G9: Antennas and Feed Lines

Before we cut any wire, we need to understand the cable that connects your radio to your antenna. That cable is called a feed line (also "transmission line"). Two kinds matter here: coaxial cable ("coax"), the round cable with a center wire inside a shield, and parallel-conductor line, two wires running side by side a fixed distance apart (also called "open-wire" or "window line").

Characteristic impedance: the "electrical thickness" of a line

Every feed line has a built-in number called its characteristic impedance, measured in ohms. It is set entirely by the line's physical shape, not by its length and not by the frequency. For a parallel-conductor (two-wire) line, the characteristic impedance is determined by two things: the distance between the centers of the two conductors and the radius (thickness) of the conductors. Spacing and wire size, that is it. Common coax is 50 ohms; "window line" (a flat parallel line with little windows cut in the plastic spacer) has a nominal characteristic impedance of about 450 ohms. Memorize that 450-ohm figure for window line.

Attenuation: feed lines eat some of your signal

No cable is perfect; every foot of feed line turns a little of your signal into heat. That loss is called attenuation. Two facts the exam wants:

• Coax attenuation rises with frequency. As you go to higher frequencies, a given coax loses more of your signal. So the same cable that is fine on 80 meters can waste a lot of power at VHF. When asked how coax attenuation changes with increasing frequency, the answer is simply: attenuation increases.
• Feed line loss is expressed in decibels per 100 feet. The decibel (dB) is a unit for comparing two power levels; here it tells you how much weaker the signal is after traveling 100 feet of cable. So the standard units of feed line loss are dB per 100 feet. (Not ohms, decibels.)

Matching: why the antenna and line should agree

Here is the central idea of the whole group. When power reaches the antenna, you want the antenna to swallow all of it. That happens only when the antenna's feed point impedance (the impedance right where the feed line connects) matches the feed line's characteristic impedance. If they differ, some of the power cannot get into the antenna and instead reflects (bounces) back down the line toward the radio. So the cause of reflected power at the feed point is precisely a difference between the feed line impedance and the antenna feed point impedance. To prevent standing waves on the line, the antenna feed point impedance must be matched to the characteristic impedance of the feed line. Match them, and the reflections disappear.

SWR: putting a number on the mismatch

When forward power and reflected power coexist on a line, they combine into a fixed pattern of strong and weak spots called a standing wave. We measure how bad it is with the standing wave ratio (SWR), written as a ratio like 1:1 or 3:1. A perfect match is 1:1 (no reflection); bigger numbers mean a worse mismatch.

The beautiful part: for a simple resistive load (a load with no reactance), you can calculate SWR by hand. You just divide the larger of the two impedances by the smaller. That always gives a number 1 or greater. Two worked cases the exam uses directly:

• 50-ohm line into a 200-ohm load: 200 ÷ 50 = 4, so the SWR is 4:1.
• 50-ohm line into a 10-ohm load: 50 ÷ 10 = 5, so the SWR is 5:1.

Notice that whether the load is bigger or smaller than the line, you always put the bigger number on top, so the answer reads 4:1 and 5:1, never 1:4 or 1:5.

SWR, loss, and a couple of clever traps

SWR and feed line loss interact in ways the exam loves to test:

• High SWR makes a lossy line lose even more. On a high-SWR line, power sloshes back and forth instead of leaving in one trip, and each extra trip burns a little more in the cable. So high SWR increases loss in a lossy transmission line. (On a perfect, loss-free line it would not matter, but real cable is never perfect.)
• A tuner at the radio does NOT change the SWR out on the antenna's line. Suppose the SWR on the feed line is 5:1, and you put a matching network (an "antenna tuner," a box of adjustable parts) at the transmitter end and adjust it to show a comfortable 1:1 to the radio. What is the SWR still doing out on the feed line between the tuner and the antenna? It is still 5:1. The tuner only fools the radio into seeing a good match; the mismatch (and its standing waves) still lives on the line beyond the tuner.
• Line loss makes the SWR at the radio look better than it really is. Because reflected power gets attenuated on its way back, a high-loss line reduces the SWR measured at the input end of the line. So higher loss reduces SWR measured at the input to the line. A flattering SWR reading can actually be a symptom of a lossy cable, not a good antenna.

Memory trick: a tuner makes the radio happy but does not heal the line; and lossy cable lies to your SWR meter by making things look better than they are.

Now the fun part, real antennas you can actually build. The two you must know cold are the dipole and the vertical (monopole). Both come from one idea: an antenna works best when it is a particular fraction of a wavelength long. (A wavelength is the physical length of one full radio wave; lower frequencies have longer waves.)

The one formula you truly need

A half-wave dipole is a straight wire, fed in the middle, that is about half a wavelength long. The handy shortcut for its length is:

Length in feet = 468 ÷ frequency in MHz

Where does 468 come from? A wave in free space would suggest about 492 for a half wave, but real wire is a touch slower and has end effects, so hams use the practical number 468. Just memorize it. Let's run the exam's examples:

• Dipole for 14.250 MHz: 468 ÷ 14.250 = about 32.8 feet, which rounds to 33 feet.
• Dipole for 3.550 MHz: 468 ÷ 3.550 = about 131.8 feet, which rounds to 132 feet.

Notice the pattern: lower frequency means a longer antenna. A 3.5 MHz dipole is enormous compared to a 14 MHz one, because its wave is much longer.

The vertical (monopole) and its quarter-wave length

A monopole is essentially half of a dipole standing upright, a single quarter-wave element working against a ground or a set of radials (wires that act as an artificial ground). Its length is a quarter wavelength, so the shortcut is roughly half of the dipole number:

Quarter-wave length in feet = about 234 ÷ frequency in MHz

• Quarter-wave monopole for 28.5 MHz: 234 ÷ 28.5 = about 8.2 feet, which rounds to 8 feet.

Radials: the vertical's hidden half

A vertical needs something to "push against," and that is the radial system. Where you put the radials depends on the install:

• For a ground-mounted vertical, the radial wires should be placed on the surface of the ground or buried a few inches below it. More radials lying on the earth means less power wasted heating the soil.
• For an elevated quarter-wave ground-plane vertical (one mounted up in the air with just a few radials), a common way to bring the feed point impedance up to about 50 ohms is to slope the radials downward. Drooping the radials raises the feed impedance to match common coax.

Feed point impedance: what changes it

The feed point impedance is the impedance the feed line sees where it attaches. For a dipole, two things move it around:

• Height above ground: as you lower a horizontal half-wave dipole toward the earth, its feed point impedance steadily decreases. At a low 1/10 wavelength up, the impedance has dropped well below its free-space value.
• Feed point location: a dipole is normally fed at the center, where impedance is lowest. As you move the feed point from the center toward the ends, the impedance steadily increases. (That is why an end-fed wire shows a very high impedance, more on that in G9D.)

Radiation patterns: where the signal goes

Different antennas spray power differently:

• A dipole in free space, looked at in the plane that contains the wire, makes a figure-eight pattern at right angles (broadside) to the antenna. It radiates best off its sides and poorly off its ends.
• A quarter-wave ground-plane vertical is omnidirectional in azimuth, meaning it radiates equally well in all compass directions (a donut shape, all the way around). Great when you want to talk in every direction.
• Height blurs a dipole's pattern. At high takeoff angles (steeper than about 45 degrees), if a horizontal dipole is less than 1/2 wavelength high, its azimuthal pattern becomes almost omnidirectional, the clean figure-eight smears out into nearly all directions. So a low dipole does not really have favored directions overhead.

Polarization and a caution about random wires

• Horizontal vs. vertical polarization: an advantage of a horizontally polarized HF antenna over a vertical one is lower ground losses. Horizontal antennas interact less with the lossy soil right beneath them.
• Random-wire antennas: if you connect a random-wire antenna (just a long wire) directly to the transmitter, watch out, your station equipment may carry significant RF current. That can mean RF burns on the gear and odd behavior, which is why most operators feed wire antennas through a proper matching unit instead.

A dipole or vertical sends power in broad patterns. A directional antenna (a "beam") instead concentrates your signal in one direction, like swapping a bare light bulb for a flashlight. More power going the way you want means stronger signals there, both transmitting and receiving. The most common ham beam is the Yagi.

How a Yagi is built

A Yagi is a row of metal rods (elements) mounted on a long support called the boom. Only one element is actually connected to the feed line, the driven element, and it is the one doing the feeding. The other elements are not connected to anything; they are parasitic, meaning they pick up energy from the driven element and re-radiate it to steer the signal. There are two kinds:

• The driven element is about 1/2 wavelength long, essentially a dipole.
• A reflector sits behind the driven element and is longer than it; it bounces energy forward.
• One or more directors sit in front and are shorter than the driven element; they pull energy forward.

So for a three-element Yagi: the reflector is longer, and the director is shorter, than the driven element. Remember it as "long in back, short in front."

Two key performance numbers

• The main lobe is the direction of maximum radiated field strength from the antenna, simply the direction the beam points strongest.
• The front-to-back ratio is the power radiated in the main lobe compared to that radiated in the opposite (rearward) direction. A high front-to-back ratio means the antenna is good at ignoring signals coming from behind, which helps you reject interference and noise from the back.

Getting more gain

Gain is how much an antenna concentrates power in its favored direction compared to a reference. Two ways to get more:

• Make the Yagi bigger. The primary effect of increasing boom length and adding directors is that gain increases. A longer boom with more directors focuses the beam more tightly.
• Stack antennas. In free space, taking two three-element Yagis and spacing them vertically 1/2 wavelength apart gives roughly 3 dB higher gain than a single Yagi. (3 dB means about double the power in the favored direction.)

Two ways to state gain: dBi vs. dBd

Gain is quoted against a reference, and there are two common references, which trips people up:

• dBd compares the antenna to a dipole.
• dBi compares it to an "isotropic" antenna, an imaginary point that radiates equally in every direction.

Because a dipole already has a little gain over isotropic, the isotropic figure always reads higher for the same antenna. Specifically, gain in dBi is 2.15 dB higher than the same antenna's gain in dBd. So an antenna rated "6 dBd" is "8.15 dBi", same antenna, bigger-sounding number. Watch for sellers quoting dBi to make an antenna look hotter.

Tuning a Yagi, and matching it to coax

• Bandwidth: using larger-diameter elements increases a Yagi's bandwidth (the range of frequencies over which it works well). Fatter tubing equals broader bandwidth.
• What you can adjust: to optimize forward gain, front-to-back ratio, or SWR bandwidth, you can change the boom length, the number of elements, or the element spacing, so the correct exam answer is all of these choices. Element spacing along the boom is one of the most powerful knobs.
• Beta (hairpin) match: this is a shorted transmission line stub placed at the feed point of a Yagi to provide impedance matching. ("Stub" means a short piece of line used as a tuning part.) It brings the driven element's impedance to match the coax.
• Gamma match: a popular matching scheme whose handy feature is that it does not require the driven element to be insulated from the boom. That makes it mechanically simple, you can bolt the driven element straight to a grounded boom.

Beyond the everyday dipole, vertical, and Yagi, hams use a whole zoo of specialized antennas, each tuned to a particular need: short-range daytime contacts, multiband convenience, low-noise receiving, mobile operation, and more. The exam touches a representative handful.

NVIS: bouncing signals nearly straight up

NVIS stands for Near Vertical Incidence Skywave. The idea is to fire your signal almost straight up so it bounces off the ionosphere and rains back down over a wide local area, filling in the "skip zone" that is too far for ground wave but too near for normal skip. For short-skip work on 40 meters in the daytime, the best NVIS antenna is a horizontal dipole placed between 1/10 and 1/4 wavelength above the ground. Low and horizontal makes the signal go up and come back down close by.

End-fed and trap antennas

• An end-fed half-wave antenna is fed at one end instead of the center. Recall from G9B that impedance rises toward the ends, so its feed point impedance is very high (thousands of ohms), which is why it needs a special matching transformer.
• Antenna traps are tuned circuits placed in an antenna's elements; their primary function is to enable multiband operation. A trap effectively "cuts off" part of the antenna on certain bands so one physical antenna can work on several.
• A disadvantage of multiband antennas in general is that they have poor harmonic rejection. Because they happily resonate on several related frequencies, they also radiate unwanted harmonics (multiples of your frequency) more easily than a single-band antenna would.

Loops, halos, and log-periodics

• An electrically small loop (less than 1/10 wavelength around) has sharp nulls (directions of minimum reception) broadside to the loop, that is, off the flat faces of the loop. Those deep nulls make small loops excellent for direction finding, you rotate the loop until a signal disappears to find its bearing.
• A VHF/UHF halo antenna (a dipole bent into a near-circle) radiates omnidirectionally in the plane of the halo, giving horizontal polarization in all directions, handy for mobile SSB work.
• A log-periodic antenna has element length and spacing that vary logarithmically along the boom. Its big advantage is wide bandwidth, it covers a huge frequency range with one antenna, trading away some of the per-element gain a Yagi would have.

Receiving and mobile antennas

• A Beverage antenna is a very long wire laid low to the ground; its primary use is directional receiving for the MF and low HF bands (like 160 and 80 meters). It is a listening antenna, prized for hearing weak signals quietly, not for transmitting.
• A screwdriver mobile antenna (a motorized whip used on vehicles) adjusts its feed point impedance by varying the base loading inductance, a little motor moves a coil so the short antenna resonates on different bands.

Stacking and a familiar nickname

• An advantage of vertically stacking horizontally polarized Yagis is that it narrows the main lobe in elevation, squeezing the beam flatter so more power goes toward the horizon for long-distance work.
• Finally, an easy one: a dipole hung from a single central support, with its two ends drooping down, is commonly called an inverted V. It looks like an upside-down V, needs only one tall support, and is one of the most popular real-world wire antennas there is.