Radiation Pattern

The three-dimensional radiation behavior of antennas is described by their radiation pattern (normally in the far field). As explained before, only an isotropic radiator would exhibit the same radiation in every spatial direction, but this radiator cannot be implemented for any specified polarization and is therefore mainly suitable as a model and comparison standard. 

Dipoles and monopoles possess directivity. An electrically short dipole in free space has a three-dimensional radiation pattern shown in Figure below with nulls in the direction of the antenna's axis.

While the radiation pattern is actually three-dimensional, it is common however to describe this behavior with two planar patterns, also called the principal plane patterns. They can be obtained from the spatial radiation characteristics by looking at a cut-plane - usually through the origin and the maximum of radiation. Spherical coordinates as shown in Figure below are commonly used to describe a location in the three-dimensional space.

The horizontal pattern (see Figure below) shows the field strength as a function of the azimuth angle ϕ with a fixed ϑ (usually ϑ = 90°).

The vertical pattern (see Figure below) shows the field strength as a function of ϑ for a fixed ϕ (usually ϕ = +/- 90 ° or 0°/180°)

Usually antenna patterns are shown as plots in polar coordinates. This has the advantage that the radiation into all possible directions can quickly be visualized. In some occasions (i.e. for highly directive antennas) it can also be beneficial to plot the radiation pattern in Cartesian coordinates - because this reveals more details of the main beam and adjacent side lobes, see figure below.

From the radiation pattern the following additional parameters can be derived (see Figure below)

• The side lobe suppression (or side lobe level) is a measure of the relation between the main lobe and the highest side lobe.
• The half-power beamwidth (HPBW) is the angle between the two points in the main lobe of an antenna pattern that are down from the maximum by 3 dB. It is usually defined for both principal plane patterns.
• The front-to-back ratio specifies the level of radiation from the back of a directional antenna. It is the ratio of the peak gain in forward direction to the gain in the reverse (180°) direction. It is usually expressed in dB.

Reference : Antenna Basic - Rohde & Schwarz

Log Periodic Antenna

Antenna Theory - Log-periodic antenna - The Yagi-Uda antenna is mostly used for long time . However, for better reception  and to tune over a range of frequencies, we need to have another antenna known as the Log-periodic antenna. A Log-periodic antenna with good impedance is a logarithamically periodic function of frequency. 

A special type of directional antenna is the log-periodic dipole antenna (LPDA), where beam shaping is performed by means of several driven elements. The Log Periodic Dipole Antenna is made up of a number of parallel dipoles of increasing lengths and spacing (see Figure 26). Each dipole is fed out of phase to the element on either side by a common feed line. The angle α formed by the lines joining the dipole ends and by the longitudinal axis of the antenna remains constant, as well as the graduation factor 𝜏 which is equal to the ratio of the lengths of neighboring elements and their spacing: 

Log Periodic Dipole Antenna Design (courtesy Rohde & Schwarz)

This antenna type is characterized by its active and passive regions. The antenna is fed starting at the front (i.e. with the shortest dipole). The electromagnetic wave passes along the feed line and all dipoles that are markedly shorter than half a wavelength will not contribute to the radiation. 

The dipoles in the order of half a wavelength are brought into resonance and form the active region, which radiates the electromagnetic wave back into the direction of the shorter dipoles. 

This means that the longer dipoles located behind this active region are not reached by the electromagnetic wave at all. The active region usually comprises of 3 to 5 dipoles and its location obviously varies with frequency. The lengths of the shortest and longest dipoles of an LPDA-Log Periodic Dipole Antenna determine the maximum and minimum frequencies at which it can be used.

Due to the fact that at a certain frequency only some of the dipoles contribute to the radiation, the directivity (and therefore also the gain) that can be achieved with LPDAs-Log Periodic Dipole Antenna is relatively small in relation to the overall size of the antenna. However, the advantage of the LPDA is its large bandwidth which is - in theory - only limited by physical constraints.

The radiation pattern, of an LPDA is almost constant over the entire operating frequency range. In the H-plane it exhibits a half-power beamwidth of approx. 120°, while the E-plane pattern is typically 60° to 80° wide. The beamwidth in the H-plane can be reduced to values of approx. 65° by stacking two LPDAs in V-shape.

Example of a V-Stacked LPDA Antenna

V-stacked LPDA antennas have E- and H-plane patterns with very similar half power beamwidths. Additionally they feature approx. 1.5 dB more gain compared to a normal LPDA.

Radiation pattern of a V-stacked LPDA - E-plane (black), H-plane (blue)

Reference:https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/premiumdownloads/premium_dl_brochures_and_datasheets/premium_

dl_whitepaper/Antenna_Basics_8GE01_1e.pdf

Antenna Impedance Measurement

Input Impedance 

One of the most significant parameters of an antenna is its input impedance: 

𝑍𝑖𝑛 = 𝑅𝑖𝑛 +𝑗𝑋𝑖𝑛 

This is the impedance present at the antenna feed point. Its real part Rin can be split up into the radiation resistance RR and the loss resistance RL 

𝑅𝑖𝑛 = 𝑅𝑅 + 𝑅𝐿 

It should be noted however that the radiation resistance, being the quotient of the radiated power and the square of the RMS value of the antenna current, is spatially dependent. This applies also to the antenna current itself. Consequently, when specifying the radiation resistance, its location on the antenna needs to be indicated.

Quite commonly the antenna feed point is specified, and equally often the current maximum. The two points coincide for some, but by no means for all types of antenna. The imaginary part Xin of the input impedance disappears if the antenna is operated at resonance. Electrically very short linear antennas have capacitive impedance values (Xin < 0), whereas electrically too long linear antennas can be recognized by their inductive imaginary part (Xin > 0).

Nominal Impedance The nominal impedance Zn is a mere reference quantity. It is commonly specified as the characteristic impedance of the antenna cable, to which the antenna impedance must be matched (as a rule Zn = 50 Ω). 

Impedance Matching and VSWR If the impedance of an antenna is not equal to the impedance of the cable and/or the impedance of the transmitter, a certain discontinuity occurs. The effect of this discontinuity is best described for the transmit case, where a part of the power is reflected and consequently does not reach the antenna (see Figure below.) However the same will happen with the received power from the antenna that does not fully reach the receiver due to mismatch caused by the same discontinuity.

Forward and reflected power due to mismatch 

The amount of reflected power can be calculated based on the equivalent circuit diagram of a transmit antenna (see Figure below).

For optimum performance, the impedance of the transmitter (ZS) must be matched to the antenna input impedance Zin. According to the maximum power transfer theorem, maximum power can be transferred only if the impedance of the transmitter is a complex conjugate of the impedance of the antenna and vice versa. Thus the following condition for matching applies: 

𝑍𝑖𝑛 = 𝑍𝑆 ∗ 𝑤ℎ𝑒𝑟𝑒 𝑍𝑖𝑛 = 𝑅𝑖𝑛 + 𝑗𝑋𝑖𝑛 𝑎𝑛𝑑 𝑍𝑆 = 𝑅𝑆 + 𝑗𝑋𝑆 

If the condition for matching is not satisfied, then some power may be reflected back and this leads to the creation of standing waves, which are characterized by a parameter called Voltage Standing Wave Ratio (VSWR). 

The VSWR is defined (as implicated by its name) as the ratio of the maximum and minimum voltages on a transmission line. However it is also possible to calculate VSWR from currents or power levels as the following formula shows: 

Another parameter closely related to the VSWR is the reflection coefficient r. It is defined as the ratio of the amplitude of the reflected wave Vrefl to the amplitude of the incident wave Vforw : 

It is furthermore related to the VSWR by the following formula: 

The return loss ar derives from the reflection coefficient as a logarithmic measure:

 

So there are in fact several physical parameters for describing the quality of impedance matching; these can simply be converted from one to the other as required. For easy conversion please refer to the table below: 

Baluns and Impedance Matching

An Antenna is normally connected to a transmission line and good matching between them is very important. A coaxial cable is often employed to connect an antenna mainly due to its good performance and low cost. A half-wavelength dipole antenna with impedance of about 73 ohms is widely used in practice. From the impedance -matching point, this dipole can match well with a 50 or 75 ohm standard coaxial cable . Now the question is : can we connect a coaxial cable directly to a dipole ?

As illustrated n figure below . when a dipole is directly connected to a coaxial cable, there is a proble: A part of the current comming from the outer conductor of the cable may go to the outside of the outer conductor at the end and return to the source rather than flow to the dipole .

A Dipole Antenna directly connected to coaxial cable 

 This undesirable current will make the cable become part of the antenna and radiate or receive unwated signal, which could be a very serious problem in some cases. In order to resolve this problem, a balun is required.

The term balun is an abbreviation of the two words balance and unbalance. It is a device that connects a balanced antenna ( a dipole , in this case) to an unbalanced transmission line (a  coaxial cable, where the inner conductor is not balanced with the outer conductor). The aim is to eliminate the undesirable current comming back on the outside of the cable. There are a few baluns developed for this important application. Figure below shows two examples . 

Two example of baluns

The sleeve balun is a very compact configuration: a metal tube of 1/4 Lambda is added to cable to form another transmission line (a coax again) with the outer conductor cable , and a short circuit is made at the base which produces an infinite impedance at the open top. The Leaky current is reflected back with a phase shift of 180 degrees, which results in the cancellation of the unwanted current on the outside of the cable. This balun is a narrowband device. If the short-circuit end is made as a sliding bar, it can be adjusted for a wide frequency range (but it is still a narrowband device). 

The second example is a ferrite-bead choke placed on the outside of the coaxial cable. It is widely used in EMC Industry and its function is to produce a high impedance, more precisely high inductance due to the large bandwidth may be obtained ( an octave or more). But this device is normally just suitable for frequencies below 1 GHz. which is determined by the ferrite properties it can be lossy, which can reduce the measured antenna efficiency.

Reference:https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/premiumdownloads

/premium_dl_brochures_and_datasheets/premium_dl_whitepaper/Antenna_Basics_8GE01_1e.pdf

Build This Simple Logic Probe Memory Circuit

Logic Probe Memory Circuit - The Circuit describe is a low cost logic probe with  memory with IC TTL 74107 Dual JK Flip Flop , Diode, resistor, and normaly closed switch provides the probe with memory function that really comes ih handy for "trapping" very fast pulse indications. See the circuit diagram. IC1 used TTL 7404. Used 5V adaptor/or battery.

reference : Popular Electronics  May 1974

Antenna Grounding Kit

Outdoor Antenna Installation Tips

Looking for more guidance or want to educate yourself before cutting the cord? Here are some tips tailored to safely installing your outdoor TV antenna.

Please note: Every time you move or reconnect your antenna, you must scan for channels on your television. Consult your television owner's manual for more precise guidelines. We include Quick Start guides with all our TV antennas and also provide them here on our website for free.

Installation tips:

1. Before installing an antenna permanently on a roof or in your attic, test reception in that area and other locations prior to installation. To perform a test, connect the coaxial cable from the antenna to your TV. Then, place your antenna in the desired location. Turn on your TV using your TV and, using your remote, complete a channel scan. Once complete, flip through your TV channels and watch for signal interruptions. Make sure your antenna is installed where you get the best signal and the highest number of available channels.
2. Higher is always better. Mount the antenna on your roof or in the attic for optimal performance. These locations are more likely to experience fewer obstacles which cause signal interference between the antenna and broadcast towers.
3. Face the front of the antenna toward the broadcast towers. Even multi-directional antennas require this to achieve the best possible reception. Don't know where your towers are? Visit our transmitter locator or download our free Antenna Point app. 
4. Check your outdoor antenna regularly for secure coaxial cable connections and signs of corrosion. Sometimes debris or humidity can interfere with reception. Where possible, cover all connections and use waterproof sealant when installing an antenna mast. (See our included sealing pads for reference.)
5. Installing your antenna near power lines is dangerous. The antenna must be at least 20 ft. (6 meters) away from all power lines. If any part of the antenna or mast assembly comes into contact with a power line, call your local power company. Do not remove it yourself.

Note: Unfortunately, sometimes antennas are returned to us in perfect working order but were returned due to faulty installation techniques. Continue reading for troubleshooting tips.

Troubleshooting tips:

Spotty reception with accessories:

1. For the best reception, make sure the coaxial cable is the correct length for your installation needs. Similar to getting your antenna up high, terminating your coaxial cable at the right length will provide better reception to your television. 
2. If you are using a splitter, diplexer, or your cable run must be longer than 100 ft., consider using a preamplifier to boost weak signals. 

Spotty reception:

1. Reflected signals are also called "multipath interference". For those living close to broadcast towers, signal loss can occur when strong signals bounce off nearby buildings and other surfaces in the area. Aim your antenna in different directions, even sometimes away from the towers, and scan for channels. If this doesn't improve your reception, your installation may require an attenuator.
   
2. Do not install your antenna near metallic objects or reflective surfaces, as this could also cause signal interference.
3. Since the switch in 2007 from analog to digital signals, receiving TV signal is "all or nothing". You won't see "fuzz" or "snow" on your TV screen if the signal is weak or there is no signal. When a digital signal is received, it will display crystal-clear on your TV. If the signal is interrupted, your TV screen will be blank.

Combining multiple antennas

When combining multiple antennas on the same mast, keep at least 4 to 6 feet of vertical separation between the two antennas to prevent interference. If you want to combine signals from a UHF antenna with a VHF antenna so there is only one down-lead going into your house, use our UHF/VHF signal combiner with a channel filter for each antenna, designed not to pick up out-of-phase signals through the other antenna. For the best results, use equal lengths of coaxial cable from the output of each antenna when connecting to the UHF/VHF combiner.

Optional grounding information

For outdoor TV antenna installations, grounding the coaxial cable will protect your equipment from voltage surges created by nearby lightning strikes but will not protect from a direct strike. Check your local electrical codes to make sure your installation is in compliance. We recommend calling a professional electrician to advise or install your antenna. We have an educational page with suggestions for grounding your antenna.

Safety precautions:

If you are installing an antenna on the roof, assemble the antenna on the ground. Installing an antenna on windy days can be especially dangerous and even slight winds create strong resistance when attempting to set up an antenna or mast. 

Antennas that are improperly installed or mounted on inadequate structures are very susceptible to wind and weather damage. This damage could become life-threatening. The owner and installer assume full responsibility for the installation and verification that it is structurally sound to support all loads (weight, wind, ice, etc.) and is properly sealed against the elements and leaks.

Antennas Direct®, Inc. is not responsible or liable for any damage or injury resulting from antenna installations or by an antenna system failure due to any unknown variable applications.