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Ham radio linear amplifiers are used where it is necessary to increase the output power from a transmitter or more usually a transceiver.
Often these linear amplifiers are needed when stations want to increase the power being transmitted to the full legal limit as many transceivers, HF, VHF and UHF do not provide this power output level.
Linear amplifiers tend to be associated more with HF operation than with frequencies above 30 MHz, however amplifiers are also available for VHF and UHF as well.
Linear RF amplifiers are required where signals that have an amplitude component are used. These amplitude components carry information and for this to be accurately preserved, the signal must be amplified in a linear fashion.
Signals including amplitude modulation, single sideband, and quadrature amplitude modulation are the main ones and for amateur radio, linear amplifiers are required for single sideband.If the RF amplifier is not linear, then the signal will be distorted and this will result in intermodulation products spreading out either side of the signal to create what is termed splatter.
To ensure that no undue levels of splatter are generated so that the minimum amount of interference is caused to other users, it is essential that all additional amplifiers remain linear.
Signals that do not carry amplitude variations such as FM signals can use non-linear amplifiers like class C amplifiers which are much more efficient and do not need the same level of heat sink capability etc. They are also cheaper to manufacture.
Reasons for installing a ham radio linear amplifier
The main reason for installing a ham radio linear amplifier is to increase the signal output, thereby becoming a larger signal on the band.
The increase in power output will be reflected as a stronger signal at the receiver and will result in more contacts being made, and also being able to be heard in pile-ups when they occur.
For example, a typical HF transceiver has an output up to 100 watts. If a linear amplifier is used to increase this to, say 400 watts. This is an increase on power of 6dB. As 6dB is equivalent to an "S" point, this represents a marked increase.
Taking the power from 100 watts to 1500 watts represents a gain of nearly 12dB and this is an increase in strength of about two "S" points.
Any station increasing its strength by two "S" points would see a significant improvement in the capability of the station, and hence the stations being contacted.
Tube & semiconductor linear amplifiers
The are both tube based and semiconductor based amateur radio linear amplifiers. In view of the very high power levels used, thermionic or tube technology is still used for many amplifiers, although semiconductors are also widely used nowadays.
Years ago vacuum time of thermionic valve technology was the only method for creating a linear amplifier with output power levels of 500 watts and above. Nowadays semiconductors are available to achieve this, although typically many transistors are operated in parallel to achieve it.
- Tube / valve amateur radio linear amplifiers: Tube / valve RF linear amplifiers have been in use for many years and have provided excellent service in ham radio RF linear amplifiers. They are robust and will tolerate mismatches, although operating under poor conditions for extended periods will shorten their life.
They also require very high voltages to be used within the amplifier - often 2 - 3 kV. These voltages can be lethal, so great care is needed when working on them. Also be aware that capacitors in the amplifiers can retain their charge for a long time. Great caution is needed, and they are best worked on after they have been switched off for a while and even then care needs to be exercised, checking all points for high voltages.
These amplifiers also need time from switch on for the tubes / valves to warm up before they can be used. Despite these characteristics, these RF amplifiers provide excellent and reliable service, often lasting for many years. Some have been service for ten or twenty years and still perform well.
- Semiconductor amateur radio linear amplifiers: Semiconductor linear RF amplifiers have a number of advantages. They do not use the very high voltages that the vacuum tube RF amplifiers need, and they are also ready to operate almost instantly - they do not require the warm-up period that the tube / valve linear amplifiers require. However they do require an accurate match. Semiconductors will not withstand the high peak voltages and currents that may appear if a poor VSWR is present and therefore they have protection circuits to reduce the power output under these conditions.
The choice of amplifier technology will depend upon the preferences of the operator and whether a semiconductor or tube based linear is going to suit best.
Points to consider before buying a linear amplifier
Although it sounds as if increasing the power level is like a great idea, there may be a few points to consider before moving ahead.
- Consider the cost: Linear amplifiers are costly items of amateur radio equipment. They can cost as much as the basic rig itself, or possibly even more.
- Improve the antenna and feeder before buying a linear: There is no point in buying and installing a linear amplifier if a poor feeder and antenna are used. IT is far more effective and also cost effective to improve the antenna and its feeder. Only then, should buying a linear being considered.
- Upgrade elements after the amplifier: The linear amplifier will significantly increase the power level and items like VSWR meters, feeders and even the antenna itself may need to be upgraded to ensure they can handle the increased power. This can be an additional cost.
- Keep antennas away from occupied areas: The use of a linear amplifier will increase the level of radiation from the antenna. Although there are still discussions about the links between radiation and health, it is always wise to keep high power levels away from where people are likely to be - raising antennas and ensuring they are away from occupied areas is an essential precaution.
- Watch for local interference: Increasing the power levels being transmitted using a linear RF amplifier can give rise to interference on local domestic appliances. Care should be taken to ensure that antennas are well away from domestic receivers, and other electronic apparatus that may be affected. This is another reason to ensure that the antennas are well removed from inhabited areas.
Many amateur radio stations use linear amplifiers very effectively. They can give a significant increase in signal strength when all other avenues have been fulfilled. Using a linear will ensure that the best possible signal is radiated within he legal limits.
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