On Sun, Feb 17, 2019 at 4:26 PM Don Wilhelm<[email protected]> wrote:
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.
Here, Don is talking about external low pass filters intended to
suppress VHF harmonics. If you live in an area where TV signals are
weak, they may still be necessary. Of course, hardly anyone receives
their TV service that way any more, and most TV stations have moved to
the UHF bands, anyway.
The low pass filters in solid state radios and amplifiers are necessary
because their outputs are untuned, and the nonlinearity of transistors
generates lots of harmonics, including low order ones. In the days of
vacuum tubes, the pi network served as an effective filter. The popular
pi-L was even better.
It might be interesting, during a major contest, to listen on the second
harmonic, say around 14.120 when there's a lot of activity on 40. Many
are using solid state amplifiers now, and even 43 dB down may be
audible. I've never positively identified one, but a lot of stations
don't come back to me for other reasons, so it would be hard to tell.
73,
Scott K9MA
--
Scott K9MA
[email protected]
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