Hi Bob,
Both tuners are nice tuners.
> I use antenna tuners in my station so as to always allow for my low pass
> filters to see 50 ohms when excursions to SSB take me away from the CW
> portions of the bands, thus optimizing low pass and band pass filter
> capabilities to their designed performance expectations. In a city
> environment proper filtering and grounding is important.
I assume you are talking about regular TVI filters. I often see claims made
that matching the filter is important for proper filter shape. The truth is
adjusting for low SWR at some frequency far below the cutoff doesn't mean a
single thing to the filter's cutoff response on another frequency!
Matching to eliminate a high SWR on some frequency other than the reject
frequency does help the filter handle rated power (it makes sure the
current and voltage isn't out of filter component limits), but that's all
matching does (in the case of a filter that has a cutoff frequency removed
from the operating frequency).
It is only the source and load impedances at or very near the cutoff
frequency of the filter that affect filter attenuation, not the impedances
at frequencies far removed from the cut-off frequencies.
> Except for the component values differing, the only difference is in the
> use of a split rotor capacitor for the input on the model SA-2040.
>
> I know what differences I observe in the use of these two units, but to
> reveal same would perhaps jade the possible answers to my question.
>
> What are the relative merits of one design over the other?
Virtually no difference. If you calculate the additional harmonic
suppression and matching benefits, (I've done that for Heathkit and others)
the effects are about zippo. The shunt C just isn't big enough to do
anything meaningful. Doesn't hurt, doesn't really help.
Lew McCoy, the guy who "originated" the SPC design, has mentioned several
times that it was a simple wiring error that got copied into print. Lew
said it appeared in print before he caught it, and said he still grins when
he sees it in popular use. If I recall correctly, both Walt Maxwell and Roy
Lewallen have covered that design and reached the same conclusion as I did.
That extra C does virtually nothing. If I'm not mistaken it's covered in
Walt's book, Reflections.
73 Tom
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