Larry wrote:
chuck wrote in news:1174053226_6783
@sp6iad.superfeed.net:
If the short antenna is matched (i.e., you use a tuner) it will be
nearly as efficient as its longer counterpart. A short vertical antenna
has very HIGH current at its base. Any loss in efficiency is due almost
exclusively to the tuner when operated over seawater. Over land, ground
losses become a more significant factor in reducing efficiency.
Any antenna shorter than 1/4 wavelength has HIGHER impedance and LESS
current. It NEVER has high current at its base. It also suffers from
having so poor an H-field generated without that big current lobe.
Hmmm. You're thinking about the capacitive reactance of a short antenna,
which can be high, and I'm thinking of the radiation resistance, the
real part of the antenna's impedance. When the short antenna is matched,
the capacitive reactance is cancelled by the tuner and you are left with
a radiation resistance that can be as small as a fraction of an ohm. by
comparison, the radiation resistance of a quarterwave vertical antenna
over seawater is about 35 ohms. The base of the antenna is always a
current node on a short vertical antenna: current is a maximum there.
The high voltages (and low currents!) occur at the tops of these
antennas; not at their bases. Where would the current flow to at the top?
If you deliver 100 watts to a short whip, it will radiate as well as a
quarter-wave vertical, assuming the same ground system, etc. There will
be MORE current at the base of the shorter antenna because power = I
squared x radiation resistance. Radiation resistance of a short antenna
is smaller than that of a quarter-wave antenna, so to keep power at 100
watts, I must INCREASE!
I would not be surprised for a 16- or 23-foot whip on a boat to
outperform a 55-foot, sloping backstay antenna at very low angles of
radiation, even with matching system losses. At 7 MHz, for example. FWIW.
A shortened antenna is NEVER anywhere near as efficient as a full 1/4
wave radiator....or more...working against a ground system. Boy, would
AM broadcasters love to have an efficient 50' antenna tower. You'll be
FILTHY RICH if you can make that work!...(c;
Keep in mind that a decrease in efficiency of 70% (1 dB) is needed
before someone at the other end would even notice it. I would imagine
that with reasonable attention to the matching components, an antenna
could be shortened to 1/8 wavelength without any noticeable drop in
signal strength, and without any noticeable change in the vertical
radiation pattern. A 1/16 wavelength whip over seawater might seem
slightly weaker to a distant station than a full quarterwave vertical
over seawater. No quarter-wave antenna with its base on the ground is
likely to outperform a 1/8 wavelength whip over seawater.
The efficiency of a resonant vertical antenna SYSTEM (even with a
radiator that is a tiny fraction of a wavelength long) is the radiation
resistance (a small number) divided by the sum of the resistances in the
system. These resistances consist of losses in the antenna wire, the
ground system, and the matching circuit (which may be inductors and
capacitive hats) x 100 (to get efficiency in %). Over seawater, "ground"
losses are insignificant, and antenna wire resistance is often
insignificant. There are indeed tuner losses, but these can often be
reduced. The point is that the important sourcea of inefficiency are not
in the antenna but in ground loss and matching circuits.
Eventually, an antenna gets to be so short that the losses (or costs,
broadly defined) incurred in feeding power to it are unacceptably high.
Too much power would be lost in the rest of the system. Nonetheless,
that short antenna will radiate all the power that is delivered to it
with high efficiency.
To maintain balance, I would mention that an antenna that is "too long"
(e.g., your 55' backstay on 15 meters) also requires matching and that
will introduce losses. There are not many antennas that will work on
every HF band without matching. Those that do tend to introduce their
own high losses.
With regard to AM broadcasters, it is not for me to say where the
economics, regulatory, and efficiency curves cross, but capacitive hats
on antennas shorter than a quarter-wave are not unheard of on those
antennas.
If you think running your HF tuner is efficient, you are sadly mistaken.
That tuner is simply a big dummy load on a short whip. It sucks!
Tuner losses are indeed significant at lower frequencies with a short
whip. And on most automobiles, ground losses are also significant. Of
course, a seawater ground and quarter-wave whip are not really
alternatives if you're in a car. ;-)
Chuck
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