> 1. Supposedly I will end up with higher radiation resistance and
> efficiency, and I assume higher effective radiated power. But what am I
> giving up?
Among other things, the time to switch over... I've just spent a couple of
weeks building a new switching arrangement for my switched L networks
without any performance increase other than that the computer can control
which band I'm on :-)
The time spent maximizing radiation resistance should be weighed against
what your ground loss resistance might be. How much, in dB, do you expect
to gain by changing your system? I think you'd have to have about 45 ohms
of loss resistance to see 1dB change in going from 25 ohms to 37 ohms
Rrad!! If you're an inveterate tweaker with a strong "it all adds up"
philosophy, you might be happy with less than 1dB improvement. But pretty
good radials and a 25 ohm antenna don't leave a whole lot of room for large
improvement.
Otherwise, I don't see much disadvantage. At 37 + j238 at the low end of
the band, just a motor drive series capacitor (370pF) should do the trick
for the adjustable 160m match. I actually do the same thing with my antenna
with a big 2000pF vacuum variable, even though it means needing to add an
extra +j44 ohms of loading coil to my base loaded 60 footer.
I have some concern about having a capacitor between the antenna and the
ground system for static buildup reasons, but so far I have not had any
problems that I can ascribe to that and it seems like it's a very popular
thing to do with extra long inverted L's, which is pretty much what I'm
talking about.
> 2. Should L-networks be located in an RF screened enclosure? I have a
> large plastic NEMA enclosure I can house the whole matching show in, but
> should I line the interioir with screening of some sort?
No, in fact, it's probably better with big coils to have them in a plastic
box. Nearby metal walls can only hurt inductor Q as far as I know, and
there's no reason not to let antenna loading coils pick up and radiate the
really tiny amount of RF that they're capable of.
73
Dan
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