We come to recognize the proportionate shape and appearance of antennas. If we see a half wavelength dipole we recognize it for the antenna it is. When we see a Ground Plane antenna we know what it is. Its just the same as when we see a Ford automobile next to a Volkswagen we know which is which. It is possible though for Ford to build a car that looks like a Volkswagen but, it’s not possible to build a dipole that looks like a Ground Plane, or a “J” antenna that does not look like the letter J! Lets investigate this, and in fact we can start with the “J” antenna as our object model.


“J”

Observe that the vertical portion of the letter J is about two times higher than the portion that forms the crook of the J, or we could say that the height of the J is three times the height of the crook. It is for this reason that the J antenna got its name.

The crook portion of a J antenna forms a “Linear Impedance Matching Transformer” or “Q-Line” transformer because of these two parallel conductors that are 1/4 wavelength long. Above this Q-Line is the radiating portion or “radiating element” that is 1/2 wavelength long.

At the bottom end of this quarter wavelength Q-Line which is electrically shorted together, there is a dead short zero Ohm impedance. One quarter wavelength above this dead short is an infinitely high impedance of thousands of Ohms. This is how any Q-Line device such as a “Bazooka Balun” works.

Now some who have read this article so far might be scratching their chins about now thinking, he said the radiating element is 1/2 wavelength long. Gee, a dipole is one half wavelength long! That’s right, a “J” antenna is merely an “end-fed” dipole! Another name for an end-fed dipole is a “Zepp”, because this form of dipole was first used on Zeppelins. So how is the more common version of a dipole different?


In the J antenna we feed the dipole on its end at the high voltage point of the antenna. If we feed it at the center at its high current point, we will see a much lower impedance or alternating current (AC) resistance. In fact the characteristic “radiation resistance” of a center fed dipole in free space is 72 Ohms. Free space by the way means that the antenna is several wavelengths above the ground, or any other conductive object. Usually free space means at least 10 wavelengths but, for practical design considerations 3 to 5 wavelengths is often times hard enough to achieve!

What happens if we feed a dipole not at the center, and not at its end but, half way in between. This sort of dipole we call a “Windom” named after the antenna’s originator. This type of dipole has a characteristic impedance or radiation resistance of 600 Ohms. This feature allows this sort of dipole to be operated on almost any frequency within several octaves of its design frequency, and always present a relatively moderate impedance and consequently a decent “SWR”.


Next let’s take a look at “Ground Plane” antennas, afterall, aren’t they just another variation on a dipole? Well, it’s certainly true that they are “current-fed” at the center of one half wavelength. If you have ever seen a Ground Plane fabricated on a chassis mount coax connector you can see how this antenna works.

You start by cutting five quarter wavelength metal rods, I have always used Brazing rod. If we were going to make such a Ground Plane for the 2 meter wavelength band we would cut these rods to about 19.25 inches. If we start by just soldering on two of them, one to the center connection, and one to one of the flange holes, we have sort of a dipole. Actually this probably looks closer to an “Inverted V” type dipole but, I think you get the picture! So, now we have one of these 1/4 wave rods connected to the center conductor of our coaxial transmission line, and one of them connected to the shield. So, why should we solder on the other three, won’t the antenna work with just these two? It would work as far as the transmitter is concerned. It would have a characteristic impedance pretty close to 50 Ohms, so the transmitter would be happy! The trouble is that without the other “radials” to form a uniform “counterpoise” , the antenna is not the “omni-directional” antenna we were seeking! If we left it looking like an Inverted-V, it would have a figure-eight radiation pattern broad-side to the two rods. If we provide three radials 120 degrees from one another, or four radials 90 degrees from one another, the antenna will have an omni-directional radiation pattern. By the way, the radials really should be about 5% longer than the radiating element. Also, if the antenna has 3 radials, they will have to be bent down at a lower more acute angle to achieve a 50 Ohm impedance match to the transmission line.


So, what’s the bottom line to all this palaver? Simply this, all antennas, any antenna can be analyzed as to its design by analyzing its current and voltage distribution. The end or tip of the antenna is always going to represent a high impedance and high voltage point. If we measure down 1/4 wavelength we will find a high current point and a relatively low impedance. If we follow this process all the way back to the feed-point we can determine all aspects of the antenna including the antenna’s aperture size, and the aperture size will tell us the antenna’s approximate gain. Every time you double the aperture size of an antenna you double its gain, which means you pick up 3 decibels of gain.

Lets check this out by looking at one last J antenna which has come to be called a “Super J”. A Super J starts with a normal looking J just like we see so many of nowadays. At the tip top of this J a quarter wavelength phase de-coupling stub is added, and then another half wavelength dipole is placed on top of the phase decoupler. Guess what happens next, we gain 3 “dBd”, or 3 dB’s above a dipole reference! In “dBi” this would be 5.2 dB’s compared to an “Isotropic” reference.


Terms

Q-Line, Bazooka Balun, or linear impedance matching transformer: All of these are electrically speaking the same thing. A Bazooka Balun only differs in that it is fabricated from two lengths of tubing, as well as a central coaxial inner conductor. These are all one quarter wavelength long!

Radiating Element: This term is both hard to closely define, and in fact is a bit of a misnomer. The vertical element in a Ground Plane is sometimes called the radiator or radiating element but, it really radiates in conjunction with other associated elements that form part of a half wavelength.

End-fed, and center fed: These terms are closely associated with the terms, “Voltage Fed and Current Fed&quot.. At the end of a half wavelength there is an infinitely high impedance and consequently an infinitely high voltage. At the exact center of a half wavelength is an infinitely high current and virtually by contrast, no voltage and a very low impedance.

Characteristic Impedance: All conductors or wires have both some amount of inductance and some distributed capacitance, this in itself provides a “lumped constant” derived impedance. In various configurations such as two wires parallel to one another, a characteristic impedance will result. Wires that are brought more closely together will have a lower impedance as parallel capacitance rises, or if they are farther apart this impedance will rise as capacitance is reduced.

Radiation resistance: All antennas have a characteristic radiation resistance because of the comparative effects of their distributed inductance and capacitance. This can also be expressed as a current to voltage ratio. Whatever this ratio is, a characteristic impedance will result. For a dipole this is 72 Ohms, for a 1/4 wave Ground Plane with radials at 90 degrees to the radiating element this is about 34 Ohms, and for a 5/8 wavelength Ground Plane its about 90 Ohms.

SWR: Standing Wave Ratio is the term given to the measurement of current or voltage distribution as imposed within the antenna. It is usually measured as a voltage and therefore the term often used is “VSWR&quot.. If an antenna has a radiation resistance of 72 Ohms and we feed it with 50 Ohm coaxial cable, the SWR will be 1.44:1. If we fed a 90 Ohm antenna directly with 50 Ohm cable the SWR would be 1.8 to 1 (1.8:1 or 90/50 = 1.8).

Phase de-coupling: When ever the aperture size of an antenna is increased we have to make provision for the additional antenna elements to work in phase with the other elements. On vertical omni-directional antennas this is done by phase de-coupling half wavelength radiators with quarter wavelength phase de-couplers.

Gain: Antenna gain is often times a controversial subject. It really shouldn’t be, for the following reason. Every time an antenna’s aperture size is doubled, its gain will double. If I properly stack one beam antenna of equal size over its predecessor I will have doubled its aperture size. If I ignore the losses imposed by the feed line and phasing network, I will have added 3 dB’s of signal gain. Don’t forget though, there’s no free lunch. If I put a 10 dB gain antenna on a 100 foot tall tower and use poor or cheap coax cable to feed it, it may well turn out that I have less signal gain than I would have had by putting a unity gain “J” up at 30 feet with good coax.

Article by Wa6BFH originally at /www.geocities.com/SiliconValley/2775/ 

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.

For someone surrounded by houses, power lines and trees, a 30m/98ft high tower is desirable. A high tower is also desirable for someone situated on perfectly reflecting ground, such as saltwater.

However, in most “real world” cases, the 30m/90ft height is less advantageous than you might think.

Why is this the case?

The angle of radiation can be so low that most of the signal is eaten up by ground loss. For the 21MHz, 24MHz and 28MHz bands, an antenna on a 30m/98ft high tower has a radiation pattern of 5 degrees or less (depending on ground conditions, the electrical height may be even higher and the angle of radiation even lower). At these low radiation angles, the ground tends to absorb a large portion of the signal.

To overcome this loss, it would be advantageous to be able to lower the antenna to increase the angle of radiation at higher frequencies — a remote-controlled variable-height tower. Hams with these towers have reported up to 12 dB “DX-gain” (e.g., on 21 MHz) by lowering the antenna from its full 30m/98ft height to 20m/66ft height.

Note that the low radiation pattern of 4-5 element beams has about 14-15 dB/i ERP “DX-gain.”. This portion of your DX radiation can be partially or totally lost, at maximum tower height.

The best (only) solution for maximum DX results at higher HF frequencies? Provide a variable-height antenna tower.

Check for the most favorable radiation angle on medium-distance and long distance DX (typically 2000-12,000 miles / 3000 – 20,000 km) with the help of the chart above.

Example:

See Figure 1. At a Radiation Angle of 23 Degrees (See Fig. 1A) your “DX-chance” at 7 MHz is 25%, while at 18 MHz it is 90%. At a Radiation Angle of 7 Degrees (See Fig. 1B) your “DX-chance” at 10 MHz is 10%, while at 30 MHz it is 50%.

While professional radio applications generally require “long-time” connections, such as an S-4 signal for 5 hours, for amateur radio applications an S9 +40 signal for just ten minutes can often be more desirable.

Conclusion: Low radiation angles (typically 7-15 degrees) are effective for “big” signals over long distances for a relatively short period of time, which is usually adequate for DX purposes. However, if long-time QSOs are your goal, higher radiation angles may be better.

Information from “Neues von Rohde und Schwarz,” Okt./Nov. 1973.