Receive-Only Antennas

Anyone who has tried 160 or 80 meters knows the feeling. The DX station is calling you, faintly, under a wall of static, and the antenna that gets your signal out does nothing to help you hear it.

Serious low-band operators solve this with a second antenna used only for listening. On these bands, hearing well and transmitting well are two different jobs.

Signal-to-Noise, Not Gain

On the highest HF bands, when the band is quiet, your receiver’s own noise can set the limit, so a more efficient antenna really does help you hear. On 160 and 80 meters it is different. Atmospheric noise from distant thunderstorms, plus whatever your neighborhood adds, is far stronger than any receiver’s own noise. An efficient antenna brings in more signal, but it brings in just as much more noise.

What helps is directivity. Noise arrives from many directions at once, while the signal you want arrives from one. An antenna that listens mainly in that one direction improves the signal-to-noise ratio even if it delivers a much weaker signal overall. You can make up the lost level with a preamplifier; you cannot remove noise that came in with the signal.

Low-band operators compare receive antennas using receiving directivity factor, or RDF: the antenna’s peak gain compared with its average gain over the whole sky around and above it. A higher RDF means better rejection of noise from everywhere else. For how receivers handle noise, see Receiver Numbers Explained.

The Beverage Antenna

The Beverage is named for Harold H. Beverage, an engineer at RCA who was developing receivers for transoceanic radio. He received his first patent, for this “wave antenna,” in 1921, and it went on to become a standard for long-wave reception.

A Beverage is a single wire run in a straight line, low to the ground, pointed at the part of the world you want to hear. It is fed through a small matching transformer at the end nearest you. At the far end, a resistor connects the wire to a ground rod. That resistor absorbs signals arriving from behind, which turns a two-way pattern into a one-way one.

• Length: at least one wavelength on the band you want, which on 160 meters means more than 500 feet. Longer gives a narrower beam.
• Height: low, typically several feet up on short supports.
• Termination: the right resistor value is the one that gives the least change in SWR across the band.
• Direction: one wire covers one direction. Many stations run two or more and switch between them.

The catch is space, and a long wire needs care. Run the wire where nobody can trip on it or drive into it, keep it far from power lines and roads, and get permission before crossing anyone else’s property. A shorter variant laid on the ground, often called a Beverage on ground, is an option for smaller lots.

Small and Terminated Loops

A small loop, only a few feet across, hears equally well off both of its faces and has two sharp nulls off its edges. That makes it a superb tool for one particular problem: a single local noise source. Rotate the loop until the noise drops into a null, and the signals from other directions remain. An electrostatic shield around the loop helps keep those nulls deep.

The output of a small loop is tiny, so it needs a good low-noise preamplifier. More turns or more area raise the output voltage.

Add a resistor in series with a single-turn loop and the pattern changes to a cardioid, a heart shape with a single null off the back. Terminated loops such as the pennant and the K9AY type use this idea. They fit on an ordinary suburban lot, and with two loops and a switch you can choose among several directions. Put a common-mode choke on every cable that connects to a small receive loop, including the coax and any preamp power, or the cables will spoil its nulls.

Active Whips

An active whip is a short vertical rod, a few feet to perhaps 20 feet tall, with an amplifier at its base that has a very high input impedance. It is small and easy to hide.

Because a whip does not reject noise by direction, its value comes from where you put it. Mounted far from the house, away from the wiring and electronics that make the noise, it can be far quieter than a transmitting antenna next to the shack. Its amplifier can overload on very strong signals, so keep it well away from your transmitting antenna and from nearby broadcast stations if you can.

Noise Cancelling and Phasing Units

A phasing unit, often sold as a noise canceller, works on a different principle. It takes the signal from your main antenna and the signal from a second sense antenna that hears mostly the local noise. You adjust the level and phase of the sense signal until it matches the noise in the main antenna but is turned upside down, and the two cancel.

These units work best against one strong, steady noise source, such as a nearby power line or a neighbor’s switching supply. The settings change with frequency, so expect to readjust when you tune. The better long-term fix is to find and cure the noise source; see How to Track Down RFI.

Protecting the Receive Antenna

A receive antenna near your transmitting antenna can pick up enough RF while you transmit to damage a preamplifier or a receiver input.

• Keep the receive antenna as far from the transmitting antenna as your lot allows.
• If your transceiver has a separate receive antenna input, read the manual to see how it is switched during transmit.
• Use a relay that disconnects the receive line and grounds it during transmit, keyed from your radio’s transmit control line.
• Consider a front-end limiter as extra protection, especially if you run an amplifier.
• Disconnect and ground receive feedlines before storms arrive and whenever the station is not in use.

Long wires collect static charge and lightning energy just like transmitting antennas. See Grounding and Lightning Protection.

Where to Start

Start by listening. Tune across 160 or 80 meters at night on your transmitting antenna and note the noise level. If one noise source dominates, a small loop or a phasing unit may be all you need. If the noise is general, try an active whip placed far from the house. If you have the room, build a Beverage aimed at the area you most want to work.

Compare each new antenna with your transmitting antenna on the same weak signals. The goal is not a louder S-meter but copying a station you could not copy before.