector.

Drive a ground stake into the earth where you want the base of your antenna to be (well away from the house). Mount the transformer box on the ground stake: its case should make good contact with the metal stake. Drive another ground stake into the earth near the place where you intend for the cable to enter the house. Mount a TV antenna "grounding block" (just a piece of metal with two F connectors on it) to the stake by the house. One easy way to attach hardware to the ground stakes is to use hose clamps.

Take a piece of 75 ohm coaxial cable with two F connectors on it (I use pre-made cable assemblies), connect one end to the transformer box, the other end to the grounding block. Bury the rest of the cable. Finally, attach a second piece of 75 ohm coax to the other connector on the grounding block and run it into the house. Use waterproof tape to seal the outdoor connector junctions.

Attach one end of your antenna to the antenna terminal on the transformer box and hoist the other end up a tree or other support(s) (don't use the house as a support: you want to keep the antenna away from the house). My antenna is 16 meters of #18 insulated wire in an "inverted L" configuration supported by two trees.

If your receiver has a coaxial input connector, you may need an adapter to make the connection; in any case, the center wire of the coaxial cable should attach to the "antenna" connection, and the outer shield should attach to the "ground" connection.

Multiple grounds and transformer coupling of the antenna should reduce the danger posed by lightning or other electrostatic discharge, but don't press your luck: disconnect the coax from the receiver when you're not using it.

How it works, in more detail:

Coaxial cable carries waves in two modes: an "outer" or "common" mode, in which the current flows on the shield and the return current flows through the ground or other nearby conductors, and an "inner" or "differential" mode in which the current flows on the inner conductor and the return current flows on the shield. Theoretically, outside electromagnetic fields excite only the common mode. A properly designed receiver is sensitive only to the differential mode, so if household noise pickup is confined to the common mode, the receiver won't respond to it.

The "characteristic impedance" of the differential mode is the number you'll see in the catalog or on the cable: 75 ohms for TV antenna coax. The characteristic impedance of the common mode depends on the distance of the line from the conductor or conductors carrying the return current: it varies from tens of ohms for a cable on or under the ground to hundreds of ohms for a cable separated from other conductors.

A wire antenna can be approximately characterized as a single wire transmission line. A single wire line has only a common mode: for #18 wire 30 feet above ground, the characteristic impedance is about 620 ohms. For heights above a few feet the characteristic impedance depends very little on the height.

If the impedances of two directly coupled lines match, waves can move from one line to the other without reflection. In case of a mismatch, reflections will occur: the magnitude of the reflected wave increases as the ratio of the impedances moves away from 1. A large reflection, of course, implies a small transmission. Reflections can be avoided by coupling through a transformer whose turns ratio is the square root of the impedance ratio.

The basic difficulty with coupling a wire antenna to a coaxial line is that the antenna's characteristic impedance is a poor match to the differential mode of the line. Furthermore, unless the line is very close to the ground, the common mode of the line is a good match to the antenna. There is thus a tendency for the line to pick up common mode noise and deliver it efficiently to the antenna. The antenna can then deliver the noise back to the line's differential mode.

Some antenna systems exploit the mismatch between the antenna's characteristic impedance and the line's characteristic impedance to resonate the antenna. If the reflection at the antenna/line junction is in the correct phase, the reflection will add to the signal current in the antenna, boosting its efficiency. While this is desirable in many cases, it is undesirable for a shortwave listening antenna. Most shortwave receivers will overload on the signals presented by a resonant antenna, and resonance enhances the signal over a narrow range of frequencies at the expense of other frequencies. It's generally better to listen with an antenna system that is moderately efficient over a wide frequency range.

In my antenna system, grounding the shield of the line at the ground stakes short circuits the common mode. The stake at the base of the antenna gives the antenna current a path to ground (while the transformer directs the energy behind that current into the coax). Burying the cable prevents any common mode pickup outside the house, and also attenuates any common mode currents that escape the short circuits (soil is a very effective absorber of RF energy at close range). Common mode waves excited on the antenna by incoming signals pass, with little reflection, through the transformer into differential mode waves in the coax.

A major source of "power line buzz" is common mode RF currents from the AC line passed to the receiver through its AC power cord. These currents are normally bypassed to chassis ground inside the receiver. They thus flow out of the receiver via the ground terminal. With an unshielded antenna feedline and a wire ground, the ground wire is a part of the antenna system: these noise currents are thus picked up by the receiver. On the other hand, with a well grounded coaxial feed these currents make common mode waves on the coax that flow to ground without exciting the receiver.

Performance:

A few years ago, I put up a conventional random wire antenna without a coaxial feed. I was disappointed that, while it increased signal levels over the whip antenna of my Sony ICF-2001, it increased the noise level almost as much. I then set up the antenna system described above; in my small yard, the base of the antenna was only 12 meters from the house. Nevertheless, the improvement was substantial: the noise level was greatly reduced. This past year I moved to a place with a roomier yard; with the base of the antenna now 28 meters away I can no longer identify any noise from the house.

The total improvement over the whip is dramatic. A few nights ago, as a test, I did a quick scan of the 60 meter band with the whip and with the external antenna system: with the whip I could only hear one broadcaster, unintelligibly faintly, plus a couple of utes and a noisy WWV signal. With the external antenna system I could hear about ten Central/South American domestic broadcast stations at listenable levels. WWV sounded like it was next door.

I have also tried the antenna system with other receivers ranging from 1930's consoles to a Sony ICF-SW55. I've seen basically similar results with all.