On Wed, 31 Dec 2003 11:30:07 GMT, Jack Burton
wrote:
On Tue, 30 Dec 2003 04:01:15 GMT, (Larry W4CSC) wrote:
On Sun, 28 Dec 2003 21:49:17 -0800, "Chuck Tribolet"
wrote:
Larry: please define "very high gain"? 9db? 6db?
The higher the "gain" of the antenna, the flatter and less "thick" its
radiation pattern. The 9db antenna has a much flatter radiation
pattern than the 6db.
So, how does the horizon work into the gain equation?
Picture the vertical antenna standing vertically. It's a high-gain
fiberglass type with many phased elements imbedded into it. Its
radiation pattern is perpendicular to the antenna in a flattened donut
shape. When you step on a donut, its outer edge moves outward as its
thickness is squeezed. That's exactly what's happening to get more
radiation to the horizon where the receiving station is located. The
phased array squeezes the fat round donut of the 1/2 wave, which
bulges out in the middle making a bigger signal perpendicular to the
antenna. This pattern moves with the antenna, always perpendicular to
it.
Now, tilt the antenna towards the receiving station. (Boat pitched?)
The flattened donut is now pointing into the water on the side towards
the receiving station. The signal level at the receiving station
drops, drastically, because this pattern is so flattened. On an FM
receiver, you hear no difference in signal "loudness" as signal
varies, UNTIL that signal drops below the receiver's noise floor,
usually a few hundredths of a microvolt, at which point the receiver
"hiss" of its FM detector gets louder and louder. As the signal
increases, again, the FM receiver "quiets", the hissing drops. This
is the only time the effect of the high gain antenna's flattened
pattern will cause communications problems. If the antenna's received
signal drops from 800 uV to 40 uV, you won't notice it. 40 uV will
keep the receiver hiss quiet. But, if we are talking about adding
this pattern tilting effect to the boat riding down into the trough of
those 30' rollers in a "worst case scenario" where it really counts,
THEN we are talking about disrupted comms. The best antenna for this
situation is NOT the 9dB fiberglass beast mounted on the side of the
helm console.....but the 1/2 wave Metz stainless whip mounted as high
up as you can get it.....with its fat pattern less effected by tilting
that's still sticking up above the waves on top of the sailboat
mast....like a beacon from the lighthouse.
FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine
even when it's rock and roll.
"In range" you'll really not see the difference on FM, which signal
strength has less effect on until it's near the fringe.
I just don't like the big, long fiberglass whips on small boats.
Neither does USCG. They got fed up with them breaking off and all use
the Metz, now. Metz brags about it on their webpage.
Interesting - all the USCG small boats around these parts use the
Shakespeare 396-1 which is a center-fed 1/2 wave.
I use one and it's been great antenna.
That's the best antenna mounted as high as you can get it. It's
end-fed by a transformer in that cylinder base. It requires no
groundplane. I just don't like the way I can't replace the broken
cable permanently mounted to it up inside.....or the screwed-on whip
because I can't slip a 34" piece of coat hanger into the end when the
whip gets busted off by that little pitchpole we did sideways to that
Perfect Strom roller in the night. The Metz whip is mounted in a
gripping ferrule. Coat hanger wire slips right in where the broken
whip comes out.