ray lunder wrote in
:
So what would a proper boat antenna look like? Examples, pictures,
name brands? Thanks as always.
Boats have no room for antennas that work good. So, there are two
compromises that work, "sort of".
One is an 18' whip coated in fiberglass to make it "yachtie", tuned from
the bottom against a seawater ground (we hope, or it's going nowhere):
http://www.boatersland.com/sha390.html
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. 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.
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. 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!
That's what the top insulator does. On "Lionheart", an Amel Sharki 41
ketch, this backstay is about 55' long, which is nicely resonant (where
the length becomes 1/4 wavelength) at 4.4 Mhz. It radiates very well
between 3-5 Mhz..maybe even wider. It also radiates very nicely as a 1/2
wavelength at 8.5 Mhz if you can get everyone away from the bottom of it
and you have a tuner that can tune very high impedances. At 8.5 Mhz, it
becomes a full dipole antenna with the tuner feeding the END of it, not
the middle as a normal dipole is fed at low impedance. There is
CONSIDERABLE voltage at 8.5 Mhz right at the bottom insulator. Keeping
hands off it is easy. Just key the mic, whistle in it and listen for the
cursing near the antenna...(c; Burn the captain and you walk the plank!
Both of these bands are very nicely around some great marine bands.
Unfortunately for sailboaters, they keep making masts, shrouds, topping
lifts, and other lines coming from the top of the masts to the bottom out
of METAL....which becomes passive elements, not directly driven, by your
backstay transmitter. The regenerated waves from any vertical metal,
wire rope, cables, shrouds, are always slightly out of phase and most
assuredly out of place with the main signal on the backstay. This causes
the reasonably nice fat donut you're looking for in a radiation pattern
into something that resembles the outside shape of an 8 leaf Shamrock
(Happy St Patty's Day, everyone!) with peaks in some directions and nulls
in others as the waves support and cancel each other around the azimuth.
These damnable metal things sticking up also absorb a lot of our radiated
signal, but are generally forgiven when the sailboat takes on diesel fuel
for these transgressions against our SSB signal. Again, it's all a
compromise.
http://www.tpub.com/content/et/14092/css/14092_35.htm
Oddly enough, those loud shore stations that haven't gone dead on marine
radio use a mast with shrouds all around it called a "conical monopole"
antenna which is VERY broadband over a wide variety of frequencies! Look
at the conical monopole on this webpage, and compare it to your
mast/shrouds/stays on the sailboat. Your boat is the TOP HALF of this
antenna! So, all is not lost. Too bad yours cannot be totally isolated
from ground and the whole rigging loaded as a giant antenna. 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
Because the feed runs right along side the mast its feeding, there is no
changing phase angle the backstay creates running out from it. A series
capacitor is tuned to balance out the inductance of the shunt wire
running up, and insulated from the mast until it is attached at the top.
Broadcasters also use shunt fed, GROUNDED towers, because the expensive
AM transmitter is NOT in series with the 10 gigaamp lightning blast that
hits the tower and goes straight to the huge ground network that's
attached directly to the bottom of the tower, underground. You can see
which type of feed your local AM stations use on their tower antenna by
looking for the shunt wire running on insulators towards the top of the
tower. Series fed will have a huge insulator at the base and no shunt
wire. Shunt fed towers are firmly grounded at the base, not insulated.
I don't know why more HF-savvy boaters don't fool around with shunt
feeding their mainmast with a shunt wire insulated from the mast on the
port or starboard side under the shrouds. They work great and give you a
fantastic excuse to put a huge ground strap from the mast to ocean
grounding blocks under the hull to protect you from lightning hits.
http://nidxa.org/kb9cry_shunt_fed_tower.htm
http://www.fybush.com/sites/2007/site-070223.html
http://www.earthsignals.com/N6TZ/
Of course, some hams are MOST useful to HF boaters with their obsession
of antennas:
http://www.k7zsd.com/antennas.htm
Very nice, indeed. No restrictive covenants in HIS neighborhood!...(c;
Larry
--
Roll up to the long checkout line....
Yell, "ICE RAID!"
It's your turn to load the grocery belt...(c;