Preamplifier
| Preamplifier | |
|---|---|
| Also called | Preamp, LNA (low-noise amplifier) |
| What it does | Amplifies weak received signals before feed line loss and receiver noise can bury them |
| Key specification | Noise figure, in decibels; lower is better |
| Typical gain | Roughly 15 to 25 dB |
| In use | |
| Usual location | At the antenna (mast-mounted) |
| Main uses | VHF and UHF weak-signal SSB and CW, satellites, moonbounce, meteor scatter |
| During transmit | Bypassed by relays, usually controlled by a sequencer |
| Seldom needed on | The lower HF bands |
A preamplifier, or preamp, is a low-noise amplifier placed ahead of a receiver to strengthen weak signals before anything else can degrade them. Most HF and VHF transceivers have one built in, switched by a button marked P.AMP or similar. In amateur radio the word more often means a separate unit mounted on the mast, right at the antenna, for VHF and UHF weak-signal work, satellites, moonbounce and other modes where every fraction of a decibel counts.
Noise figure
Every amplifier adds a little noise of its own. Noise figure (NF) measures how much, in decibels; a perfect, noiseless amplifier would have a noise figure of 0 dB. Modern preamps for 2 meters and 70 centimeters reach well under 1 dB.
In a chain of stages, the first stage matters most. If it has low noise and enough gain, the noise of everything after it, including the receiver, has little effect. Loss ahead of the first stage, on the other hand, adds directly to the noise figure, decibel for decibel. That is why a preamp belongs at the antenna rather than in the shack. Suppose the coax loses 3 dB and the receiver's noise figure is 5 dB: the system noise figure is about 8 dB. Put a preamp with a 0.5 dB noise figure and 20 dB of gain at the top of the mast, and the system figure drops to under 1 dB. The same preamp at the radio end could do nothing about the 3 dB already lost in the cable.
Where a preamp helps, and where it doesn't
On VHF and above, the noise arriving from the antenna is low, especially when it points at a quiet sky, so the receiver's own noise sets the limit on what can be heard. A good mast-mounted preamp can reveal signals that were completely inaudible without it. That is why preamps are standard equipment for SSB and CW on 2 meters and up, for receiving satellite downlinks, and for meteor scatter and moonbounce.
On HF the situation is reversed. Atmospheric noise and man-made noise from power lines and household electronics are usually far stronger than the receiver's own noise, especially on the lower bands. A preamp raises the signals and the noise together, so nothing becomes easier to copy, and the extra gain makes the receiver more likely to overload on strong signals, producing false signals called intermodulation. A preamp can still help on the upper HF bands and 6 meters at a quiet location, or with small receiving antennas whose output is very low. FM work through local repeaters rarely needs one.
Switching and sequencing
A preamp cannot survive transmitter power. A mast-mounted preamp therefore sits between two relays that route the signal around it on transmit. Some low-power units sense RF and switch themselves, but stations running an amplifier use a wired keying line and a sequencer. On transmit, the sequencer switches the preamp out first, waits for the relays to settle, and only then enables the amplifier and the RF; on receive, it reverses the order. If RF arrives before the relay has finished moving, the preamp is likely to be destroyed. Each preamp has a maximum power rating for its bypass relays, which must not be exceeded.
Choosing and installing
- Use only the gain needed. Enough to overcome the feed line loss and the receiver's noise is plenty. Extra gain only invites overload.
- Consider filtering. Nearby FM broadcast, paging and other strong transmitters can overload a wideband preamp. Many preamps include a filter for the band they serve.
- Satellite stations transmitting on 2 meters while receiving on 70 centimeters face a particular problem: the third harmonic of a 2-meter uplink can fall in the 70-centimeter satellite segment, and a strong uplink can overload the preamp. Filtering and antenna separation help.
- Power usually comes up the coax through a bias tee, a small device that adds DC to the line, or through a separate cable.
- Weatherproof everything, including the connectors, and add lightning protection where the cables enter the building.