Why receiving antennas are judged differently
Which is generally true for 160- and 80-meter receiving antennas?
Atmospheric noise is so high that directivity is much more important than losses
This is the insight that makes receiving antennas a separate discipline. On the low bands the noise arriving from the sky vastly exceeds the receiver’s own noise, so making the antenna more efficient raises signal and noise together and gains you nothing. What helps is rejecting noise from directions you do not want.
Hence the figure of merit:
What is receiving directivity factor (RDF)?
Peak antenna gain compared to average gain over the hemisphere around and above the antenna
Not gain over a dipole, and not front-to-back ratio. RDF compares the peak with the average over the whole sky, because noise arrives from the whole sky. An antenna with modest gain and a genuinely quiet pattern has a high RDF and hears better than a high-gain antenna that also hears everything else.
The Beverage
A long low wire, terminated, and one of the great low-band receiving antennas.
When constructing a Beverage antenna, which of the following factors should be included in the design to achieve good performance at the desired frequency?
It should be at least one wavelength long.
At 160 metres that is over 160 metres of wire — which is why Beverages belong to people with fields. Longer is better; directivity improves with length.
What is the function of a Beverage antenna’s termination resistor?
Absorb signals from the reverse direction.
Without it the antenna is bidirectional; with it, unidirectional. Same mechanism as the terminated rhombic from the wire-antennas lesson.
What indicates the correct value of terminating resistance for a Beverage antenna?
Minimum variation in SWR over the desired frequency range.
An elegant test. Correctly terminated, the wire behaves as a matched transmission line — so its input impedance is nearly constant with frequency. Sweep it and adjust the resistor for the flattest response. Optimising for lowest SWR at one frequency gets the wrong answer; flatness across the range is the criterion.
Loops for direction finding
What challenge is presented by a small wire-loop antenna for direction finding?
It has a bidirectional null pattern.
A small loop has two nulls, 180 degrees apart, so a bearing is ambiguous — the source is in one of two opposite directions and the loop cannot say which. Resolving that ambiguity is the central problem of loop direction finding.
Two fixes:
What is the function of a sense antenna?
It modifies the pattern of a DF antenna to provide a null in only one direction.
A small vertical whip added to the loop, combined in the right phase and amplitude. The loop’s figure-eight plus the whip’s omnidirectional response gives a cardioid — one null instead of two.
What type of radiation pattern is created by a single-turn, terminated loop such as a pennant antenna?
Cardioid
What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?
A single null
Unambiguous. And note again that DF works on nulls rather than peaks — a null is sharp, a peak is broad, so the bearing is far more precise.
Loop construction
What is the purpose of placing an electrostatic shield around a small-loop direction-finding antenna?
It eliminates unbalanced capacitive coupling to the antenna’s surroundings, improving the depth of its nulls
A small loop responds to the magnetic field. Any electric-field coupling to nearby objects — a hand, a body, a wall — unbalances the loop and fills in the null, which destroys the only thing the antenna is for. A shield (split, so it does not form a closed turn) removes the electric coupling and leaves the magnetic response intact.
How can the output voltage of a multiple-turn receiving loop antenna be increased?
By increasing the number of turns and/or the area enclosed by the loop.
Voltage is proportional to turns × area × rate of change of flux. Both are yours to choose, and a multi-turn loop is how you get usable output from a physically small antenna.
Check yourself
- Why does making a 160 metre receiving antenna more efficient often not help?
- You take a bearing with a small loop and get two possible directions. What do you add?
- How do you know a Beverage’s termination resistor is the right value?
Answers
- Atmospheric noise dominates on the low bands, so more efficiency raises signal and noise together. Directivity — rejecting noise from unwanted directions — is what helps.
- A sense antenna, which converts the bidirectional figure-eight into a cardioid with a single null.
- Minimum variation in SWR across the frequency range of interest — flatness, not a minimum at one frequency.