On Wed, 10 Nov 2004 03:16:47 GMT, Chuck wrote:

Well do I have egg on my face!

Gary, you are correct, of course, in stating that there is not a lot of
difference between the vertical radiation patterns of half-wave and
quarter-wave antennas. Surely not the differences I was alluding to.

And so my statements to the contrary were just plain wrong.

While I was writing half-wave, I was thinking of something longer, like
3/4 wave. I should have been more careful and I do apologize.

My point, however, is just as valid. Many sailboats sport 45' backstay
antennas and that is close to 3/4 wavelength in the 15 MHz range. A 3/4
wave antenna has maximum vertical radiation at 45 degrees! I would say a
16- or even an 8-foot whip would be very competitive with such a
backstay antenna at the lower radiation angles needed for transoceanic
communication.

At higher marine frequencies, 3/4 wavelength is obviously even less than
45 feet.

Of course, the 3/4 wave will be efficient and easy on the autotuner.

I'll try to keep my brain in synch with my typing, henceforth.

Chuck


Hi Chuck,

That 3/4 wavelength antenna pattern you are looking at I will bet is
for a horizontal antenna 3/4 wave high. The pattern for a vertical
antenna is different. Also when you see antenna patterns that show
main lobe radiation angles you need to look closely at them to see how
many db down the signal really is at the desired angle. It does not
disappear entirely at any angle. Although there sharp notches in the
pattern at times where the signal is highly attenuated it is rare that
the signal is completely eliminated at that small angle.
Also with longer antennas, multiple lobes are created rather than a
single lobe as seen with a shorter antenna. Many times those multiple
lobes can be a help in filling in angles that may be otherwise missed.
Sometimes the nulls can work against you too.

With a sloping antenna such as a backstay, while the radiation angle
may be raised in one direction because of a long antenna it also is
lowered in the opposite direction because of the higher angle lobe.

On a boat you usually have little control of where the antenna goes
and the angle at which it runs.

The lowest radiation angle may not always be the best for the path you
are trying to work either. For very long distances low angles are
usually better but medium and shorter distances may be better with a
little higher radiation angle.

A note about the low frequencies:
If you are working surface wave communications below 3 mhz a vertical
antenna is essential. Only vertical polarization works in that mode
and is very reliable night and day over the given range.
Horizontally polarized signals cancel out and you get no surface wave
with them. AM broadcast stations are an example of this type of
propagation. Surface waves follow close to the earth on the low
frequencies. On higher frequencies they are quickly attenuated. I am
sure that Bruce can attest to the reliable communication on the low
band.

Doug's 23 foot whip may work very well on the higher bands as it is
more vertical than a backstay and probably more in the clear. But it
will not be a good performer on the lower bands.

Another note on short antennas: That 23 foot whip that Doug uses is
less than an 1/8 wavelength on 4 mhz.
A quarter wavelength vertical has a radiation resistance of around 36
ohms. Shorten it to an 1/8 wavelength and the radiation resistance
does not drop in half but goes down to around 6 ohms! That antenna
radiation resistance is in series with the ground system resistance
which is usually quite high. It may be in the order of 20 to 30 ohms
in many cases. Guess where most of the power goes.

Regards
Gary