Tag Archives: Wireless Communications

Yagi-Uda Antenna/Parasitic Array

The Yagi-Uda antenna is a highly directional antenna which operates above 10 MHz and is commonly used in satellite communications, as well as with amateur radio operators and as rooftop television antennas. The radiation pattern for the Yagi-Uda antenna shows strong gain in one particular direction, along with undesirable side lobes and a back lobe. The Yagi is similar to the log periodic antenna with a major distinction between the two being that the Yagi is designed for only one frequency, whereas the log periodic is wideband. The Yagi is much more directional, so it provides a higher gain in that one particular direction that it is designed for.

The “Yagi” antenna has two types of elements: the driven element and the parasitic elements. The driven element is the antenna element that is directly connected to the AC source in the transmitter or receiver. A reflector element (parasitic) is placed behind the driven element in order to split the undesirable back lope into two smaller lobes. By adding directive parasitic elements in front of the driven element, the radiation pattern is stronger and more directional. All of these elements are parallel to each other and are usual half wave dipoles. These elements work by absorbing and reradiating the signal from the driven element. The reflector is slightly longer (inductive) than the driven element and the director elements are slightly shorter (capacitive).

It is well known in transmission line theory that a low impedance/short circuit load will reflect all power with an 180 degree phase shift (reflection coeffecient of -1). From this knowledge, the parasitic element can be considered a normal dipole with a short circuit at the feed point. Since the parasitic elements reradiate power 180 degrees out of phase, the superposition of this wave and the wave from the transmitter leads to a complete cancellation of voltage (a short circuit). Due to the inductive effects of the reflector element and the capacitive effects of the director antennas, different phase shifts are created due to lagging or leading current (ELI the ICE man). This cleverly causes the superposition of the waves in the forward direction to be constructive and destructive in the backwards direction, increasing directivity in the forward direction.

Advantages of the Yagi include high directivity, low cost and high front to back ratio. Disadvantages include increased sizing when attempting to increase gain as well as a gain limitation of 20dB.

yagi

VHF and UHF

The RF and microwave spectrum can be subdivided into many bands of varying purpose, shown below.

radiospec

On the lower frequency end, VLF (Very Low Frequency) tends to be used in submarine communication while LF (Low Frequency) is generally used for navigation. The MF (Medium Frequency) band is noted for AM broadcast (see posts on Amplitude modulation). The HF (shortwave) band is famous for use by HAM radio enthusiasts. The reason for the widespread usage is that HF does not require line of sight to propagate, but instead can reflect from the ionosphere and the surface of the earth, allowing the waves to travel great distances. VHF tends to be used for FM radio and TV stations. UHF covers the cellphone band as well as most TV stations. Satellite communication is covered in the SHF (Super High Frequency) band.

Regarding UHF and VHF propagation, line of sight must be achieved in order for the signals to propagate uninhibited. With increasing frequency comes increasing attenuation. This is especially apparent when dealing with 5G nodes, which are easily attenuated by buildings, trees and weather conditions. 5G used bands within the UHF, SHF and EHF bands.

Speaking of line of sight, the curvature of the earth must be taken into account.

los

The receiving and transmitting antennas must be visible to each other. This is the most common form of RF propagation. Twenty five miles (sometimes 30 or 40) tends to be the max range of line of sight propagation (radio horizon). The higher the frequency of the wave, the less bending or diffraction occurs which means the wave will not propagate as far. Propagation distance is a strong function of antenna height. Increasing the height of an antenna by 10 feet is like doubling the output power of the antenna. Impedance matching should be employed at the antennas and feedlines as losses increase dramatically with frequency.

Despite small wavelengths, UHF signals can still propagate through buildings and foliage but NOT the surface of the earth. One huge advantage of using UHF propagation is reuse of frequencies. Because the waves only travel a short distance when compared to HF waves, the same frequency channels can be reused by repeaters to re-propagate the signal. VHF signals (which have lower frequency) can sometimes travel farther than what the radio horizon allows due to some (limited) reflection by the ionosphere.

Both VHF and UHF signals can travel long distances through the use of “tropospheric ducting”. This can only occur when the index of refraction of a part of the troposphere due to increased temperature is introduced. This causes these signals to be bent which allows them to propagate further than usual.

Feed Lines (HAM Radio)

The following are questions that are used for the HAM Radio Technician level license. The title of this section of questions is: “Feel Lines: types of feed lines; attenuation vs. frequency; SWR concepts; matching; weather protections; choosing RF connectors and feed lines.”

Question 1.

Why is it important to have a low SWR in an antenna system that uses coaxial cable feed line?
A. To reduce television interference
B. To allow the efficient transfer of power and reduce losses
C. To prolong antenna life
D. All of these choices are correct

SWR or Standing Wave Ratio refers to the efficiency of an antenna. A low SWR means that the loss will be reduced in the system. Therefore, the correct answer is B.

 

Question 2.

What is the impedance of the most commonly used coaxial cable in typical amateur radio installations?
A. 8 ohms
B. 50 ohms
C. 600 ohms
D. 12 ohms

The correct answer is B. 50 Ohm transmission lines are very common in most RF systems. Television systems use 75 Ohm lines.

 

Question 3.

Why is coaxial cable used more often than any other feed line for amateur radio antenna systems?
A. It is easy to use and requires few special installation considerations
B. It has less loss than any other type of feed line
C. It can handle more power than any other type of feed line
D. It is less expensive than any other types of feed line

The correct answer is A, considering it’s ease of use.

 

Question 4.

What does an antenna tuner do?
A. It matches the antenna system impedance to the transceiver’s output impedance
B. It helps a receiver automatically tune in weak stations
C. It allows an antenna to be used on both transmit and receive
D. It automatically selects the proper antenna for the frequency band being use

An antenna tuners are used if the SWR is too high for a radio to operate properly. It matches the antenna’s impedance to the impedance of a transmitter. Automatic tuners also exist. Correct answer: A.

 

Question 5.

What generally happens as the frequency of a signal passing through coaxial cable is increased?
A. The apparent SWR increases
B. The reflected power increases
C. The characteristic impedance increases
D. The loss increases

Coaxial cables work well within certain frequency ranges, however most are not rated to go above certain ranges. You’ll need to pay more for coaxial cable that can handle higher frequencies. The reason for them not working well at high frequencies is due to loss. Correct answer: D.

 

Question 6.

Which of the following connectors is most suitable for frequencies above 400 MHz?
A. A UHF (PL-259/SO-239) connector
B. A Type N connector
C. An RS-213 connector
D. A DB-25 connector

Type N connectors are used above 400 MHz. Correct answer: B.
N

 

Question 7.

Which of the following is true of PL-259 type coax connectors?
A. They are preferred for microwave operation
B. They are water tight
C. They are commonly used at HF frequencies
D. They are a bayonet type connector

PL-259 type coax connectors are used in UHF, HF applications. Correct answer: C.pl259

 

Question 8.

Why should coax connectors exposed to the weather be sealed against water intrusion? A. To prevent an increase in feed line loss
B. To prevent interference to telephones
C. To keep the jacket from becoming loose
D. All of these choices are correct

The correct answer is A. Water intrusion can cause an increase in loss. Correct answer: A.

 

Question 9.

What might cause erratic changes in SWR readings?
A. The transmitter is being modulated
B. A loose connection in an antenna or a feed line
C. The transmitter is being over-modulated
D. Interference from other stations is distorting your signal

Standing wave ratio is important for the measuring the efficiency of equipment. Modulation or interference therefore will not have an effect on the SWR. Loose connections may however cause an issue with SWR. Correct answer: B.

 

Question 10.

What electrical difference exists between the smaller RG-58 and larger RG-8 coaxial cables?
A. There is no significant difference between the two types
B. RG-58 cable has less loss at a given frequency
C. RG-8 cable has less loss at a given frequency
D. RG-58 cable can handle higher power levels

RG-58 and RG-8, although similar are different in that the RG-8 coax cable has less loss per length. Correct answer: C. The following table lists losses per feet:

coax1

 

 

Question 11.

Which of the following types of feed line has the lowest loss at VHF and UHF?
A. 50-ohm flexible coax
B. Multi-conductor unbalanced cable
C. Air-insulated hard line
D. 75-ohm flexible coax

Air-insulated hard line coax has the lowest loss with added insulation. Correct answer: C.