Study › Amateur Extra › E9H

E9H

ANTENNAS AND TRANSMISSION LINES

Receiving antennas: radio direction finding (RDF) techniques; Beverage antennas; single- and multiple-turn loops

E9H011 of 11

When constructing a Beverage antenna, which of the following factors should be included in the design to achieve good performance at the desired frequency?

Why A Beverage is a traveling-wave receiving antenna: a long wire strung close to the ground (often just a few feet up) that picks up the tilted wavefront of a low-angle signal arriving along its axis. For the wave to build up usefully along the wire, the wire has to be electrically long, at least one wavelength and typically one to several wavelengths for good directivity and front-to-back ratio. That is why Beverages are practical on 160 and 80 meters, where a wavelength of wire is a few hundred feet, and why they are receive-only antennas with poor efficiency.

Watch out The choices calling for a quarter wavelength or a height of more than a wavelength invert the design: a Beverage is deliberately mounted low, near the ground, and is long, not short. A four-sided loop describes a quad element, a completely different antenna.

Beverage = Bee Line: Big and Low. One wavelength or more of wire, just above the ground.

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

E9H022 of 11

Which is generally true for 160- and 80-meter receiving antennas?

Why On 160 and 80 meters the received noise floor is set by atmospheric noise (static, lightning crashes) rather than by the receiver, so extra antenna loss attenuates the noise and the signal equally and the signal-to-noise ratio is unchanged. What does improve reception is a pattern that rejects noise and interference arriving from other directions. That is why very inefficient but highly directive antennas like Beverages, small loops and short verticals in arrays work well for low band receiving.

Watch out The choice about mounting 1/2 wavelength high describes what is needed for a transmitting antenna's takeoff angle, and at 160 meters that would be about 80 meters up; receiving Beverages deliberately lie close to the ground. Low loss feedline is likewise unnecessary for the same noise-limited reason.

Low bands: noise-limited, not loss-limited. Trade efficiency for directivity.

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

E9H033 of 11

What is receiving directivity factor (RDF)?

Why RDF is a figure of merit for receiving antennas on the low bands, where the limit is atmospheric and man-made noise arriving from all directions rather than the receiver. It is defined as the antenna's peak gain minus its average gain taken over the whole hemisphere around and above the antenna, expressed in dB. A high RDF means the antenna rejects noise from every direction except the one you want, so the signal-to-noise ratio improves even if absolute gain is low, which is why a short Beverage or a small terminated loop can hear better than a big dipole.

Watch out Forward gain compared to gain in the reverse direction is the front-to-back ratio, which only looks at one direction behind the antenna and says nothing about noise arriving from the sides or overhead. Comparisons to isotropic or to a dipole are just gain references, dBi and dBd.

Noise comes from everywhere, so RDF compares peak gain to the average over the whole hemisphere.

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

E9H044 of 11

What is the purpose of placing an electrostatic shield around a small-loop direction-finding antenna?

Why A small loop for direction finding works off the magnetic component of the wave, and its sharp nulls occur when the loop plane points at the station. Stray capacitive (electric field) coupling to nearby objects, and to the operator's hand, unbalances the two halves of the loop and adds an unwanted vertical-antenna response that fills in the nulls. A Faraday (electrostatic) shield around the loop, with a small gap so it is not a shorted turn, blocks that electric-field coupling while letting the magnetic field through, keeping the nulls deep and symmetrical.

Watch out The shield does not improve gain or matching; small shielded loops are quite inefficient, and their value is the sharpness of the null, not signal strength. Bandwidth and out-of-band rejection come from the tuning circuit, not the shield.

Shield the E-field, keep the H-field: deep nulls need a Faraday shield with a gap.

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

E9H055 of 11

What challenge is presented by a small wire-loop antenna for direction finding?

Why A small loop responds to the magnetic field and has a figure-8 (cosine) pattern with two deep nulls exactly 180 degrees apart. Because the nulls are sharp, they give a precise bearing line, but that line points two ways, so you cannot tell whether the signal came from ahead or behind. The usual cure is to add a sense antenna (a vertical whip) whose omnidirectional response combines with the loop to make a cardioid pattern with a single null, resolving the ambiguity.

Watch out The choice saying there is no clearly defined null gets it backwards: the small loop's nulls are deep and sharp, which is exactly why it is favored for direction finding. Loops are also very useful on MF and HF, not just VHF and up.

Loop nulls are sharp but come in pairs. Add a sense antenna to turn the figure-8 into a cardioid with one null.

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

E9H066 of 11

What indicates the correct value of terminating resistance for a Beverage antenna?

Why A Beverage is a traveling-wave antenna: energy runs down the long wire and is absorbed by the resistor at the far end, so no standing wave forms. When the terminating resistance matches the wire's characteristic impedance (typically 400-600 ohms over ground), the feed point impedance stops being resonant and stays nearly constant with frequency. So the practical test is to sweep the band and adjust the terminating resistor for the flattest, least variable SWR.

Watch out The front-to-back choice has it backwards: proper termination gives maximum front-to-back ratio, since the resistor absorbs the rearward wave instead of reflecting it. The DC current choice is a distractor because the antenna carries RF, not DC.

Terminated = traveling wave = no resonance, so impedance stays flat. Tune the resistor for flattest SWR across the band.

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

E9H077 of 11

What is the function of a Beverage antenna's termination resistor?

Why A Beverage is a long wire (one wavelength or more) strung low over ground, and it responds to waves traveling along it from either end. Placing a resistor roughly equal to the wire's characteristic impedance (typically 400 to 600 ohms) at the far end, to ground, absorbs the energy picked up from signals arriving off that end instead of letting it reflect back down the wire. The result is a unidirectional pattern with good front-to-back ratio, which is what makes terminated Beverages so useful for low-band receiving.

Watch out Front-to-side ratio is set by the wire's length and height, not the termination; an unterminated Beverage is bidirectional, so the resistor fixes the back lobe, not the sides.

Terminate the far end and the back signal dies in the resistor: termination equals front-to-back.

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

E9H088 of 11

What is the function of a sense antenna?

Why A small loop has a figure-eight pattern with two deep nulls 180 degrees apart, so you cannot tell which of two opposite bearings the signal came from. Adding a sense antenna, typically a short omnidirectional vertical, mixes its signal with the loop output in the right phase and amplitude to produce a cardioid pattern with just one null. That single null resolves the 180-degree ambiguity and tells you which direction the transmitter really is.

Watch out The idea that it increases sensitivity is backwards: combining the sense antenna with the loop actually shallows one null and generally costs a bit of signal. Its job is pattern shaping for bearing resolution, not gain or multipath cancellation.

Sense antenna makes 'sense' of which way: figure-eight plus vertical equals cardioid, one null only.

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

E9H099 of 11

What type of radiation pattern is created by a single-turn, terminated loop such as a pennant antenna?

Why A small loop by itself has a figure-8 (bidirectional) pattern, but adding a resistive termination to one side of the loop introduces a traveling-wave component that combines with the loop response. The two components add in one direction and cancel in the other, producing a cardioid: one broad lobe forward and a deep null off the back. Pennants, flags and EWEs are all low-noise receiving antennas that use this trick to null out interference or noise from one direction.

Watch out Bidirectional describes the untuned, unterminated small loop with its two opposite nulls; the whole point of the termination resistor is to break that symmetry and leave a single null.

Terminate the loop and you kill one lobe: figure-8 becomes cardioid (heart-shaped, one null).

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

E9H1010 of 11

How can the output voltage of a multiple-turn receiving loop antenna be increased?

Why A small receiving loop works by magnetic induction: the voltage induced is proportional to the number of turns times the area enclosed times the rate of change of the magnetic field (V = 2π f N A B / λ-related constant). So doubling the turns or doubling the enclosed area roughly doubles the output. That is why direction-finding loops are wound with many turns on a fairly large form.

Watch out Using high impedance wire for the coupling loop does nothing useful; the coupling loop just transfers energy to the feed line, and adding resistance only adds loss. Permeability of a shield is not a voltage-boosting factor either, since the shield is there to reject the electric field component.

Loop voltage goes up with N times A: more turns, more area, more signal.

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

E9H1111 of 11

What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?

Why A cardioid (heart-shaped) pattern has broad response over most directions but one deep, narrow null. Nulls are far sharper than peaks, so you can pinpoint a bearing much more precisely by rotating for minimum signal than for maximum. The cardioid is usually made by combining a small loop with a vertical sense antenna, and because there is only one null, it resolves the 180-degree ambiguity that a plain loop's two opposite nulls leave you with.

Watch out The choice about a very sharp peak describes what a high-gain beam like a long Yagi gives you, and peaks are inherently broader and harder to read than nulls; a cardioid's maximum is quite broad.

Loop alone = two nulls (ambiguous). Loop + sense = cardioid = ONE null = one bearing.

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

Powerwerx SS-30DV 30A DC Power SupplySponsored · View on Amazon →Yaesu FT-991A HF/VHF/UHF All Mode TransceiverSponsored · View on Amazon →

← E9G All groups E0A →