Thanks for the additional information, Ian. My principle reason for
suggesting a bandpass filter for VHF is that most of the commercial VHF
transmit filters available in the US are of the bandpass type.
I note, too, that hetrodyne or PLL-type transmitters (translate: virtually
all modern transmitters) can have mixing products or other unwanted
emissions that could appear above or below the transmit frequency. These
are almost always suppressed quite well, so they are "legal," but they could
cause interference to a nearby TV. On HF, the mixing spurious emissions
generally do not cause any TVI problems, the TV channels are all higher in
frequency than the transmitter, so the low-pass filter eliminates TVI
nicely, whether it was caused by harmonics or other spurii.. A VHF or UHF
transmitter could have mixing or PLL-generated unwanted emissions that are
lower in frequency, interfering with a neighbor's TV reception. The
bandpass filter can also help with "phase-noise" type problems.
I personally would opt for the bandpass filter if I were buying a
commercial filter, based on availability and the fact that it woudl filter
any and all emissions outside the ham band. I would try the stub notch
filter first if I were going to try a homebrew solution and it appeared that
my TVI was caused by a specific harmonic.
73,
Ed, W1RFI
A low-loss "cavity" bandpass filter that will help with both receive
intermod or unwanted emissions from the transmitter is available from:
DCI Digital Communications Inc.
Box 293, Hummingbird Bay
White City, SK S0G 5B0
Canada
Phone: 800-563-5351 (from the US only)
306-781-4451
Fax: 306-781-2008
Email: dci@dci.ca
Web Site: http://www.dci.ca/
----------
>From: Ian White, G3SEK
>To: rfi
>Subject: Re: [RFI] Low pass filters
To: <rfi@contesting.com>
>Date: Monday, January 19, 1998 10:47PM
>----------------------------------------------------
>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
>kilowatt.
>
>
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.demon.co.uk/g3sek
>
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>
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