Larry wrote:

SNIP

Above 15 Mhz, it's somewhat of an antenna. Below 15 Mhz, the further
down you go the worse it gets because it's just TOO SHORT! We put these
awful lossy "tuners" between the bottom of the whip and the radio to
match the impedance, at the cost of losing a lot of power. Being so
short at lower HF frequencies, its radiation pattern looks more like an
inflated hot air balloon, radiating mostly straight up...not out towards
the receivers over the horizon.

Actually, vertical antennas (even very short ones) radiate almost
nothing directly overhead. Over seawater, vertical antennas radiate
better at low angles toward the far horizon than just about any
horizontal antenna.

These antennas at low frequencies have
almost no antenna CURRENT, which makes the required H field (the magnetic
radiation part of the radio wave). They have lots of voltage, E-field,
but E-field cannot exist without H-fields perpendicular to them so they
radiate poorly. You need BOTH.

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.


Some of the "cure", about as much as you'll get, is to use a LONGER
antenna with the tuner doing less of the work.

Ironically (as is often the case with natu if bread and crackers are
exposed to the same environment, the bread becomes hard and the crackers
soft. Go figure.) at the higher frequencies, a longer vertical antenna
may actually radiate less toward the horizon (low angles). So an antenna
that is the "right" size for 4 MHz may not work as well on 15 MHz as a
shorter one! An antenna "too long" for a frequency will develop lobes
and nulls in the vertical plane which may be detrimental to your desired
propagation path. Yet another need for thoughtful compromise.

One comes with sailboats.
We call it the "backstay", a nice sloping piece of wire we can insulate
from ground on the bottom (series feeding it's called because the tuner
is installed in SERIES between ground and the bottom of the backstay,
because of the insulator. We insulate the top to keep the antenna
current from continuing up the backstay into the mast and going DOWN the
mast...which makes a wave that CANCELS the wave made by the backstay!

Well, if the mast is insulated at the base (some are) what you have is
something like an inverted "V" (end-fed) and there will be some
directionality, but cancellation is far too strong a term to describe
it. Even if the mast base is grounded, the small amount of
directionality would probably not be noticed in normal operation.

SNIP


On Geoffrey's former boat, an Endeavour 35 sloop, I used to load the port
shroud through the ungrounded chainplate that lead right into the port
storage cabinet over the settee...and ran a ground wire down under the
cushions to the ground strap back to the engine I installed. This
arrangement SHUNT fed the mainmast, which wasn't grounded properly to
anything at its step. Shunt feeding has been used since the beginning of
radio. We hams have been shunt feeding out "masts" for decades:
http://www.qsl.net/w9rb/webdoc9.htm

Shunt-feeding the mast (and back- and forestays) doesn't require the
base of the mast to be insulated. I've been shunt-feeding a grounded
mast on a 34' Tartan for years and never felt the need for a better
antenna. Since I use a manual tuner, it was impractical to feed the
bottom of the backstay with a tuner in the cabin. Because the lower ends
of the forestay and backstay are ungrounded, they act as a top hat,
making the mast appear longer (electrically) than it is. Since the stays
are not symmetrical, they doubtless provide some often needed horizontal
radiation.

I must say that on some frequencies, particularly the 40 meter band, the
feedpoint impedance of the system was wildly high. I ended up using
some additional outboard reactances to tame it. Everywhere else, tuning
was reasonable. An autotuner may have handled it OK. A key advantage is
that you can install an SSB in minutes without worrying about insulating
the backstay, etc.


Chuck



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