General Course, Module 7: Antennas That Work the World

7.1Feed Lines and SWR, With the Gloves Off

Start where Technician left off, and go deeper on three fronts.

Characteristic impedance is geometry. For a parallel-conductor line, it is set by the distance between the conductor centers and the radius of the conductors. Ladder-style window line runs about 450 ohms, and its selling point is loss: open-wire line loses far less than coax, especially when SWR is high.

SWR gets its causal definition: reflected power at the feed point is caused by a difference between the feed line impedance and the antenna feed point impedance. Purely resistive mismatches make the arithmetic simple, just the ratio of the two: a 50-ohm line into a 200-ohm load gives 4:1; into a 10-ohm load, 5:1 (always expressed as the larger number first). And the cure is stated the same way: preventing standing waves means matching the antenna feed point impedance to the line's characteristic impedance.

Now three truths about SWR that separate General understanding from Technician understanding:

• The tuner does not fix the line. With 5:1 SWR on the feed line and a matching network at the transmitter adjusted to show the radio 1:1, the SWR on the feed line itself is still 5:1. The tuner satisfies the transmitter; the line keeps living its mismatched life. Module 7 of the Technician course called the tuner a diplomat, and here is the proof.
• High SWR increases loss in a lossy line. Reflected power makes extra round trips, and every trip pays the line's toll again. On low-loss line, high SWR is nearly free; on lossy coax, it compounds.
• Line loss lies to your SWR meter. Loss attenuates the reflected wave on its way back to the meter, so higher loss makes the SWR measured at the transmitter end read lower than the truth at the antenna. A long run of cheap coax showing 1.5:1 may be hiding a 4:1 disaster at the feed point. Flattering meters and honest antennas are found at opposite ends of lossy cable.

Coax attenuation itself still rises with frequency, and feed line loss is specified in decibels per 100 feet, the unit on every cable datasheet you will ever read.

7.2Dipoles and Verticals by the Numbers

The formula taped to Ray's bench: length in feet = 468 / frequency in MHz for a half-wave dipole, half that (234 / f) for a quarter-wave vertical. Run the pool's three:

• Dipole for 14.250 MHz: 468 / 14.25 = about 33 feet.
• Dipole for 3.550 MHz: 468 / 3.55 = about 132 feet.
• Quarter-wave monopole for 28.5 MHz: 234 / 28.5 = about 8 feet.

Cut long, trim to resonance, exactly as the Technician rule (shorter = higher frequency) predicts.

Patterns and height are where dipoles get interesting:

• In free space, a dipole's pattern is a figure-eight at right angles to the antenna, the broadside rule from Technician drawn precisely.
• But height rewrites the pattern. A horizontal dipole less than 1/2 wavelength high becomes almost omnidirectional in azimuth at high elevation angles. On 80 meters, "low" is anything under 130 feet, which is why nearly every 80-meter dipole in the world is effectively an omnidirectional cloud warmer, and why that is exactly what NVIS wants (Section 7.4).
• Height also moves the feed point impedance: it steadily decreases as a dipole drops toward 1/10 wavelength above ground. And along the wire itself, impedance increases as the feed point moves from center toward the ends, from about 70 ohms at center to thousands at the tip. Hold that thought two sections.

Verticals have their own account book:

• A ground-mounted vertical lays its radial wires on the surface or buried a few inches down.
• An elevated ground-plane brings its feed point to approximately 50 ohms by sloping the radials downward.
• The pattern is omnidirectional in azimuth, the all-directions workhorse.
• The comparative ledger: a horizontal antenna's advantage over a vertical at HF is lower ground losses, since verticals launch their signal along the lossy earth while horizontals bounce off it.

One cautionary antenna closes the section: the random wire fed directly from the transmitter. It works, but with a documented price: station equipment may carry significant RF current, because the "counterpoise" completing the antenna is your radio, your mic cable, and occasionally your lips on 40 meters. The Technician course's RF-in-the-shack symptoms start here; the cure is a proper feed line and choke.

7.3The Yagi, Engineered

At Technician the Yagi was "the antenna with the most gain." Now the design brief.

The anatomy: a driven element approximately 1/2 wavelength long, a reflector behind it that is longer, and one or more directors ahead of it that are shorter. Long pushes, short pulls, and the pattern focuses forward.

The design levers, each with its price:

• More gain: increase the boom length and add directors. Gain is bought by the foot.
• More bandwidth: use larger-diameter elements. Fat elements are electrically broad.
• Fine tuning: element spacing along the boom can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth. All of these trade against each other; a Yagi is a negotiation, not a formula.
• More still: stack a second identical Yagi 1/2 wavelength away and gain rises about 3 dB, one free doubling for the price of a second antenna. Vertically stacking horizontal Yagis narrows the main lobe in elevation, squeezing radiation toward the horizon where the DX lives.

The measurement vocabulary:

• The main lobe is the direction of maximum radiated field strength.
• Front-to-back ratio compares the power in the major lobe with the power in the opposite direction, the spec that quiets the noise behind you.
• Gain references matter: the same antenna's gain in dBi (versus an isotropic point source) reads 2.15 dB higher than its gain in dBd (versus a dipole). Neither number is wrong; advertisers simply prefer the bigger one. Check the suffix before you are impressed.

Feeding a Yagi has its own tricks, because a Yagi's natural feed impedance sits below 50 ohms:

• The beta or hairpin match: a shorted transmission line stub at the feed point providing the impedance match.
• The gamma match: its signature advantage is that the driven element does not need to be insulated from the boom, which is why so many commercial beams bolt straight together.

7.4The Specialists

The last group is the catalog of antennas built for one job each, and every entry cashes in something from earlier in the course.

• The NVIS antenna. Module 2 taught NVIS propagation; here is its hardware: a horizontal dipole placed between 1/10 and 1/4 wavelength above ground is most effective for daytime short-skip on 40 meters. The low height that ruins DX (Section 7.2's omnidirectional high-angle cloud) is precisely the point: fire straight up, rain down regionally. Deliberately hanging an antenna low is the most counterintuitive good idea in emergency communications.
• The end-fed half-wave. Its feed point impedance is very high, thousands of ohms, exactly as Section 7.2's moving-feed-point rule predicts at the wire's end, and exactly why your EFHW needs the 49:1 transformer whose arithmetic Module 3 settled. Three modules, one antenna, full circle.
• Antenna traps exist to enable multiband operation: tuned circuits along the element that electrically shorten it on higher bands. The multiband convenience carries one disadvantage worth knowing: an antenna resonant on several bands has poor harmonic rejection, happily radiating a harmonic that lands on another of its bands, which is why multiband antennas raise the stakes on your transmitter's low-pass filtering. The documented tradeoff of multiband antennas generally: they radiate harmonics more readily, so transmitter cleanliness matters more.
• The inverted V: the everyday name for a dipole hung from a single central support, the most-built HF antenna on Earth because trees outnumber towers.
• The halo: maximum radiation omnidirectional in the plane of the halo, the trick for horizontally polarized mobile VHF work.
• The log-periodic: element length and spacing vary logarithmically along the boom, buying its signature advantage, wide bandwidth, a Yagi-looking antenna that covers 3-to-1 frequency ranges at the cost of gain per element.
• The screwdriver mobile antenna: adjusts its feed point impedance by varying the base loading inductance with a motor, the Technician course's loading coil made tunable from the driver's seat. It will return in Module 8's mobile section.
• The Beverage: a very long, low wire used for directional receiving on MF and the low HF bands. It hears wonderfully and transmits terribly, and serious 160-meter operators run separate antennas for each direction of listening.
• The electrically small loop (under 1/10 wavelength around): its pattern has nulls broadside to the loop, and turning a null onto a noise source is the loop's superpower, for direction finding and for portable operators fighting local QRM.