[RFI] Low pass filters

Ian White, G3SEK Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk
Mon, 19 Jan 1998 22:47:52 +0000

Ed, W1RFI wrote:

>The main purpose of a low-pass xmit filter (or bandpass for VHF 
>transmitters) is so that when you DO have an interference problem, you can 
>point to it with pride.  :-)  Most cases of consumer interference are caused 
>by fundamental overload, not transmitter harmonics.

Ed's statement might promote a common error about the difference between
harmonic filtering requirements for HF and VHF transmitters.

There is NO difference! A lowpass (or harmonic notch) filter is usually
the correct solution for both. 

The only case where a bandpass filter is the right solution is when
there are spurious signals close to the wanted frequency. This is quite
rare in a well designed transmitter, because bandpass filtering is
usually applied at low level in the exciter.

VHF bandpass filters are primarily designed for receiving, and have low
loss in order to preserve noise figure. The low losses also allow them
to be used after a transmitter, but this is usually NOT a good
engineering solution.

If the only requirement is to attenuate harmonics, the right solution is
ALWAYS a lowpass filter or a harmonic notch filter - at any frequency.

One important difference at VHF is that the percentage frequency change
is usually much smaller than at HF. This means that the harmonic
frequencies are essentially fixed, so harmonic notch filters using coax
stubs can perform very well indeed. See my web pages for details of the
harmonic notch filters by G4SWX which have excellent performance, can be
built in minutes, cost literally a few pennies, and can handle a

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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