[Elecraft] Semi-OT: seeking circuit designs for 2 meter LPF

[Tom W8JI]

> 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