Topband: QEX article and T-vertical improvement

[Dan Zimmerman N3OX]

> 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