Study › Amateur Extra › E9D

E9D

ANTENNAS AND TRANSMISSION LINES

Yagi antennas; parabolic reflectors; feed point impedance and loading of electrically short antennas; antenna Q; RF grounding

E9D011 of 12

How much does the gain of an ideal parabolic reflector antenna increase when the operating frequency is doubled?

Why A parabolic dish's gain depends on its aperture measured in wavelengths: G is proportional to (pi*D/lambda)^2, or equivalently proportional to frequency squared for a fixed dish diameter. Doubling the frequency halves the wavelength, so the (D/lambda) ratio doubles and the gain goes up by a factor of 4. A power ratio of 4 is 10*log(4) = 6 dB.

Watch out The choice that says 3 dB is the trap: 3 dB is only a doubling of power, but gain scales with the square of frequency, not linearly, so the increase is 6 dB.

Dish gain goes as frequency squared: double f, times 4 power, 6 dB. Same 6 dB if you double the dish diameter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D022 of 12

How can two linearly polarized Yagi antennas be used to produce circular polarization?

Why Circular polarization is just two linear waves at right angles to each other whose fields are 90 degrees out of time phase, so the resultant field vector rotates one full turn each RF cycle. To build that with Yagis you mount two identical arrays on the same boom axis with one set of elements horizontal and the other vertical, driven elements at the same boom position, and feed them in quadrature (commonly by adding an extra quarter wavelength of line to one). Which one leads by 90 degrees determines whether the result is right-hand or left-hand circular, and this crossed-Yagi arrangement is standard for satellite work because it tolerates spacecraft spin and Faraday rotation.

Watch out Stacking two Yagis in parallel planes keeps both sets of elements in the same polarization, so phasing them only steers or shapes the pattern (in phase gives stacking gain, 90 degrees gives a beam tilt); you never get rotation without spatial perpendicularity.

Circular = crossed plus quarter: elements 90 degrees apart in space, feed 90 degrees apart in phase.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D033 of 12

What is the most efficient location for a loading coil on an electrically short whip?

Why On an electrically short whip the current tapers from maximum at the base to zero at the tip, and radiated power depends on the area under that current distribution. Putting the loading coil partway up, typically near the center, keeps a higher, more uniform current flowing in the whole lower section instead of letting it taper off immediately, which raises the radiation resistance and efficiency. The tradeoff is that a center coil needs more inductance and has higher loss than a base coil, so in practice center loading is the usual best compromise.

Watch out Base loading, the choice that says as low as possible, is the easiest to build and is common on mobile whips, but it is the least efficient because the current above the coil falls off quickly. Loading at a voltage maximum (the tip) would require an impractically huge coil.

Raise the coil, raise the current: center loading beats base loading for efficiency.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D044 of 12

Why should antenna loading coils have a high ratio of reactance to resistance?

Why An electrically short antenna has a small radiation resistance, so the loading coil's series loss resistance competes directly with it for the transmitter's power. Efficiency is roughly radiation resistance divided by (radiation resistance + loss resistance), so any ohms burned in the coil are heat instead of radiated signal. A high ratio of reactance to resistance, which is just the coil's Q, means the coil supplies the needed reactance with minimal loss resistance.

Watch out The choice about minimizing Q has it backwards: X/R is the definition of Q, so a high ratio means a high-Q coil. Radiation angle is set by the antenna's height and geometry, not by coil losses.

Q = X/R. High-Q loading coil = low loss ohms = more watts radiated, fewer watts heating the coil.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D055 of 12

Approximately how long is a Yagi's driven element?

Why A Yagi is built around a resonant dipole as its driven element, and a resonant dipole is a half wavelength long. The parasitic elements are sized relative to it: the reflector is roughly 5 percent longer and the directors a few percent shorter, which is what steers the pattern forward. In feet, a half-wave driven element is approximately 468 divided by the frequency in MHz.

Watch out The 234 divided by frequency formula gives a quarter wavelength in feet, which is the length of one leg of a dipole or of a ground-mounted vertical, not the whole driven element; 1005 divided by frequency is closer to a full wavelength and is used for loop antennas.

Driven element = dipole = half wave. 468/f(MHz) feet; 234/f is only one leg.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D066 of 12

What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?

Why An electrically short antenna has a lot of capacitive reactance and a small radiation resistance; adding inductive loading cancels the reactance but the resulting circuit has a high ratio of reactance to resistance, meaning high Q. Bandwidth is roughly the resonant frequency divided by Q, so raising Q narrows the range over which SWR stays low. That is why a loaded 40 m mobile whip may cover only tens of kilohertz while a full-size dipole covers hundreds.

Watch out The idea that coil placement leaves bandwidth unchanged is wrong: moving the coil up the radiator can improve efficiency and slightly change the Q, but no placement escapes the fundamental high-Q, narrow-bandwidth behavior of a short antenna.

Short antenna plus loading coil equals high Q equals narrow SWR bandwidth. Small antenna, small bandwidth.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D077 of 12

What is an advantage of top loading an electrically short HF vertical antenna?

Why An electrically short vertical has a current maximum at the base that tapers rapidly to zero at the tip, so very little of the structure carries useful current. Adding a capacitance hat or top loading coil at the top forces the current distribution to stay high over more of the radiator, which raises the radiation resistance relative to the fixed ground and conductor loss resistance. More of the power fed to the antenna therefore leaves as radiation instead of heat, which is the definition of improved radiation efficiency. Top loading also puts any loading inductance where the current is smallest, minimizing coil I-squared-R loss compared with base loading.

Watch out Lower Q is tempting because top loading does broaden the usable bandwidth a bit, but the Q of a short vertical stays high regardless, and that is a side effect rather than the principal advantage; higher losses is exactly the opposite of what top loading achieves.

Top loading keeps current high up the whole radiator: more current radiating, less heating the ground.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D088 of 12

What happens as the Q of an antenna increases?

Why Q is the ratio of stored reactive energy to energy dissipated per cycle, and for any resonant circuit, including an antenna, the usable bandwidth is roughly the resonant frequency divided by Q. So raising Q sharpens the resonance and narrows the frequency span over which SWR stays low. Physically short, heavily loaded antennas have small radiation resistance and large reactance, giving high Q and very narrow SWR bandwidth, while fat elements or cages lower Q and broaden the response.

Watch out Increased bandwidth is exactly backwards: Q and bandwidth are inversely related. Gain and common-mode feed line current are separate issues, set by the antenna pattern and by feed line balance or choking, not by Q.

BW = f0 / Q. High Q means a sharp, narrow peak: more Q, less bandwidth.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D099 of 12

What is the function of a loading coil in an electrically short antenna?

Why An antenna shorter than a quarter wavelength (or half wave for a dipole) looks like a small resistance in series with capacitive reactance, so it cannot be fed efficiently. Adding a series inductor, the loading coil, supplies an equal and opposite inductive reactance that cancels the capacitive part, leaving a purely resistive feed point at the operating frequency. That is what resonance means: X_L = X_C so the net reactance is zero.

Watch out The choices about lowering Q or losses are backwards: an electrically short loaded antenna has a low radiation resistance and high stored energy, giving high Q and narrow bandwidth, and the coil's own wire resistance adds loss rather than reducing it.

Short antenna = capacitive; add inductance to cancel it. Coil cures the capacitance, but Q goes up and bandwidth goes down.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D1010 of 12

How does radiation resistance of a base-fed whip antenna change below its resonant frequency?

Why Radiation resistance measures how effectively the antenna converts current into radiated power, and for a short vertical it scales with the square of the antenna's electrical length: roughly 40*pi^2*(h/lambda)^2 ohms for a short monopole over a good ground. Operating below the resonant frequency makes the whip a smaller fraction of a wavelength, so h/lambda drops and the radiation resistance falls, often to just a few ohms or less. That low resistance is why loaded short verticals lose so much power in coil and ground losses, since those losses are then comparable to or larger than the radiation resistance.

Watch out The idea that it becomes imaginary confuses radiation resistance with the feed point reactance: an electrically short whip does develop a large capacitive (imaginary) reactance below resonance, but radiation resistance itself stays a real number.

Radiation resistance follows (height/wavelength) squared: shorter in wavelengths means lower R.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D1111 of 12

Why do most two-element Yagis with normal spacing have a reflector instead of a director?

Why In a two-element parasitic array the parasitic element can be tuned either inductively (longer, a reflector) or capacitively (shorter, a director). At the spacings normally used for two-element beams, roughly 0.15 to 0.25 wavelength, the reflector arrangement produces more forward gain than a director at the same spacing, and it also gives a higher feed point impedance and wider usable bandwidth. A director only becomes competitive at very close spacing, around 0.1 wavelength, where the radiation resistance drops sharply and the bandwidth becomes very narrow.

Watch out Front-to-back and front-to-side ratios in a two-element Yagi are actually modest and can be better with a closely spaced director, so pattern figures are not the reason builders choose a reflector; the choice about SWR confuses feed point impedance convenience with the main design goal.

Two elements, normal spacing: the reflector is the gain winner.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E9D1212 of 12

What is the purpose of making a Yagi's parasitic elements either longer or shorter than resonance?

Why A parasitic element has no feed line; it is excited by the field from the driven element, and the current that flows in it depends on its reactance. Making an element longer than resonance makes it inductive (lagging current) so it re-radiates as a reflector, while making it shorter makes it capacitive (leading current) so it acts as a director. Those phase shifts, combined with the spacing delay, cause the re-radiated fields to add in the forward direction and cancel toward the rear, producing gain and front-to-back ratio.

Watch out Loss reduction is tempting because off-resonance elements sound inefficient, but element length here is chosen for current phasing, not efficiency; mechanical balance and wind torque have nothing to do with electrical tuning.

Long = lagging = reflector behind, short = leading = director ahead. Length sets phase, phase sets the beam.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

Nagoya NA-771 Dual Band Handheld AntennaSponsored · View on Amazon →Icom IC-7300 HF/50MHz Base TransceiverSponsored · View on Amazon →

← E9C All groups E9E →