[RFI] Is a low pass filter necessary?

Roger (K8RI) k8ri at rogerhalstead.com
Fri May 11 00:19:18 PDT 2012

On 5/10/2012 11:41 PM, John wrote:
> I was on an antennas reflector recently asking about the proper placement
> of a low pass filter and several members asked why am I using one at all.
> They said with the modern radios (I have an Icom 706 original) a low pass
> filter really isn't necessary, that most modern rigs already contain one
Most modern rigs contain many. The finals will amplify 28 MHz as well as 
1.8.  2nd harmonic of 1.8, is 3.6  Solid state high power amps are the 
same way.  Basically their band switching is changing the LP filters on 
the output.
> and the spurrious emissions are already greatly reduced.

Normally LP filters would have nothing to do with spurious emissions.

>   So, what is the
> opinion of list members-with a modern rig, does having a low pass filter in
> line really help to solve any additional problems?

Additional problems?  Additional problems are unlikely to be harmonics 
in today's rigs.

Typically the answer is no. For one thing most solid state rigs do their 
band switching by changing LP filters on the output so they are already 
The same is true for solid state amps.
Even today's legal limit tube amps are normally pretty clean, BUT many 
of the marginal designs end up being driven a bit too hard when the 
owner tries to match the advertiser's figures.  Then they could 
certainly use a LP filter.

However, once in a while I run across a station that I really wish had 
one<:-))  It may be the way they are set up, but "typically* the answer 
is still no.

Just listen on the harmonics of each band.  You are likely to hear a 
station or two.  CW is easy to read. SSB is usually pretty distorted.  
Fortunately digital TV is more immune to the interference than the old 
analog sets.


Roger (K8RI)
> John AF5CC
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