On Wed, 10 Nov 2004 19:07:19 GMT, Chuck wrote:

Hello again, Gary. Thanks for your reply.

There are a few points still not settled. But first we need to separate
the objective importance of radiation at various vertical angles from
the objective reality of how much energy different antennas radiate at
these angles. There seems to be a reluctance among some in the group to
acknowledge that vertical radiation angles are important enough to
warrant an influence on antenna design decisions. I'm going to leave
that alone and just talk about how various antenna designs radiate.

They may be important but there is usually little you can do about it
on a boat.


My first "exhibit" is figures 54, 59, and 60 from the ARRL Antenna Book
(20th edition), Chapter 16. Vertical radiation patterns for a
quarter-wave, transom-mounted whip are compared with those for a typical
backstay antenna at 20 meters.

Figure 60 shows the backstay at 15 meters but there is no corresponding
quarter-wave whip figure for that frequency. Fortunately, however, we
are on fairly solid ground by assuming the vertical pattern of a
quarter-wave 15 meter whip will be quite similar to that of a
quarter-wave 20 meter whip. Thus, we can compare figure 54 to figure 60.

I believe that the ARRL patterns show the whip to be an unambiguously
better low-angle radiator than the backstay. From these patterns, I can
easily imagine situations in which the backstay would nonetheless be a
better choice. I can just as easily imagine situations in which the
quarter-wave whip would be a better choice. Just based on the vertical
radiation patterns alone.

They only show the vertical angle at an azmuth of 90 degrees. Don't
know if that is the best or worse direction for the vertical angle.

If you look closely at those patterns you willl see that the backstay
vertical pattern is much broader than the whip. That is an advantage
when working various distances that require different take off angles.

Very low angles are usually only good for very long haul
communications. Want to talk to China or Japan?
Shorter range, around the US, usually require higher angles to do the
job.

Talk to some of the hams that have stacked beams on tall towers. Often
the lower antenna, with it's higher take off angle is superior to the
higher antenna on shorter paths.

Also remember that just because the maximum of the lobe may be at 30
degrees, dosn't mean that it is dead at 15 or even 10 degrees. It may
only be down a couple of db at lower angles.



But that's not all! The versatility of the backstay antenna at other
frequencies and the attendant complication of a tuner could be compared
to the simplicity of the whip, its physical independence from the mast,
and its lack of need for a tuner. These are other considerations that
might affect one's choice. Heck, they're not even mutually exclusive!
Just measurably different.

A whip with no tuner is good for only one frequency. And then it
requires some sort of matching network to make it work. Might just as
well put in a tuner and make use of it on other frequencies too.


Regarding the alleged necessity of a vertical antenna for "surface wave
type communications," please consider the US Marine Corps' take on this
in their Antenna Handbook (MC RP 3-40.3C, page 4-40):

"NVIS propagation is simply sky wave propagation that uses antennas
with high-angle radiation and low operating frequencies. Just as
the proper selection of antennas can increase the reliability of a
long- range circuit, short-range communications also require proper
antenna selection. NVIS propagation is one more weapon in the
communicator’s arsenal.

To communicate over the horizon to an amphibious ship on the
move, or to a station 100 to 300 kilometers away, the operators
should use NVIS propagation. The ship’s low take-off angle
antenna is designed for medium and long-range communications.
When the ship’s antenna is used, a skip zone is formed. This skip
zone is the area between the maximum ground wave distance and
the shortest sky wave distance where no communications are possible.
Depending on operating frequencies, antennas, and propagation
conditions, this skip zone can start at roughly 20 to 30 kilometers
and extend out to several hundred kilometers, preventing communications
with the desired station.

NVIS propagation uses high take-off angle (60° to 90°) antennas to
radiate the signal almost straight up. The signal is then reflected
from the ionosphere and returns to Earth in a circular pattern all
around the transmitter. Because of the near-vertical radiation angle,
there is no skip zone. Communications are continuous out to several
hundred kilometers from the transmitter. The nearly vertical angle
of radiation also means that lower frequencies must be used. Generally,
NVIS propagation uses frequencies up to 8 MHz."

Sorry for the poor formatting. NVIS is what you get with a horizontal
dipole on the deck of a non-metal hull that I had mentioned. I really
doubt that you can get reliable daytime 3 MHz communication using 150
watt transmitters and antennas connected to 50 foot masts and at
distances of hundreds of kilometers. But with NVIS, it is routine.

The "surface wave" propagation that I was talking about, that requires
vertical polarization, is not the same thing. NVIS is still dependent
on ionosphere reflections and is at the mercey of the ionosphere.
Daytime may kill the signal.
With surface wave propagation it is there all the time, night or day.
It is what broadcast stations depend on. It is very usefull on the 2
mhz marine band with proper antennas. The signal follows the surface
of the earth rather than being reflected from the ionosphere.


The other point has to do with the vertical radiation pattern of a
3/4-wave vertical. You will agree, I believe, that the VERTICAL pattern
of the 3/4-wave vertical over perfect ground is "one-half" of the
HORIZONTAL pattern of a 1.5 wavelength dipole in free space. (Split the
dipole with a plane perpendicular to the wire's axis and then rotate the
plane through 90 degrees so the wire is vertical. You can throw away the
image beneath the plane to make it look like the usual patterns.) It
follows, then, that the lobe of the 3/4 wave antenna in the vertical
plane will peak at 45 degrees. Of course, over real ground the pattern
will be different. I doubt though that real ground will LOWER the
vertical radiation pattern. In any case, my statement has nothing to do
with the the height of a horizontal dipole above ground.

No, you can not just split the pattern of a horizontal antenna and
rotate it to get a vertical pattern. In free space yes, on the ground
no. The earth has a large effect on it. Reflections from the earth add
and subtract to determine the pattern.

If you look at the vertical patterns from a horizontal antenna at
different heights above ground you will see drastic changes in the
vertical pattern.
There is not a lot of information printed on vertical radiators of
different lengths. Folks often confuse the horizontal patterns with
what a vertical pattern would be.


As an "exhibit" on this point, I offer a meager quote from Low Band
DXing (3rd edition), page 9-51:

Note that going from a 1/4 wave vertical to a
1/2 wave vertical drops the radiation angle from
26 degrees to 21 degrees. More important, however,
is that the 3-dB vertical beamwidth drops from 42
degrees to 29 degrees. Going to a 5/8 vertical drops
the radiation angle to 15 degrees with a 3-dB beamwidth
of only 23 degrees. But notice the high-angle lobe
showing up with the 5/8 wave vertical. If we make the
vertical still longer, the low-angle lobe will disappear
and be replaced by a high-angle lobe. A 3/4 wave vertical
has a radiation angle of 45 degrees.

So the humble contribution I've been trying to make is that longer
antennas are not always better than shorter ones. They are sometimes
better and sometimes worse. But they are always different. Whether the
difference is worth considering pretty much depends on the nature of the
difference.

I agree. Large ships usually have several different types of
antennas. However, in most boat installations you usually only have
one shot at it. One antenna is all there is room for. I would opt for
as much wire as I could get up in that case. More wire will give much
improved performance on the low bands with a moderate compromise on
the high bands.


Time to move on, I think.

Regards,

Chuck


Regards
Gary

















Gary Schafer wrote:
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