Where's the Current?
Last month, the article entitled An Uncommonly Good Vertical raised a lot of questions that deserve answers. In addition, I want to use this month’s article to present some antenna basics to help explain why that antenna radiates so well, and to lay the ground work for future articles which define specific antennas based on this concept for other bands with outstanding performance.
Background
How well does this antenna perform? I think the following will answer that question very well:
On 5-5-98 at about 6pm WD9IAN (Byron) just north of Chicago made the following statement in response to my CQ (from Eatonton, Georgia) – “I have been tuning the band for an hour and you are the strongest signal I have heard.” I told him I had a piece of wire running out the window and he commented – “Boy, if you can do that with a piece of wire, imagine what you could do with an antenna”. That statement was made in jest but it also portrays the fact that “a piece of wire” just does not qualify as an acceptable antenna in the minds of most Hams. Most seem to believe that if it does not have the appearance of an expensive monument, it can’t perform well.
So, then what is the definition of an antenna? W5QJR’s definition is “An antenna is a conductor cut to a specific length, then located and oriented in a manner to cause it to perform the desired antenna function.” (Note- the “conductor” could be any conductor, including a tower or a beam element—or if you are Mother Nature, you could cause the air to be a conductor of Lightning. Evidence that it is a good conductor results in large noise pulses in your receiver.) After “designing” your ultimate antenna, then it is time to decide the method to connect the antenna to the radio. A matching network may be needed.
The Reference Antenna
Here is the antenna under discussion:
I took a piece of magnet wire (about #16) about 65 feet (19.8 m) long (1/2 wavelength on 40 meters) and put insulators on both ends. I then went to K-Mart and bought a fancy slingshot for about $10, a roll of 20-pound fish line, and a bag of 1/2 ounce sinkers. The slingshot placed the sinker on a trajectory that pulled the fish line over the desired branch of a selected tree. The fish line was then used to pull the wire up to the desired height, after I had secured the low end of the wire. The only real problem—I should have used a sinker with more weight—it would have been easier for the weight to pull the fish line over a limb that has rough bark. A wire was brought into the shack from the low end of the wire. Also, a ground lead was brought in. To make the ground lead low inductance, the shield of a piece of coax was used. The ground is a piece of bare wire [about 60 feet (18.3 m) long] buried along the foundation of the house, because it was an easy way to hook to ground.
To match this antenna to the radio, a tuner is used. Mine is a MFJ Deluxe Versa Tuner II. It does a good job of matching but the paper washers that insulate the Antenna Matching capacitor shaft must be replaced. The paper washers get burned and the carbon path then is a good conductor that shorts out the antenna. You will smell, see and hear the burning when it happens. Find some made of Teflon or plastic – for about a penny a piece.
It would have been better to mount the tuner at the base of the antenna rather than in the Ham Shack, but it was convenient and works well, so why not? Well, to tell the truth, there is some RF on the radio, but not enough to get burned. I can tolerate that rather than having to go out and adjust the tuner when QSYing from one end of the band to the other. Someday I will build a remote tuner.
New Stuff
Back to the subject at hand. Look at Figure 1 and note that as the RF signal travels along a wire there is a current maximum at 90 degrees, another at 270 degrees, then it repeats for each cycle.
Looking at the figure, there is a current maximum and two minimums for each 180 degree portion of the cycle. Since there are 360 degrees in a cycle, 180 degrees is a half cycle. A length of wire equivalent to 180 degrees at the operating frequency is commonly referred to as a 1/2 wavelength. This is the fundamental antenna element.
The current maximum is in the center of a 1/2 wavelength antenna. A common 1/4 wavelength vertical also has the current in the center of the antenna, but since half of the antenna is buried (radials form an image) the maximum current is at ground level. Mobile antennas suffer efficiency due to the high current in the skin of the car, which has high loss due to eddy currents in the steel. This is why the small loop antenna was developed—it has high current in a good low loss conductor—thus the efficiency is high. Other small antennas use loading coils to make the conductor a 1/2 wavelength electrically, as do many multi-element beam antennas. The bottom line is, all fundamental: antennas are 1/2 wavelength electrically.
Current and Radiation
So what is the relationship of the current and the radiation from the antenna? Here is basically what happens: The transmitter causes current to flow in a conductor—the current creates a magnetic field surrounding the conductor—as the magnetic field changes it causes an electric field to be created (fundamental concept of a generator). At about 1/3 wavelength from the antenna, the magnetic field and electric field have the necessary space and phase relationship to properly interact and create an electromagnetic wave, which is what we refer to as the radiation from the antenna (The text books call this the Poynting Vector). Since the actions are reciprocal, radiation impinging on the antenna will cause current to flow into the receiver antenna terminals. Therefore, an antenna can both transmit and receive. Now, look at Figure 2 and re-read the paragraph above.
The magnetic field covers a large volume. The antenna is like the primary of a transformer—if there is ferrous material in the vicinity of the antenna, it is the same as having a lossy resistance connected across the secondary of the transformer. That resistance is reflected back to the primary of the transformer (the antenna) as a loss resistance. Characteristics of the soil near the antenna can also affect the antenna by reflecting loss resistance into the antenna as in a transformer.
In addition to the reflected loss, the impedance of the antenna is a function of where you connect to it and the operating frequency relative to the natural resonant frequency of the antenna. At the location of current maximum is the low impedance point of the antenna. Conversely, where the current is minimum, the impedance is maximum. A typical dipole has an impedance close to the impedance of coax at the center of the dipole. The ratio of the antenna impedance to the impedance of the coax is referred to as the VSWR.
My vertical (or more correctly, my sloper) for convenience, is end fed at the base of the antenna where the current is minimum, voltage is maximum, and therefore, the impedance is high. Because the low end is close to ground, the impedance is lower than in free space, and is about 3300 ohms. A matching circuit between the transmitter/receiver and antenna is necessary for maximum power transfer.
Summary
The above is for the purpose of letting you know:
1) that the current varies along the antenna,
2) all resonant antennas are 1/2 wavelength electrically,
3) where the current is maximum is also where the radiation is maximum,
4) where the current is maximum is where the impedance is lowest,
5) the antenna can be fed at any point to achieve the desired impedance,
6) the antenna is like a transformer, thus it’s surroundings are critical, and the height and orientation of the antenna are major factors that determine the radiation pattern.
Take a good look at your antenna and determine where the current maximum is. Is that where you want it to be to achieve the performance you want?
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 29th April 2024