A WORD ABOUT SPURIOUS EMISSIONS
Current FCC regulations require amateur equipment operating between 30 MHz and 225 MHz to have all out of band spurious emissions down greater than 60 dB from the carrier, or intended in band signal. (-60 dBc).
It's a sorry fact that much of the amateur equipment operating at these frequencies does not even come close to meeting the -60 dBc spurious requirement. For example, a very popular 8877 2m amplifier design from the 1970's that has been duplicated by many hams and for sale amplifier builders, typically has 2nd harmonic emissions of only -30 to -35 dBc and third harmonic emissions of about -35 to -40 dBc!
Lunar-Link Systems amplifiers have become the amplifier of choice for many multi-operater VHF contest groups due to LA-Series Amplifiers' extremely low spurious emissions. The 3rd harmonic of a 2m amplifier can cause huge receive problems with the 432 MHz station. Similarly, a strong 3rd harmonic from the 432 MHz amplifier will cause problems with the 1296 receiver. Out of band spurious signals can also create problems interfering with other RF services who may be operating on one of those harmonic frequencies.
All current Lunar-Link Systems amplifiers easily meet the -60dBc spurious requirement, even the LA-72, 432 MHz amplifier which has no FCC mandated spurious emissions requirements. LA-x2 amplifiers incorporate both an internal low pass filter and a shorted 1/4 wave stub in their output circuit. Typical harmonic outputs are down over 70 dBc, that is 10 times weaker than the FCC requirements.
A tip for multi-operator contest stations is to run their Lunar-Link amplifiers slightly over loaded. By increasing the loading you will sacrifice a percent or two in efficiency, but harmonic emissions can often be reduced by another 5 dB. In addition, an over loaded amplifier will run with lower grid current and be easier on the tubes. To "overload" your amplifier, first tune up normally at 1500 watts output, that is just alternately peak the Plate Tune and Plate Load for maximum output. Next turn the Load knob counter clockwise slightly until you see the grid current drop by about 25%. Then re-peak the Plate Tune control for maximum output. Finally, increase the drive power until you again have 1500 watts output.
Below is a typical spectral display of a LA-22, 144 MHz amplifier running 1500 watts output. Eimac 3CPX800A7 tubes are used and the driver is a Kenwood TS-2000. The operating parameters are: Vb = 2620 VDC, Ib = .905 A, Ig = 18 ma, Drive Power = 46 watts, Apparent Efficiency = 63.3%.
Lunar-Link Systems is not responsible for typographical errors
Lunar-Link Amplifiers © 2002
Web design by K1FO, HTML by K2AH ®2002
03