Re: low pass filters

Posted by Don Wilhelm on Feb 17, 2019; 11:25pm
URL: http://elecraft.85.s1.nabble.com/low-pass-filters-tp7649039p7649040.html

A low-pass filter will not change RFI either coming into the shack or
going out.  There are low-pass filters in all transceivers produced by
Elecraft (and most other manufacturers), and they are in both the
transmit and receive path.
The current FCC requirement is for the 2nd harmonic be suppressed by 43
dB over the fundamental.
Low pass filters come from a past era when the FCC requirements were not
as stringent and TVs responded to those higher order harmonics from
amateur transmitters.  That is no longer the case.

Considering receive, we do have many sources of unintentional radiation
which is present in the HF spectrum, and other than locating and having
the offending device turned off, there is not much we can do about it.
Although if it is interfering with licensed operation, there is recourse
by cooperation with the FCC.  The products giving problems range from
inexpensive switching power sources (read wall-warts) to control
circuits in many household appliances and other consumer devices, and
even some lighting products.

If you are operating in an environment like Field Day where there are
multiple transmitters operating in close proximity, bandpass filters and
single band antennas are the normal "cure" as well as physical
separation of the antennas.

73,
Don W3FPR



On 2/17/2019 5:59 PM, Richards wrote:
> Good day -   What is the consensus concerning low-pass filters to
> minimize RFI both coming in and going out  of the shack?   Is there a
> type or size, brand or model that comes highly recommended?  Is it
> prudent to employ one, regardless of whether or not there is a known,
> identified problem , i.e., just in case?  Does it assist reception as
> well as transmission?
>
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