Posted by
Ron D'Eau Claire-2 on
Mar 24, 2007; 8:26pm
URL: http://elecraft.85.s1.nabble.com/Vetical-Antennas-tp444325p444330.html
A half wave needs no ground at all. Since virtually no current is flowing
into the antenna when fed at the end, no current needs to flow into a
ground. From my second-story window I use a parallel tuned circuit with the
power fed from the rig in via a link at one end, and the 1/2 wave antenna
connected to the other end. An L-network would work just as well. In that
case, since the high-impedance is at the antenna end of the inductor, that's
where the capacitor goes.
An L-network is a simple parallel-tuned resonant circuit with the connection
between the tuning cap and the inductor opened to connect the low-impedance
circuit - in this case the low-impedance coax connection to the rig. The
high-impedance load at the other end is across the tuned circuit that
includes the rig in series with the coil and capacitor. Here's a rough
schematic:
>---coil---|-----> high impedance antenna
50 ohm |
<---cap----|
A 5/8 wave does draw current at the base and it'll look inductive since it
is longer than its naturally resonant 1/2 wave. You'll need capacitive
reactance equal to the inductive reactance to bring the 5/8 wave radiator to
resonance. Again that can be done with either L-network or a link-coupled
parallel tuned network. When the load shows inductive or capacitive
reactance, that reactance detunes the circuit, so you adjust the value of
the coil or cap to compensate and bring the system, including the antenna,
back to resonance while providing the required impedance transformation.
That's what is happening when you adjust an L-network or a parallel tuned
link-coupled network for a low SWR on the 50-ohm link to the rig.
Whether you'll need to worry about a balun or feed line choke will depend
entirely upon whether there's RF currents flowing on the outside of the coax
and, if so, whether they are a nuisance.
You are quite right that in both cases the demands upon a low-impedance
ground system are greatly reduced over that needs of a 1/4 wave (or shorter)
vertical for equal efficiency. With the antenna oriented vertically, you'll
still have the added far-field ground losses inherent in such antennas
compared to a horizontal antenna at a good height (0.2 wave or higher). But
you can't do anything about that short of locating next to the sea shore
<G>. Even so, a vertical will generally outperform a low (0.2 wave high or
less) horizontal, especially for lower angles of radiation generally
desirable for DX. The horizontal really shines for DX when it approaches 1/2
wavelength above ground. Under those conditions you'll see 3 dB to 6 dB gain
over a vertical - equivalent to doubling or quadrupling your transmitter
power. Down at 0.2 wave above the ground, the horizontal will show a very
strong vertical lobe that isn't present with the vertical, although most
verticals show some radiation straight up anyway. That strong vertical lobe
from the low horizontal is what provides the short skip or "NVIS"
characteristic of those antennas that is generally superior to verticals.
Below 0.2 wavelength the amount of current induced in the ground by a
horizontal causes excessive losses that reduce its gain. That's why
verticals really outshine low horizontals on the lower-frequency bands. To
be a really effective DX antenna, an 80 meter dipole needs to be >70 feet up
and something around 120 feet up is ideal. A 40 meter dipole needs to be
about half that height. For a lot of us, that simply isn't practical.
Ron AC7AC
-----Original Message-----
I'm just catching up reading this thread and have some questions:
What is the best way to feed a 1/2 wave or 5/8 wave vertical at ground
level? I'm guessing a link coupled matching network would be better than an
L match and would perhaps obviate the need for a feedline choke? What would
constitute an efficient 'ground' for such an antenna? I assume the
requirement for lots of close in radials is not so important as it would be
for a 1/4 wave vertical?
I await any comments with interest.
Thanks and 73 Fraser G4BJM
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