Lesson 28

Transmission Lines

It’s time to connect your antenna to your transceiver. That is done using transmission lines, frequently called feedlines. We’ll focus on two types of transmission line here. We’ll use the term “coax” to refer to a coaxial cable with a plastic dielectric insulator. 

The other type of feedline we’ll cover is ladder line. It’s informally called “TV Antenna Wire.” That’s because it looks like what homes used with the antenna on their roof for local stations. Formally it’s “parallel conductor transmission line.” Why use a ladder line? Because it offers lower loss when compared to some coax.

Ladder line feedline

No matter the type of feed line, the velocity factor of the wire or cable will play a role. Velocity factor is the velocity of the wave in the transmission line divided by the velocity of light in a vacuum. It’s a number between 0 and 1, with 1 being the velocity of light in a vacuum. Simply, how fast does a wave travel over a wire versus light through the air. You can remember this as the only answer that is divided by velocity of light in a vacuum.

To figure the velocity factor in a coaxial transmission line we need to consider three parts. The copper core or the “center conductor” and a shield are the conductors. They get separated by a plastic or foam insulator. The insulators are made from dielectric materials. Coax can have a velocity factor of between 0.6 to 0.9 of the speed of light. The biggest effect on that performance is the insulating dielectric material  

Here’s a view of the difference between types of dielectric materials. It’s significant between foam dielectric coaxial cable and solid dielectric cable. Assuming all other parameters are the same, foam dielectric benefits include:

  • lower safe maximum operating voltage limits
  • lower loss per unit of length and
  • higher velocity factor

All these are correct when asked about foam dielectric benefits. 

Every wire has both a physical length, as well as an electrical length. The physical length is what could be calculated with a tape measure. The electrical length is a representation of how long the wire performs based on the velocity factor. The electrical length of a coax cable is typically longer than its physical length. Why? Because electromagnetic waves move more slowly in a coaxial cable than in air. Remember this is slower because the velocity factor will always be less than 1. At the perfect velocity of factor of 1 the wave would be traveling at the speed of light in a vacuum.

It’s not difficult to calculate the physical length and electrical length of wires. You’ll be asked to do one for the exam.  

Find the approximate physical length of an air-insulated parallel conductor transmission line. The line is electrically ½ wavelength long at 14.10 MHz.

Here’s the math. First convert the frequency to wavelength. To do that divide 300 by the frequency in megahertz. 300 / 14.10 = 21.28 meters.

That’s a full wavelength calculation, so divide by two for ½ wavelength. 21.28 / 2 = 10.64 meters.

Here’s a helpful and interesting fact. An air insulated parallel conductor transmission line has a velocity factor of almost 1! 

So our final answer is 10.6 times 1 equals 10.6 meters.

Before we completely go away from feedline types, let’s not forget microstrip transmission lines. It’s not something you are going to buy in a roll at a hamfest. Microstrip feedlines are precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies.

Troubleshooting Feed Lines

From time to time, you will need to understand why a feedline is not performing like you expect. One of the first things to do is understand if the feedline is open or shorted. “Open” means not connected. “Shorted” means cross connected. 

One of the next few questions shows up as one of the hardest questions on the exam based on our practice tests. Let’s take a deeper look at impedance readings when different things happen to your feedline. We’ll use a meter with a signal generator. 

Line Length Line Status Generator Shows
1/8-wavelength Open Capacitive reactance
1/8-wavelength Shorted Inductive reactance
1/4-wavelength Open Very Low Impedance
1/4-wavelength Shorted Very High Impedance
1/2-wavelength Shorted Very Low Impedance

Let’s imagine that Kaleb has a piece of coax running through his walls, and he needs to test it. Using a generator, like some antenna analyzers, here’s what he should look for. 

If he disconnects the cable at one end, and puts the generator on the other he’ll get one of two readings. On a 1/8-wavelength transmission line, if it’s open all the way, the generator will show a capacitive reactance. If there is a short, the generator will show an inductive reactance.

Kaleb moves on to a 1/4-wavelength line. If he connects his generator and it’s open, it will read very low impedance. A short will show very high impedance to a generator.

On his 1/2-wavelength transmission line, a short presents a very low impedance to a generator.

Here’s the real helpful part. You can mix and match these readings. Kaleb is still testing his 1/4-wavelength transmission line.   He sees an inductive reactance on the generator. This might mean he has a short somewhere near the middle of the transmission line.

Matching Antennas to Feed Lines

You have a beautiful transceiver and have invested in quality feedline. Now it’s time to match the impedance of your feedline to your antenna. Mismatches cause signal loss.  The better the match, the more efficient your signal will move to and from your antenna. The reflection coefficient describes the interactions of a load and a transmission line.   

You saw baluns on many of the wire antenna illustrations in earlier lessons. These handle the matching of the balanced side of the antenna to the unbalanced side of the feedline. Balun is a word that came from merging the terms “balanced” and “unbalanced.”  

For a Yagi antenna there are several ways you might match your antenna to your feedline. The first we’ll review is the gamma match. Here’s how it works as a matching system. Connect the shield of the coax to the center of the antenna and the conductor a fraction of a wavelength to one side. You frequently see these on short VHF and higher Yagi antennas. 

A Gamma match configuration may also have a series capacitor included. It’s used to cancel unwanted inductive reactance. You may also see a gamma match used to shunt-feed a grounded tower at its base. 

Another type of Yagi connection is the Delta match. It connects to the driven element in two places. They are spaced a fraction of a wavelength each side of the element’s center. This creates a triangle shape, or “delta.”

An additional Yagi matching option is the hairpin match. It’s a U-shaped piece of cable used to match driven elements of a yagi antenna. One popular use is in the design of tape measure yagis used to work satellites. Think of a hairpin, also called a beta match, like a coil or inductor. To use a hairpin matching system, an antenna’s driven element must be tuned to a capacitive (driven element electrically shorter than 1/2 wavelength) feed point impedance.  This is so the capacitance and inductance may cancel each other out. When matching a driven element using a beta or hairpin match, the driven element needs to be insulated from the boom. 

Let’s switch to a different type of impedance matching system. The stub match uses a short length of transmission line. It’s connected in parallel with the feed line at or near the feed point. You can remember this because a STUB uses a SECTION of transmission line.

Here are some matching scenarios for different types of antennas.  

For a loop antenna, you can match using transmission lines with different impedances. Let’s say your loop antenna has a feedpoint impedance of 100 ohms. You’re trying to match that to a 50 ohm feedline. In this case using a transmission line with a 75 ohms impedance would be suitable for matching.  

A phased array antenna has multiple driven elements. To ensure signals arrive in phase to those elements, phasing-lines are used. That allows you to control the antenna’s radiation pattern. We want the antennas of the phased array to operate together like musicians in a concert.

Feed line impedance is important in microwaves too. What kind of matching does a microstrip feed line use? Many have a Wilkinson divider to divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance. The key to remember with the Wilkinson divider is the equal split between two 50-ohm loads.

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