Amplifiers
| Amplifiers | |
|---|---|
| Also called | Linear amplifier, linear, amp |
| What it does | Raises a transceiver's output to a higher power, typically from 100 W to between about 500 and 1,500 W on HF |
| US power limit | 1,500 W PEP output on most bands, lower on some bands and for some license classes |
| Types | Solid-state and vacuum tube |
| Operation | |
| Drive power | Usually tens of watts up to 100 W; some models are designed for low-power radios |
| Control connections | Keying (transmit/receive) line, ALC, often band data |
| Gain from 100 W to 1,000 W | 10 dB, a little under two S-units |
| VHF/UHF | Small solid-state "brick" amplifiers for handheld and mobile radios |
An amplifier, or linear amplifier, takes the output of a transceiver and raises it to a higher power. On HF, the usual arrangement is a 100-watt HF transceiver driving an amplifier to somewhere between about 500 and 1,500 watts. "Linear" means the amplifier reproduces its input faithfully, only larger. That matters for single sideband and other modes whose amplitude varies, because a nonlinear amplifier distorts them and spreads the signal far beyond its normal width.
What an amplifier buys
Going from 100 to 1,000 watts is a gain of 10 decibels. Most S-meters are scaled at about 6 dB per S-unit, so the station at the other end sees the signal rise by a little under two S-units. That is the difference between a signal lost in the noise and one that is copied easily, which is why amplifiers are popular for DXing, contesting and the 160-meter band. An amplifier does nothing for reception, though. A better antenna improves both directions at once, and most experienced operators improve the antenna first.
Solid-state and tube amplifiers
Tube amplifiers use one or more transmitting tubes and a high-voltage power supply, often several thousand volts. The operator adjusts two controls, usually called tune and load, for each band and sometimes for different parts of a band. Tube amplifiers are rugged, forgiving of a moderate mismatch and repairable, and many decades-old models are still in use. Their high-voltage supplies are lethal, and the capacitors can hold a dangerous charge after the amplifier is unplugged.
Solid-state amplifiers use power transistors and need no tuning. Many change bands automatically by reading data from the transceiver, come on instantly and are compact. They need a good match, and their protection circuits shut them down when the SWR, drive or temperature is too high. For that reason many include, or are sold with, an automatic antenna tuner; as with tuners built into transceivers, these generally handle moderate mismatches rather than any length of wire. Amplifiers that run from 13.8 volts exist but draw very heavy currents, so most high-power models use their own mains-powered supplies, and most legal-limit amplifiers are best run from a 240-volt circuit.
Connecting and driving
- Keying line. The transceiver tells the amplifier when to switch into transmit, usually through a jack on the back of the radio. Check that the amplifier's keying voltage and current are within the radio's rating; some older amplifiers put more voltage on that line than a modern radio can switch, and need an interface.
- Drive level. Set the transceiver's power to the level that gives the amplifier its rated output, and no more. Overdriving produces a distorted, wide signal that splatters onto neighboring frequencies, and it can damage the amplifier.
- ALC. Automatic level control is a voltage the amplifier sends back to reduce the radio's power if it is overdriven. It is a safety net, not a substitute for setting the drive correctly.
- Duty cycle. FT8, RTTY and other digital modes transmit at full power for the whole transmission. Many amplifiers are rated for less output on these modes than on SSB.
- Station components. Feed lines, tuners, baluns and connectors must all be rated for the power.
Rules and responsibilities
US amateurs may run up to 1,500 watts PEP (peak envelope power) output on most bands, but the rules also require using no more power than needed for the contact. Some bands have lower limits, including 200 watts PEP on the 30-meter band and special rules on the 60-meter band, and a Technician is limited to 200 watts PEP on the HF segments open to that class.
HF amplifiers sold in the US must be certified by the FCC under rules designed to keep them from being sold for, or easily adapted to, Citizens Band and other non-amateur services. Licensed amateurs may build or modify a limited number of amplifiers for their own stations. The details have changed over the years, so check the current rules before building, modifying or importing one.
An amplifier also changes the station's RF exposure picture, so the evaluation must be redone. More power means more risk of interference to the household's and neighbors' electronics, and the operator is responsible for a clean signal and for working with neighbors to solve problems.
VHF and UHF amplifiers
On the VHF and UHF bands, small solid-state amplifiers, often called bricks, boost a handheld's few watts to 30 or 50, or a mobile radio's output to 100 watts or more. They run from 13.8 volts and need suitable current. Many include a receive preamplifier and an SSB setting for weak-signal work, satellites and moonbounce. The US certification rules apply only to amplifiers that operate below 144 MHz. Exposure limits are strictest between 30 and 300 MHz, which includes the 2-meter band, so a brick amplifier and a nearby antenna call for particular care.