Steve, KK7UV wrote:
I currently have a wire T-vertical with a
75-ft. vertical portion and a
75-ft. topload wire over about 60
radials.
Feedpoint is around 25 + j0 at 1.825 Mhz, which I match
with a toroid 1:2
unun.
The latest issue of QEX has an
article about maximizing radiation
resistance by putting the current
maxima halfway up the vertical.
EZNEC shows me that by extending my
T topload wire to 140-ft. long, the
current maxima will move up to
the middle of the vertical, and my
feedpoint will be around 37 +
j238 at 1.825 Mhz. I would replace the
unun with a switched or
adjustable L-network for matching across the whole
band, and perhaps
other L-networks for 80 or 40.
A couple questions for the
group....
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?
The short
answer is time and money:
In addition to the comments about
tuning with L-networks or series capacitors and need for longer radials,
the EZNEC 4.0 version shows that the radiation pattern does not change
when going from a 75 foot Tee top to a 140 foot Tee top. (same
3db lobe width and same maximum radiation elevation of 24
degrees).
However, the model shows that the gain for the 75 foot Tee
top is 1.19 dbi and the gain for the 140 foot Tee top is 1.11 dbi, so one
is theoretically loosing 0.08 dbi gain.
I guess this is because the
longer top loading wires are actually "covering up" a little of
the radiation. Therefore I would stay with the 2:1 UnUn feed and the
75 foot top loading.
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?
I agree that the UV
resistant NEMA plastic enclosures without shielding is the way to go.
73 & GL
George K8GG
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