> There's a good design for a kilowatt-capable 6 meter low > pass filter in the ARRL Handbook (2007), but I haven't > found a design for a 2 meter LPF filter that I really > like. The best I've found so far is at: > > http://rf-transmitter.blogspot.com/2008/09/200w-lowpass-filter-for-150-mhz-2-meter.html > > which seems fine except it's only rated for 200 watts.
I would not think it is fine, or any filter is fine. At 150 MHz even fractions of an inch of error in ground length or signal routing can make a filter unsuitable, as can coupling between traces or ground trace currents on circuit boards. The type of component makes a huge difference in performance, and VHF filtering requirements are particularly stringent. Even how you do the sheetmetal or ground connectors can make or break suppression. (This stuff actually shows up on upper HF, so it is not just VHF that requires layout care.) > > It should be noted that the amplifier itself (due to push > pull design) suppresses even order harmonics very deeply, > and at 144 mHz suppresses 3rd order harmonics -40dB or > more, so it doesn't really need a low pass filter, except > that (my understanding) the FCC require a LPF on any > amplifier. > Most class AB solid state amplifiers I have tested suppress the second harmonic only around 20-30 dB, sometimes less. None are suitable to meet HF specs, let alone FCC VHF specs. Personally, I think you would have to use a strip line filter with a good layout and good backplane, and use surface mount capacitors, to make a suitable filter. That's what I always have had to do. I have been successful with wire hairpins and multilayer metal clad capacitors using shielded boxes with chambers for each filter section, but attention has to be paid to bonding of shield walls and harmonic currents in shield walls. The capacitors need to have very low series inductance. The idea you should have a filter because the FCC mandates one is not correct at all. They set the emissions purity, and care less if you have a filter or not so long as you suppress harmonics and spurious properly. Here is the current rule for low VHF: "(e) The mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency between 30-225 MHz must be at least 60 dB below the mean power of the fundamental. For a transmitter having a mean power of 25 W or less, the mean power of any spurious emission supplied to the antenna transmission line must not exceed 25 µW and must be at least 40 dB below the mean power of the fundamental emission, but need not be reduced below the power of 10 µW. A transmitter built before April 15, 1977, or first marketed before January 1, 1978, is exempt from this requirement." 73 Tom ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
