[Elecraft] low pass filters

[K9MA]

> On Sun, Feb 17, 2019 at 4:26 PM Don Wilhelm<donwilh at embarqmail.com>  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

k9ma at sdellington.us