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Shiawassee Amateur Radio Association [SARA]

Established: January, 1958 an ARRL Affiliated Club since 1961

"Whiskey 8 Quack Quack Quack"

Meets at: James P. Capitan Center, Lower Level; 149 E. Corunna Ave.; Corunna, MI 48817 Monthly: 2nd Tuesday @ 7:00 PM

Club station located in the James P. Capitan Center - Lower Level.
Grid Square EN72wx   Latitude: 42.9819 N   Longitude: -84.1164 W   Alitude: 760 ft.

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SARA's ANTENNA FUNDAMENTALS

All radios require some sort of antenna, whether they are receiving or transmitting. The antenna is the interface between electric elements in the radio and the elcetro-magnetic [EM] fields in space. The antenna elements establish radio frequency [RF] alternating electrical currents (and voltages) which interact with the physics of the antennna and it's surroundings. The physical parameters (physics) of the antenna establish the electro-magnetic field paramaters which are being received or transmited. This continues into the study of "fields' and "field theory" which can be very difficult to master.

Antennas are among the first considerations when setting up a radio station. Antennas have a large effect on establihing an effective communication system, yet they are often not what many initially consider as 'very important for consideration'. Antenna systems deserve a very serious technical approach as you begin your efforts. You can communicate very well with low power (QRP) if you use a good antenna system. If you use a poor antenna design process, a 'legal limit' power station will still have issues trying to communicate even for short distances. Antennas can (and should) be the best place to concentrate your early efforts for improving communication efficiency. We include the antenna to transmission line impedance match as a part of the antenna design. For efficiency, the impedance at the transmission line connection should have the same impedance as the transmission line (lowest loss condition for the line).

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For efficient radio transmission the antenna elements need to establish the strongest EM field possible for the situation. For receiving signals the EM field needs to produce the largest electrical circuit changes possible. These are 'reciprical' effects, they always move together and improving one improves the other. Communication reliability and quality are influenced directly on how well the antenna launches or collects EM waves. When electrical elements are remote to the antenna, a tranmission line is used to connect the antenna with the electrical circuit elements. This then introduces transmission line characteristics into the system analysis. When viewed as a system, both the transmitting antenna on one end and the receiving antenna on the other have large impacts on the efficiency. Look for 'theory of reciprocity' in antenna literature - start HERE. The goal is an antenna (and matching network) should present the transmission line with a 'matched' impedance value.

Certain concepts and terms must be further understood to enable choosing the right antenna(s) for a particular radio communication circuit. Defining several basic terms and relationships will help you further understand antenna system fundamentals. These terms/content include: radio wave formation, radiation fields and patterns, polarization, directionality, resonance, reciprocity, impedance, bandwidth, gain, and take-off angle. To start we will view the system as a transmitting RF antenna concept, changing the antenna electric parameters into EM RF radiation in an efficient manner.

For effective communications all of the following must exist: alternating electric energy in the form of a transmitter, an electric antenna conductor (a wire / element), an electric RF current flowing through the element, and the generation of both electric and magnetic fields in the space surrounding the antenna element. When an alternating electric current flows through a conductor, electric and magnetic fields are created around the conductor. If the length of the conductor is short compared to a wavelength (less than 0.05%), the electric and magnetic fields will generally die out within a distance of one or two wavelengths. As the conductor is lengthened, the intensity of the EM far fields enlarge. Thus, an ever increasing amount of energy escapes into space via the EM field. When the length of the antenna element approaches one-half of a wavelength most of the energy will escape in the form of electro-magnetic radiation. If the circumference of the field reaches two wavelengths or more, it is considered as establishing the 'far field', if less than two wavelengths it is considered as 'near field'. Interactions are different between the 'near' and 'far' fields making determinations a little more difficult to explain. the near and far field are not a clearly defined transition - they blend over some distance. Usually a 'transmission line' connects the transmitter electronics to an antenna's input. So considerations on transmission lines is added to the study. For low loss, the antenna connection must have the same impedance value as the transmisson line characteristic impedance.

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At (near) the end of the antenna element, electrons flow tends to stop and the flow gets reversed or reflected back into the element (reverse flow). Some flow can be considered as charging a capacitor with one plate being the antenna end and the other plate as a 'far field electric' effect. This leads to defining 'capacitive end effects' for the antenna element and radiating system. As the reflected electrons travel back down the element they have a different electrical phase compared to the electrons coming up the element (forward flow). The net effect is that the summed current at a defined point varies along the element. The radio power for an element appears nearly equal along it's entire length, thus the RF voltage must inversely vary compared to the current {P = E * I}. If power is constant, then the product of voltage and current is constant, when E increases then I must decrease and the reverse relationship must foolw. Smaller current means higher voltage and lower voltage means higher currents. This evolves directly from Ohm's Power Law, the product of voltage and current are equal to the RF power, again nearly a constant along the element length. The current at the end is approaching zero, meaning the RF voltage at the end is approaching a maximum value. As the reflected electron flow gets close to the end of the opposite antenna element it is relected back towards the initial end where we started. The length, at where this reverse flow becomes 'in-phase' with the initial forward phase, is defined as the half wavelength resonant point. When this occurs, the forward and reverse values add constructively (regeneration) and greatly increase the resulting EM field strength. This increased far field radiation strength is what we want for efficient RF tramsmission. The RF frequency at which the source power is 'in phase' with the antenna's half wavelength resonance causes a regenerative process making for an oscillating effect along the antenna element. The forward and reverse current adding constructively along the entire antenna element occurs only when the frequency of the RF signal and the current propagation velocity match up with the physical length of the antenna element. The EM field energy is created by the sum of the fields around the oscillating antenna element. Note that the preceding discription has not described the exact connection between the electric elements and the antenna element. This is intentional, it is why passive directors and reflectors can impact the antenna characteristics withou having a direct electrical connection to the 'driven' element. It uses just the presence of an RF current in any antenna element, the RF frequency and the length of the antenna element are important. A radio wave is a moving EM field that has velocity in the direction of it's travel. The electric intensity and magnetic intensity fields are at right angles to each other as they move away from the antenna element. These effects require design study to understand and are left to an individual student (read that as complex and difficult to fully understand).

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The lines of force for the electric field define the radiated wave's polarization. The electric field lines are parallel to the antenna element's direction. The magnetic field lines are perpendicular to the electric lines, thus perpendicular to the elment's direction. Thus if the antenna element is vertical to the earth's surface the antenna is a 'vertical antenna' and if the element is horizontal to the earth's surface it is a 'horizontal' antenna. If the direction of the propagating field rotates along the direction of propagation, the wave is elliptically or circular polarized and has a rotational direction (right hand or left hand rotation as looking from the antenna element). At low and medium frequencies, vertical polarization will provide more consistent performance as 'ground wave' is the predominate radiation mode. The earth's surface acts like a good conductor at these frequencies and tends to short out the electric field component giving a greater loss to the EM field as the distance increases. At high frequencies 'skywave' becomes the predominate propagation mode. As the ionoshere reflects RF waves, the polarization becomes more elliptically polarized. The preferred antenna polarization does not seem to be either vertiicl or horizontal dominated in the HF frequencies. At VHF & UHF it is best if the same choice is used at each antenna location. Usually a 3+ dB loss difference is incured if the polarizations are different (this effect can change signal strength as much as a gain/loss of 20 dB (for range) differences in the real world).

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Reciprocity is the antenna property that states that receiving characteristics are just (and always) the exact inverse of transmitting characteristics. Whatever is 'best' for transmitting is also the 'best' for reception. This is true for ALL antenna characteristics including initial EM wave formation, directional characteristics, efficiency, etc. As you cannot control the other end of the communication link, the best you can do is improve your 'near end' link efficiency and polarization.

The feedpoint for an antenna determines the antenna input impedance. The impedance of an antenna is equal to the ratio of the voltage to the current at the point on the antenna where the feed is connected to the antenna (feed point). If the feed point is located at a point of maximum current, the antenna impedance is usually in the 20 to 100 ohms range. If the feed point is moved to a maximum voltage point (near the ends), the impedance is as much as 500 to 10,000 ohms. The input impedance of an antenna depends on the conductivity or impedance of the surrounding ground. For example, if the ground is a simple stake driven about a meter into earth of average conductivity, the impedance of the monopole may be double or even triple the quoted values. Because this additional resistance occurs at a point on the antenna circuit where the current is high, a large amount of transmitter power will dissipate as heat into the ground rather than radiated as intended. Therefore, it is essential to provide as good a ground or artificial ground (counterpoise) connection as possible when using a vertical whip or monopole. The amount of power an antenna radiates depends on the amount of current which flows in it. Maximum power is radiated when there is maximum current flowing. Maximum current flows when the impedance is minimized which is when the antenna is resonated, so it's input impedance is has a "pure resistance" value. (When capacitive reactance is made equal to inductive reactance, they cancel each other, and impedance then equals a pure resistance.) Note this is at the antenna feedpoint which by definition is where any transmission line is connected. Care should be given to match this impedance of the transmission line being used.The antenna and transmission line efficiencies depend heavily on these effects - usually this is the differnce between a 'good' or 'bad' antenna system.

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If the tranmission line impedance is the same as the antenna impedance, they can be directly connected, but if not than some 'matching' electrical components will be required for best efficiency. These added components should be adjusted to provide the correct impedance transformation between the antenna and the transmission line (called tuning). Transmission line losses are quoted in terms of wavelengths of transmission line length. At low frequencies, fractions of a wavelength can be involved, at VHF dnd UHF many multiples of wavelengths are usually the normal outcome. Thus matching the line impedance and antenna impedance at the antenna becomes very critical. How critical becomes more of an effect as the frequency rises. Additionally, matching the input transmission line impedance to a transmitting/receiving electronics unit become very important for low loss circuit design. Modern transmitters can use a detection circuit and lower the units of power based on a degree of mismatch. If line input SWR is higher than about 1.5:1 power usually starts reducinging and at 2.0:1 the power can be reduced to very low values. So, matching at both ends of a transmission line become very important for good efficiency of the antenna system. Many beginners us 'matching units' at the transmitter end of the circuit to reduce the transmitter from reducing output power but ignore the antenna end of the transmission line matching. The result is low efficiency in the system and the poor performance that results. The more experienced ham will pay attention to matching the transmission line at both ends! (it is one of his secrets for station performance).

Antenna Gain, Directivity, Bandwidth, Take Off Angle, and Ground Effects are additional issues any practical antenna system needs to consider. Each has it's own set of issues and solutions to be investigated and implemented. Making correct choices will give you maximum system performance. Please consult some good handbooks or search online to learn about each of these areas. Understanding antenna topics will assist you in making good choices in your designs and result in much more favorable system performance.

Coax that connects things! topic divider

 

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