Reading Assignment

Read Chapter 3 pages 4-8 to 4-14 in your text before you continue.

Ok we have these great antennas but we have to get the transceiver hooked up to the antenna. Antennas are not perfect and one antenna won't work on the whole band

Feedlines

Ordinary electrical cables are ok to carry low frequency AC, such as household power, which reverses direction 120 times per second. However, they cannot be used to carry currents in the radio frequency range or higher, which reverse direction millions to billions of times per second, because the energy tends to radiate off the cable as radio waves, causing power losses.

Radio frequency currents also tend to reflect from discontinuities in the cable such as connectors, and travel back down the cable toward the source. These reflections act as bottlenecks, preventing the power from reaching the destination.

ion Transmission lines use specialized construction such as precise conductor dimensions and spacing, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. Common types of transmission line include ladder line, coaxial cable & rigid hardline. The dimensions of feedline connector determine the Characteristic Impedance of the cable. When the cable is terminated at the antenna end by an impedance equal the characteristic impedance of the transmission line, maximum power is transferred to the antenna and back at the source (transmitter) the impedance appears to be equal to the termination impedance of the antenna. Therefore maximum transfer of power from the transmitter to the feedline is achieved.

The only loss of power in a well matched system is because of the losses in the transmission line itself. This loss in the transmission line is lost as heat just like you heated a resistor. The loss in any cable system will be more the higher the frequency being applied and transferred happens to be. This is well documented in the specifications of the feedline. Typical feedline impedance is 50 or 300 ohms.

The point on the driven element of the antenna where the feedline is attached is called the Feed Point. It is usually in the middle of a dipole antenna with 1/4 wavelength of antenna element on each side and more often than not 50 ohms impedance.

The antenna will have an Impedance. or AC resistance at its feed point on the frequency it is designed to be operated. Typically it is 50 ohms, but can vary. If the feed point is, say 300 ohms and you are feeding it with a 50 ohm cable hooked to a 50 ohm transmitter, then you will have to use an impedance matching device between the antenna and the feedline.

Coax cable is the most used transmission line because it is easy to use and requires few special installation considerations. The bad boy of antenna transmission lines is moisture in the connectors, so care should be used to keep moisture out. Use silicone based grease in the connections so there is no space for moisture to accumulate. Over time the jacket of coax will crack and it will have to be replaced.

The lowest loss will be an air-insulated hardline, however, it is rarely used by hams because of cost and difficulties in the installation. This type of feedline will usually have a dry nitrogen system connected to the feedline to keep a positive pressure so moisture will not get into the air space and make the system unworkable.

Refer to your text book Section 4.3 on page 4-8 & 4-9 to view more cables.

Connectors

ion

Coax connectors are used to connect via coax transceivers, antenna tuners, swr-meters, antenna transmission (feedlines) and antennas together.


PL259 Connectors

The PL259 UHF Connector is used to connect most HF antenna connections. Note a UHF connector is used for HF frequencies.

PL259

For UHF frequencies (above 400 MHz) the 'N' Connector is more commonly used. This is because of the losses associated with the PL259 and associated coax at VHF and UHF frequencies.

The higher the frequency, the greater the losses in a given length of feed line.

Other connectors shown are the BNC connector, generally used for low power level & test cables.

Most of the other connectors shown are adapters to connect different connectors and coax to each other.




ion Soldering traces on a printed circuit board, hard wired circuit or connectors is an art. You must use enough heat to melt the solder itself, and the components being solder so that the solder flows freely. Allow the components to cool completely before any movement of the components or you will end up with a cold-solder joint. This cold-joint is distinguished by a dull finish rather than a nice shiny surface. Use Rosin Core Solder to solder electronic components. Never use Acid-Core solder commonly used in plumbing repair.

Standing Wave Ratio

SWR Standing Wave Ratio is a measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission line, into an antenna.

In an ideal system, 100% of the energy is transmitted. This requires an exact match between the source impedance, the characteristic impedance of the transmission line and all its connectors, and the antenna's impedance.

An ideal transmission line would have an SWR of 1:1, with all the power reaching the destination and no reflected power.

A problem with transmission lines is that impedance mismatches in the cable tend to reflect the radio waves back toward the source end of the cable, preventing all the power from reaching the destination end.

ion Any power lost in the feedline itself is converted to heat

A high SWR represents high reflection, with much of the power reflected back down the cable.

A SWR of 2:1 or higher will cause the protection circuits in most solid-state transmitters begin to reduce transmitter power to prevent damage to the radio.

The SWR of a transmission line is measured with an instrument called an SWR meter, and checking the SWR is a standard part of installing and maintaining transmission lines.

ion A even better instrument to determine if a feedline and antenna are properly matched is a Directional Wattmeter sometimes called a tru-line wattmeter. The directional wattmeter shows the actual power going forward to the antenna and the actual power not radiated and being reflected. An SWR meter is only a relative measurement and it can give lower than actual reflections depending on its placement in the circuit.

In the past, during the CB craze, when there was much misunderstanding of impedance mismatches, some believed they could fix the mismatches by changing the length of the feedline. Actually they were just fooling the SWR meter. The transmission line length has nothing to do with impedance mismatches.

However it is true that some losses occur in feed lines and there is more loss in longer lengths and this is higher as the frequency used is higher. Therefore it is better to keep the transmission line as short as practical.

Baluns

A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced (single-ended), and the reverse. They are also often used to connect lines of differing impedance. The origin of the word balun is bal(ance) + un(balance).

A balun is used for three purposes: 1. Isolate the transmission line from the antenna, 2. Provide balanced output current to the antenna & 3. Properly used as a matching network.

1. Feedline Isolation: Current-type baluns are especially effective in reducing RF current on outer surface of a coaxial cable's shield. There are different RF currents flowing on or within a coaxial cable. There is a current flowing on the center conductor of the cable. Due to the skin-effect, there are currents flowing on the cable's braided second conductor (its shield) which surrounds the inner conductor. Without a device to isolate the antenna from the feed line, the outer surface of the coax's shield is part of the antenna. On its way back down the coax, some current is radiated, some is conducted back to the transmitter and on to station's ground system, house wiring, etc.

ion The use of a 1:1 current balun on dipoles and directional (Yagi, Beam etc.)antennas is highly recommended.

2. Current baluns avoid the bad habits that conventional 'Voltage-type' baluns exhibit. 'Voltage-type' baluns try to produce equal and opposite voltages at the baluns balanced port regardless of the load impedance. Since low impedance antennas are current fed, a balun that produces equal and opposite currents at its output over a wide range of load impedances is desirable.

3. Some antennas do not have a characteristic impedance of 50 ohms. The most used antenna feedline is 50 ohms. So, if an antenna has an impedance of 300 ohms, the impedance needs to be matched. There are also times when 300 ohm balanced feedline is used and hooked up to a 50 ohm feed point impedance antenna. A properly designed balun can provide this matching in addition to isolation.

A good tutorial on Current Baluns can be found at RadioWorks.com

Antenna Matching

ion If you have an antenna, transmission line in good shape and it works good why do we need to do antenna matching? Well an antenna designed for a specific frequency is not a perfect match for a different frequency. In other words no antenna is perfect all across the band of frequencies we might want to operate.

What we do is use what is commonly called an Antenna Tuner or Antenna Matcher which matches the impedance of the transmitter to the impedance of the antenna &ammp; feedline. Another way to way to express it is: Lets say the antenna exhibits an impedance of 70 ohms at this new frequency. The transmitter is 50 ohms. An impedance mismatch for sure. This happens because the antenna is now too short or too long for this new frequency.

The antenna impedance matcher or tuner will match these impedances so the transmitter will see an exact 50 ohms and the antenna looking back will see an exact 70 ohms. Now maximum power is transferred and none or little is reflected back.

The way the tuning is done is Inductance or Capacitance is switched in or out for the coarse match and then fine tuning is accomplished by using a variable inductor and/or capacitor, tuning for minimum SWR.

You can easily see that the correct name for the box should be an impedance matcher not an antenna matcher, but the term is ingrained into our hobby, but you now know the difference.

But the key to having a good system is to have a good antenna to begin with. You might match a transmitter to a bad antenna, but the antenna is still bad.

Problem solving: sometimes you will have erratic SWR readings and you won't be able to tune it properly or you will tune it but it will not hold. Look at the obvious point that could cause a problem. Erratic readings are probably caused by a loose connector or water in the system. High SWR readings occurring over time may mean the feedline is starting to fail. Maybe its time to take the antenna down remake all the connections and inspect all the aspects of the antenna and feedline system.

ion Automatic Impedance Matching devices are available to make your ham radio life much easier. Especially if you use a multi-band antenna and operate on multiple bands.

When you change bands and you have an "Manual Antenna Tuner" you would now have to adjust the course and fine adjustment until you achieve proper impedance matching.

You could memorize the settings for each frequency you use, but that still takes time and fine tuning to get it right.

An Automatic Tuner will do two things: One automatically match your transmitter/receiver to your antenna & Two remember the settings, so when you return the tuning is ever quicker - seemly instantaneous.

You can find antenna matchers and auto tuners at many sources. Ham Radio Outlet and Universal are two well known outlets.

Antenna Analyzers

ion An antenna analyzer is a device used for measuring the efficiency of antenna systems. The antenna analyzer is essentially a bridge where the impedance of two legs are compared. When the two impedances are the same the bridge will be balanced. Using this circuit it is possible to either measure the impedance of the antenna, or it is possible to adjust an antenna's physical length until it has the same impedance desired.

All of this impedance matching is done at a specific frequency or band of frequencies.

You can also determine the bandwidth of the antenna. What this means, is will the antenna perform over the entire band or part of the band where you want to operate.

The antenna analyzer can 'fine-tune' your antenna. Remember, a good antenna system properly installed and tuned is the most important part of your ham radio station.

Unlike an SWR meter which measures the standing wave ratio in the cable feeding the antenna, a complex-impedance antenna analyzer requires little or no transmitted power applied to the antenna. The SWR meter works by measuring the reflected wave from the antenna back to the transmitter, which is minimal when the antenna is balanced. A badly tuned antenna can produce enough reflected power to damage the transmitter. Since an analyzer can be used without transmitting, there is no risk of transmitter damage.

Many amateur radio operators use antenna analyzers to optimize an existing antenna or troubleshoot antenna related problems such as diagnosing a break in coax cable and finding faulty antenna components.

QUIZ 10

Lets learn by doing the quiz and learn what we need to reinforce by our answers to the questions. More practice.

You can take the quiz as often as you wish.

Take Quiz 10

Review

In this module you have been introduced to antennas feedlines and antenna matching. Another big bite to chew. What a large subject to master. Don't be intimidates because antenna experimentation is one of the facets of Ham Radio that is the most fun. Start out small, make a J-pole or a simple Ground Plane, its really all you need to hit the local repeater. Then after learning how to swim, go after some DX on simplex (don't remember - look it up in the term glossary). Look at the recommended reading and do some internet searches to gain more knowledge on the subject matter..

Recommended reading

Frequently Asked Questions About Antenna Systems and Baluns by Jim Thompson, W4THU

The ARRL Antenna Compendium from, of course, The ARRL.

First Steps in Ham Radio - a series of articles intended to help you get started.

ARRL Technical Information Service - numerous links and articles on a variety of technical topics

Understanding Basic Electronics - introduction to the electronics of radio

View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.

Review the Term Glossary in Chapter 10 of your Ham Radio License Manual

Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.