Lectrosonics ALP690 Active LPDA Wireless Microphone Antenna

Lectrosonics ALP690 Active LPDA Wireless Microphone Antenna – Directional UHF Antenna with Adjustable RF Gain

The ALP690 is a high-performance active antenna for wireless microphone receivers, built by Lectrosonics for location and studio production where receiver antennas have to work at a distance from the action and often at the end of long coaxial cable runs. It is an LPDA, a log periodic dipole array, which produces a directional reception pattern with genuine forward gain, and inside the same housing sits an RF amplifier whose gain can be adjusted in fine steps, together with selectable bandpass filtering. That combination lets an RF coordinator place the antenna where coverage is best rather than where the cable happens to reach.

The passive antenna element alone delivers 7 dBi of gain, equivalent to 4 dBd over a dipole, across a 450 to 850MHz passive passband, and it keeps working in passive mode if no power is present. With DC bias supplied on the coaxial cable, the active electronics switch in: the filtered passband narrows to either 470 to 608MHz or 470 to 700MHz, and amplifier gain can be set anywhere from -6dB to +12dB. Settings are made on a membrane keypad with an LED display built into the antenna body.

Key features

• Directional LPDA pattern with 7 dBi (4 dBd) passive gain
• RF amplifier gain adjustable from -6dB to +12dB in 1dB steps
• Selectable active filtering: 470 to 608MHz or 470 to 700MHz
• High third order intercept of +27dBm at the input for strong-signal handling
• Powered by DC bias on the 50 ohm BNC coaxial connection
• Lightweight 355g skeletal construction that reduces wind loading outdoors

Why adjustable gain matters

Antenna gain is not free performance; it has to be matched to the losses and the RF environment of each installation. Long coaxial runs, splitters and multicouplers each subtract decibels, and the ALP690’s positive gain exists to replace exactly those losses, no more. Setting gain by calculation, adding up cable and splitter losses and compensating for them, keeps the receiver’s input at the level it was designed for. Too much gain in a strong-signal environment invites intermodulation, which is why the range extends below zero: negative settings can offset the antenna’s own pattern gain to dipoles-equivalent level, or tame the output when high-power transmitters work close to the antenna. The +41dBm output intercept figure shows the amplifier itself stays linear under demanding conditions.

Filtering and interference control

The selectable bandpass filters suppress RF above and below the working band, which in practice means television transmitters, communications traffic and other out-of-band energy reach the receiver with far less strength. The 470 to 608MHz setting suits the post-repack US television landscape, while the wider 470 to 700MHz setting serves export regions. Because the LPDA pattern is directional, with significant rejection to the rear, aiming the antenna at the performance area also rejects interference arriving from behind it; vertical mounting gives a wide horizontal coverage pattern for stages and presentation spaces.

Construction, power and mounting

The antenna elements are copper traces on a 0.133 inch glass epoxy laminate with a durable finish, formed into an open skeletal shape that presents little area to the wind on outdoor shoots. It measures 27 x 33 x 4cm and weighs 355 grams. Power requirements are 8V to 16V DC bias on the centre pin of the coaxial cable, polarity protected, at 1.5W maximum, supplied by a compatible receiver, an active multicoupler or an inline bias-T; with no bias present the antenna reverts automatically to passive operation. The mounting block provides 1/4-20 and 3/8-16 threads plus a 5/8-27 microphone stand thread, and it can be rotated 90 degrees for different orientations.

Where the ALP690 is used

Film and television location work, live events, houses of worship and installed venues all use antennas of this type when transmitters roam a wide area and receivers sit in a rack some distance away. The ALP690 suits those jobs because its gain, filtering and pattern can each be set for the venue rather than accepted as fixed, turning antenna placement from a compromise into a planned part of the RF design.