Getting Started with Antenna Basics ~ A Primer for New Hams
A new individual becoming Ham a radio operator faces a complex amount of physics conceptual knowledge to master. The new Ham requires a good understanding of these physics concepts in order to work efficiently with antennas. This basic understanding is a great foundation to start the educational growth process on the topic. Basic knowledge allows one to expand further into future details and become totally proficient on the topic. Antenna theory is an area that follows this form of an educational process. In order to use a radio for communication, you must use an antenna. So it follows that these antenna physics concepts become important to every amateur radio user. As such, there usually will be lots of antenna discussions with other hams early in a ham's life. The following information is meant to establish a good starting point for the new ham on antenna physics..
Every ham quickly visually perceives antenna circuitry and feed line concepts, a wire or tube with perhaps a transformer (balun or unun), connecting wiring (transmission line) attaching to the radio equipment and BANG, a radio station setup is formed. It is fortunate that antenna physics are pretty easy to conquer.
Antenna performance is a different concept area for most hams starting out and requires us to understand various terms. The process of optimizing of the antenna system's performance is usually cheaper and easier to implement than adding additional electronic equipment (new higher power transmitters and/or amplifiers). Antennas are easy to home-brew and operate compared to new equipment. One usually can quickly find any improvement in system performance with changes in the system. The learning process gets involved with new terms like antenna gain, efficiency, capture area and effective heights. These are not usually terms a beginning ham already has much knowledge about. This article will establish the new concepts needed and help eliminate some confusion in these areas. The concepts will allow the new ham to discuss antenna topics with others by supplying a good basis in antenna physics.
Reciprocity of Antennas
Antennas work both for transmitting and receiving direction in exactly the same manner. This "forward/backward" in exactly equal physics characteristic is called 'reciprocity'. It derives from a physics concept that many beginners seem to find difficult to accept. It is however a true and falls directly out of the physics mathematics. The antenna converts one type of energy into another type of energy and the laws of energy conversion implies energy coming in will equals the energy going out, always true, without regard to the direction of the transfer. One type of energy is electromagnetic wave (EM wave power) and the second is electric at power (volts and amps). It is these two energy types that the antenna is the conversion device we us to effect the transfer. Receiving take EM wave as the input and outputs electrical power. Transmitting take electrical power as the input and outputs an EM wave. The effect is antenna reciprocity (always).
The antenna is the important energy converter between these two types of energy. In understanding antennas it becomes necessary to have a basic knowledge of both electrical field properties and electromagnetic fields. It turns out that electrical properties and magnetic properties are easier for most of us to understand when compared to field theory. So usually a beginning ham's knowledge of EM waves (fields) lags behind the electrical energy understanding. It is due to the normal educational path that educators use to teach these topics.
Electromagnetic Fields
Even the professionals have trouble with fields! They define fields as "action at a distance" and truly "get lost" when trying to exactly explain what a field really is. Electric attraction and repulsion and magnetic attraction and repulsion effects have been known for thousands of years. But humanity waited for James Clerk Maxwell {13 Jun 1831 ~ 05 Nov 1879}, a Scottish physicist and mathematician, to establish the theoretical work that allows the actual mathematical formulas to be utilized. His work shows that a very natural and definite relationship exists between the two fields (magnetic and electric). The two mathematical expressions used (formulas) have become known as "Maxwell's Equations". It is this foundation upon which the mathematical EM theory has been built that allows us to use these theories. You can spend additional time in studying these equations and their use. An understanding of those will add to your antenna theory knowledge, but are beyond our basic knowledge requirements.
Traveling Waves
There are two components of traveling energy waves (fields), one electric and one magnetic. If we place ourselves at a point (in time and space) and watch the waves travel past, we would see a gradually changing EM waves, actually a sine wave, of first electrical and then magnetic portions with the energy being 'transformed' from one type of energy to the other as the wave passes by. One type of EM wave increases as the other decreases, a kind of bouncing back and forth between the two types (being 90 degrees out of phase). The fields are perpendicular to each other, traveling outward in a spherical wave front. The frequency of the transformation between these two types is when the completing of exactly one sine wave passing by our point is the definition of 'one wavelength' long.
The speed of transmission or speed of our wave is at the 'speed of light' [SOL] in free space. Scientists have agreed that SOL is: 299,792,458 million metres per second in 1983. We can convert this to: metres/millisecond, miles/second, feet/second, inches/microsecond or what ever velocity units we desire. It is really, very fast! Just under 300,000 kM/second, about 186,000 miles per second.
The relationship between frequency and wavelength can be expressed algebraically:
C = F * Lamda
where:
C = the constant for the speed of light or (299,762,458 million metres / second)
F = Frequency (Hertz)
Lamda = Wavelength (metres)
by using algebra we can rearrange the formula to solve for frequency or wavelength in terms of the other two.
Light is a form of EM radiation and follows the same basic considerations. Different colors have a different frequency and thus wavelength. Light obeys the exact same mathematical formulas. Using the 1,314,900 seconds per year, one light year is 3.942 x 10^11 kilometres per year (about 2.446 x 10^11 miles per year). Assuming a universe age of 13 billion years you can multiply to get a distance to the "edge" of the universe. (That is a big big number!)
Power Density
Using the same observation point we used for our traveling wave discussion and placing a light source (or an antenna) at that point. Letting time advance we would observe an EM field expanding in spherical shape into the surrounding space. It will be moving away at the SOL. We call this a spherical wavefront. The surface area of our sphere will depend on the distance from our source, this distance is the radius of our expanding sphere,
Recalling from geometry that the surface area of a sphere is:
S = 4(pi)R^2
where:
S = surface area of sphere (our wavefront) (metres squared or m^2)
pi = 3.14159...
R = Radius of sphere (metres)
Assuming our EM wave source energy is known constant with respect to time. Our wavefront is expanding into space with an expanding surface area, thus the power of our EM wave in a given surface area is diminishing due to the distance growing.
The amount of electromagnetic power contained in a unit of surface area on the wavefront is called the "power density" of the EM wave.
D = P / 4(pi)R^2
where:
D = Power Density (watts/m^2)
P = Radiated Power (watts)
pi = 3.14159...
R = Radius of sphere (metres)
We can understand from the above as the wavefront moves away from the source, the power density decreases with square of the distance from the source. The decrease of power density is called "spherical divergence". It means the EM wave density is decreasing as it moves away from the source. Thus a convenient way to measure the amplitude of the EM field magnitude is the measurement of the power density.
Field Strength
The EM field is capable of transmitting power from one place to another it is reasonable that the power is derived from the electric and magnetic components of the EM field. A change in the transmitted power (at our source) will result in a result in a corresponding change in the power density at our distant measuring point. The electric field strength, magnetic field strength and power density have relationships that can be expressed mathematically.
Quite simply they are:
D = E * H
where:
D = Power Density (watts/m^2)
E = Electric Field Strength (volts/metre)
H = Magnetic Field Strength (amperes/metre)
These are 'far field' equations and force a definition of 'near field' and 'far field' zones. Generally when the radius (distance) is greater than a couple of wavelengths, we consider this as 'far field' and when less than a couple of wavelengths is the 'near field'. In the ' near field' other considerations must be included.
In the far field a simple relationship between the electric field and magnetic field strengths:
E= Zs * H
where:
E = Electric Field Strength (volts/metre)
Z = impedance of space (377 ohms)
H = Magnetic Field Strength (amperes/metre)
{Please note the similarity to Ohm's law.}
Combining these last two equations we obtain two new expressions for EM power density:
D = E^2 / 377 and D = 377 H^2
where the terms are as previously defined
EM Field Summary
To understand antennas we have now covered the necessary details of EM theory required. The concepts of point sources, electric and magnetic fields, frequency, wavelength, power density, field strength, and spherical divergence have been explained. Power density, electric field strength, and magnetic field strength are particularly important as they are what we use to measure the EM fields. Making a measurement of one allows us to know the value of other two by using the simple equations presented.
The reciprocity properties mean the concept are identical for transmitting or receiving. Lets discuss further the transmitting case and then come back to the receiving case.
Equivalent Circuit - Antenna
Power flows from a transmitter output into the antenna to allow radiation via the EM wave. The antenna loads the transmitter and appears to the transmitter as an impedance. If the antenna is carefully 'tuned' to resonance, the load appears as a pure resistance. Today's ham uses sophisticated equipment, like a vector network analyzer [VNA] to measure the precious antenna complex impedance. For now we will use the resonant case for discussion to simplify the discussion. The complex impedance case can be used with an increase in the mathematical complexity but does not add to the basic understanding. What we are doing is using resonance to make the equivalent value just a simple resistor, i.e. no electrical reactance values. Thus, an antenna looks like a simple series resistance to the transmitter. The value is called the antenna input impedance resistance [Rr]
Radiation Efficiency
In practice not all of the power supplied to the antenna terminals is converted into EM waves. There are other losses in the antenna system. Resistance in the antenna system materials convert some of the electrical energy into heat which spreads into the surrounding environment. We add a second resistor in series with the antenna input resistor to represent the energy loss which is converted to heat. We will call this element 'loss resistance' [Rl].
To be a 'good' antenna we wish to insure that the radiation resistance is high and the loss resistance is low. This allows most of the input power to be placed into the radiated EM wave. We mathematically express the 'radiation efficiency' [Nu] as:
Nu = (Rr + Rl) / Rr
This maximizes the amount of radiated energy when Nu is high. Converting the value of Nu into a percent is just multiplying the value by 100. thus, we arrive at an 'efficiency percentage' value for this in our antenna.
An Isotropic Antenna
An isotropic antenna is one that radiates energy equally in all directions. It is like our point light source and has no actual spacial dimensions - It is not a real physical thing, but adds to our antenna understanding. It becomes a convenient reference as we consider antenna gain.
Antenna Gain
There are two types of antenna gain, Antenna Power Gain [Gp] and Antenna Directive Gain [Gd]. They are related mathematically by:
Gp = Nu * Gd
As an example, lets assume and antenna has a Nu of 50% and a directive gain of 6 dB (factor of 4), then the power gain would be 50% times 4 equaling a factor of 2.00, which is 3 dB power gain. As higher power density in the EM wave is the most common goal, power gain tends to be favored in most antenna discussions. Older ham practice was to compare antennas using a dipole as the reference, but a dipole has a power gain factor of 1.64 (2.15 dB) with respect to an isotropic antenna. Thus it becomes important to know what the 'reference antenna ' type is being used when one compares multiple antenna designs. A gain of 6 dB has no meaning without stating what it is referenced against. Simply a higher number as gain value may not be actually higher levels of EM wave. {Yes - in the past advertisers have changed what an antenna was being referenced to in order to obtain a higher gain number. Hoping that just the number would be used by the newer ham for making his choices.}
Transmitting Antenna Summary
The information presented establish the important conclusions with respect to transmitting antennas. The importance of knowing which antenna gain, power gain and directive gain, we are trying to deal with AND the type of reference we are using for comparison values, Isotropic or dipole or ??. The use of antenna 'gain value' without either of these two consideration has no meaning. If someone is not stating or supplying these two items, give them no credibility.
The addition of non-resonant antenna design can be accomplished into into the above construct. The internet connected computer can assist with additional information, calculators, analysis software. There are great tools out there for little or no expense but they do not change the basic information provided here.
Receiving Antennas
Some additional concepts enter into our discussion when we add receiving antennas to our basics. Noise impacts our ability to define the best antenna. It applies most when we are trying receive a EM wave. 'Capture Area' and 'Effective Height' concepts will then be presented. These terms are of historical interest so we will add them in for added information.
Noise Impact
Below about 30 MHz, atmospheric noise (lightening and electrical sources) becomes the limiting factor for communications. Conversely, above 30 MHz, the internal noise in the receiver is the major limiting factor. Everything in the universe has resistance to electron motion. Conductors have low values while insulators have very high resistance values. Free electrons in various materials have random motion which increases as the material's absolute temperature rises. This random noise has a RMS voltage [e]associated with it.
e = (4 k T R B)^-2
where:
k = Boltzmann's constant (1.38 x 10^23 / joule1
T = temperature (degrees Kelvin)
R = Resistance of the material (Ohms)
B = Bandwidth (Hertz)
The noise power is expressed by converting noise voltage to power by using: e^2 / Resistance
Pn = k T B
where:
Pn = noise Power (watts)
k = Boltzmann's constant
T = temperature (Kelvin)
B = Bandwidth (Hertz)
Note: The noise power is not dependent of the material resistance. This means that everything produces noise and the amount varies with the temperature and the bandwidth we are measuring. A Gaussian distribution of noise voltage is usually used in the analysis. There are many sources for reference if you wish to continue this area of study. We will not continue beyond here for this discussion.
These relationships change which of the antenna gains is most important for further consideration. Maximizing communication path efficiency (our true goal in communication system design) suggests that directivity gain is most important below 30 MHz and power gain is most important above 30 MHz. As the system noise environment changes from atmospheric (distant) dominance to internal receiver sources as we change the frequency.
If below 30 MHz, we sacrifice some EM power in the receiving portion of the system by using good antenna directive gain characteristics, we can minimize atmospheric noise. A much smaller receiving antenna can be used in a tunable manner (lowest reception noise) and get a gain in communication path efficiency when the antenna has the desired directive gain (pattern steering). Thus small, directional receiving antennas using a 'receiving antenna' connection offered on many modern transceivers can offer significant benefits in you station efficiency.
Thus, if we can reduce the noise a fair amount, while only suffering a small loss of power gain, you can obtain an improved signal to noise ratio [S/N or SNR]. Increasing S/N will make communication operations much better. This detail means it is not sufficient to just design for the highest antenna power gain, but must consider system wide structure in our search for optimum communication efficiency.
Capture Area
The EM energy an antenna can capture is useful concept. The maximum EM power is what we wish to achieve in the antenna system. Power is the summing of energy versus time. The EM energy delivered to an antenna is implying that the area we are integrating over (capturing) should be maximized. Capture area is a term that is related to the power gain and the wavelength [Lamda] is are interested in.
A = Gp * (Lamda^2)/(4 (pi)
where:
A = Antenna Capture Area (metres squared)
Gp = Antenna Power Gain
Lamda = wavelength os interest (metres)
pi = Constant (~3.14159...)
This equation shows that capture area decreases as the square of the wavelength. Large capture areas are essential as frequencies increase, think VHF and UHF. Thus, as we go to higher and higher frequencies, smaller Lamda, the power gain must be increased to maintain a given capture area. The result, at higher frequencies the power gain forces antenna designs to larger sizes (longer booms, more elements, etc.). Final result is that the useful antennas have sizes comparable to lower frequency antennas.
The term capture area has become quite important in discussing VHF and UHF antenna effectiveness.
Effective Height
Historically electric field strength [E] was used to specify the EM field level. Today, "power density" seems to be the preferred method to define the EM field. The term effective height [L] was defined as the voltage measured at the antenna terminals with no load connected.
L = Antenna Open-circuit Terminal Voltage / Electric Field Strength [E]
where:
L = Antenna Effective Height
This term is a little ambiguous for two reasons. The first it gives the voltage (signal amplitude) at the antenna terminals. This varies on the placement of the terminals. A dipole being center fed has a much lower voltage than the same design being fed at a larger distance from the center, much higher voltage. The same power into the antenna with different impedance at the measuring terminal location. The modern off center fed [OCF] dipole is the design here defined versus a center fed dipole. The EM power is what we should be interested in a system analysis and power density provides a more consistent measurement unit.
A second reason, the term "height" implies a measurement above ground, which it is not. Perhaps a better term would have been length, which has seen some usage.
Among professionals, the term capture area, especially at VHF and UHF, has almost completely replaced effective height. Among amateurs the older term can still be found in use, so we have included it here.
Conclusion
The knowledge presented here should give a new ham the ability to discuss antenna designs and make them 'capable hams' in comparing antenna literature. Improving a ham's station efficiency is a long time goal of most all hams and never goes obsolete. My journey has lasted 57 years to this date and I still endeavor to improve my station and make informed choices in system design.
WB8GUS ~ June, 2025