SB-102 with Lo-Pass filter

[Robert P. Okas]

Hi Allen,

    A PI-net is indeed a lowpass filter. It darned well better be since
it's also used to match the output of a Class-C final to the load. We all
know that Class C is anything but linear! I haven't done this, but it
would be an interesting exercise to compare the output spectra of a
transmitter with and without an outboard LPF.

    I have a Drake TVI filter that I once tried with my DX-100. I had an
SWR meter inline between the TX and the filter and couldn't get the SWR
down to 1:1 with a dummy load. Since I had no TVI complaints when running
80m AM, I elected to leave the filter out.

    I also run an SB-102 and the antenna is a 40m dipole, ladder line fed,
with an MFJ tuner. The neighborhood has cable and I'm guessing that 50% of
the residents are subscribers. I've had no TVI complaints and minimal
problems getting into my own (cable-less) TV. At another QTH with a cable
subscription, I couldn't stay out of my TV when operating 20m SSB. Heck,
the sound of my voice was even coming through the telephone receiver! And
this was an old-style, non-electronics phone!

    At the old QTH, I had a less than ideal grounding situation.
Presently, I have an 8' rod in ground with about 10' of #10 stranded wire
between the earth and the rig. This seems to have the greatest impact on
avoiding TVI.

    Hope this helps.

73,
Bob - W3CD
Redwood City, CA


On Wed, 15 Jul 1998, Allen & Matt wrote:

> Hi Group--
>
> I've been led to believe that one should always use
> a low-pass filter on the output of the rig to help
> minimize TVI--especially when the neighborhood does
> not have cable.
>
> Is this really true?
> (Assume a doublet antenna, balanced feedlines, ant. tuner.)
>
> As most older rigs, this one has pi-out. Is that, in
> itself, low-pass?
>
> Does the filter really make a difference *in the real
> world*?
>
> How about you SB-users who have no lo-pass filter!
> Do you have particular problems with TVI?
>
> Thanks for info and help!
>
> Rev. Allen
> --
> * * * * * * * * * * * * * * * *
> 73 de Allen, KJ6XH, and Matt                  West Sacramento, CA
>                email:     mattall at cwnet.com
>                http://users.cwnet.com/mattall
>
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