All a low pass filter is capable of is passing the frequencies before the
cutoff frequency and rejecting the ones after it. This kills any interference
from the 2nd, 3rd, harmonincs to the higher frequencies above the cutoff
frequency. The filter is designed around 50 ohms in and out unless something
special. I cant figure what they were talking about harmonics matching, unless
they meant say 14 Mhz 2nd harmonic being 28 Mhz or 10 meter interference? A
filter with a lower cutoff frequency then could be used but most are designed
around a cutoff frequency of 30 Mhz that goes in line that I know of.
Best,
Will
*********** REPLY SEPARATOR ***********
On 8/16/05 at 4:34 PM Don Havlicek wrote:
>How does the amp produce trash that is not present at the input?
>If your amp does this, it's a piece of trash and should be redesigned,
>not 'filtered'.
>As for impedance matching 'at the harmonics' .. that's gobbledegook. A
>well-designed filter will attenuate as it is designed to attenuate IF it
>is inserted between a source and a load with correct impedance at the
>frequency IN USE.
>Don N8DE
>
>Jim Brown wrote:
>> On Tue, 16 Aug 2005 16:10:22 -0400, Don Havlicek wrote:
>>
>>
>>>Why don't hams place the LP filter between the Xcvr and the Amp?
>>
>>
>> Yes. That allows a 100w filter to do the job. Doesn't protect
>> against trash from the amp, but it helps the receiver. And if you're
>> running barefoot, put it between the amp and the tuner.
>>
>> BUT -- the same cautions about impedance matching being necessary
>> still apply, and that's the fly in the ointment. Not only must you
>> be matched at the operating frequency for the filter specs to apply,
>> you must also be matched at the harmonics! The chances of that
>> occuring are slim to none unless you happen to be working into an
>> antenna that is resonant at the harmonics. We DO use some of those,
>> but it certainly isn't all that we use, and it would only apply to
>> single band filters.
>>
>> Jim Brown K9YC
>>
>>
>>
>
>
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