[RFI] Is a low pass filter necessary?

Dale Svetanoff svetanoff at earthlink.net
Thu May 10 21:35:46 PDT 2012


Today's rigs are, in fact, designed with internal filters (low pass and/or
band pass) so that the rigs can meet the spurious and harmonic requirements
set forth by FCC and other regulatory bodies.  Without the filters,
harmonic content of the output would be very high.  With the exception of
finals running true Class A or AB1, there are significant harmonics present
in the unfiltered output of solid state PAs.  Now, add in the effects of
other semiconductors in the signal path, such as PIN diodes (used for T-R
switching), and you have a nice spectrum of output signals riding along
with the desired signal.

One way to visualize this issue is to study the schematic diagram of your
rig.  I don't own a 706, but I do own an Alinco DX-70TH.  The many filter
networks are easily seen on the schematic, and in the case of the '70TH, I
can tell when networks are being switched in or out by listening for the
soft "clicks' of the relays that Alinco used for filter network switching. 
(Relays are better than PIN diodes because they don't create more
harmonics.)  I think it has something like 16 such relays.

I also have several tube rigs that see frequent usage, as I enjoy operating
with them.  In most cases, I do place low pass filters in the output of
those tube rigs, especially any that are running Class C finals (such as my
Heathkit DX-40).  You only have to look at the output of one of those rigs
once with a spectrum analyzer to see why these old beasts and analog TV
sets usually did not get along too well.

So, bottom line: If you run modern solid state rigs barefoot, there should
be little need for an extra low pass filter.  If you run such rigs into a
vacuum tube linear amplifier, you may want to consider a low pass filter on
the output of the amp just in case any over-drive occurs and
non-linearities develop within the amp.  If your amplifier is also solid
state and meets FCC regs, it should be OK to go as-is without extra

Remember, low pass filters can only help reduce high order harmonics in the
output signal of a transmitter.  They can not provide relief from RF
overload if the fundamental signal from the transmitter is getting into TV
sets, computer systems, telephones, audio equipment, and so forth.  When
that happens, the choices are: 1) Reduce power output; 2) Provide greater
physical separation distance between the antennas and the device(s) being
interfered with; 3) Install shielding, by-passing, or RF absorbing devices
to the victim device and its associated interconnects. 

Hope this helps.

73, Dale

> [Original Message]
> From: John <af5cc at fidmail.com>
> To: <rfi at contesting.com>
> Date: 5/10/2012 10:41:24
> Subject: [RFI] Is a low pass filter necessary?
> 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
> and the spurrious emissions are already greatly reduced. So, what is the
> opinion of list members-with a modern rig, does having a low pass filter
> line really help to solve any additional problems?
> John AF5CC
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> RFI at contesting.com
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