Antenna Ratings

[sk]

Know anywhere I can find ratings to help decide which antenna?? I have a 23
foot center console. Generally range 10-15 miles offshore and plan to mount
the antenna on my

 

[Harry Krause]

Larry: please define "very high gain"? 9db? 6db?

FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine
even when it's rock and roll.

I have an 8' "Digital" brand antenna hooked up to my VHF on my 25'
Parker pilothouse. Works fine in the rock and roll of the Atlantic Ocean.

Visit the boat at:

http://photos.yahoo.com/hakrause

 

[Jack Burton]

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?

"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.

Later,

Tom
S. Woodstock, CT

 

[Shortwave Sportfishing]

~~ major snippage ~~

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.

Yes, that is true enough on a sail boat because you are talking about
height in that case which can conquer a multitude of conditions nine
foot antenna or three foot base loaded (end fed) stainless steel.

I'm speaking about a small boat as in, oh say, a 23 foot CC Ranger
with a T-top or a 32 foot Contender with a T-top. I would think that
even in your scenario, the angle of the signal to the horizon when it
leaves the antenna would work the same base load or center fed and
cause signal capture problems at the receiving end regardless.

By the way, neither of those boats would ever be in a situation with
30 foot rollers - if they show up, I'm long gone. :>)

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.

How can an antenna work without a ground plane? At the frequencies
we're discussing, the ground effect in FM is about the same as it is
in AM if I understood your discussion points (deleted from this post)
correctly.

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.

Wellm to each their own I guess. :>)

Later,

Tom
S. Woodstock, CT

 

[Shortwave Sportfishing]

Modulation has no effect on antenna physics. The object is to fit at
least 1/2 wavelength onto a conductor. Where it is fed is of no
consequence to radiation, but does effect the feedpoint impedance.

As to the "ground plane".....

~~ snippage ~~

Well, you kind of danced around the answer, but I'd still like to know
how the ground plane effects the radiation angle which logically would
also have an effect on reception of a signal. On page 3-9 of the ARRL
Antenna Handbook (16th addition - sorry, it's the latest I have at
hand at the moment) states:

"The total current in the antenna consists of two components. The
amplitude of the first is determined by the power supplied by the
transmitter and the free-space radiation resistance of the antenna.
The second component is induced in the antenna by the wave reflected
by the ground. This second component, while considerably smaller than
the first at most usefull antenna heights, is by no means
insignificant."

So it would seem that the "ground plane/wave" is not an umimportant
consideration when considering antennas.

Further on that same page, is the following:

"Changing the height of the antenna above the ground will change the
current flow assuming that the power to the antenna is constant."

Again, it would appear that the "ground plane/wave" is not
insignificant.

Now, as I understand it, at VHF frequencies, the methodology of
providing energy to the antenna (loading/feed) is not as important to
the generation/reception of the signal as is height. In fact, if I
read the pattern charts correctly, the height of the antenna has more
to do with the lobe pattern (the donut you were discussing) than the
method of feeding the antenna.

Yes/No?

Later,

Tom
S. Woodstock, CT

 

[Gary Schafer]

"Changing the height of the antenna above the ground will change the
current flow assuming that the power to the antenna is constant."

Again, it would appear that the "ground plane/wave" is not
insignificant.

Now, as I understand it, at VHF frequencies, the methodology of
providing energy to the antenna (loading/feed) is not as important to
the generation/reception of the signal as is height. In fact, if I
read the pattern charts correctly, the height of the antenna has more
to do with the lobe pattern (the donut you were discussing) than the
method of feeding the antenna.

Yes/No?

Later,

Tom
S. Woodstock, CT

The method of feeding the antenna has nothing to do with the take off
angle of an antenna.

The stuff you are reading in the handbook about ground effect on
vertical angle are discussing HF antennas where due to the low height
compared to wavelength and the propagation method at hf the ground has
a great effect.

An antenna at VHF is usually several wavelengths above ground and
ground has little effect on take off angle. Given a 1/4 wave ground
plane antenna with radials at 90 degrees, the ground plane of that
antenna will be the controlling factor. It does tilt the angle up a
little from what a 1/2 wave dipole would be.

If you take the ground radials and bend them down and folded over the
coax feed line so that they are vertical you then have a half wave
center fed vertical dipole antenna.
This particular version would have the coax going up through the
center of the folded down ground radials. A tube or pipe could be
substituted for the folded over ground radials. In most cases that is
exactly what is done. A metal tube is used in place of the radials.
This is commonly referred to as a "coaxial sleeve antenna". Just about
all of the fiberglass type VHF antennas have some form of this type of
ground plane in them.

Another type of 1/2 wave vertical antenna that does not need a ground
plane is one that is fed at the end of the 1/2 wave length rather than
at the center as above. The METZ type half wave is one such type. It
uses a coil at the feed end to transform the low impedance of the coax
to the high impedance feed end of the antenna.
At a high impedance feed point the current is much less than it is at
a low impedance feed point (center of 1/2 wave) so not much of a
ground plane is needed. In this case the coax shield acts as the
ground plane for the 1/2 wave antenna. It is not the best ground plane
but again not much is needed in this case. The high current point is
in the center (quarter wave point) of the antenna.

Once you get a few wavelengths above ground additional height does not
much effect the radiation lobes (pattern) of the vhf antenna. But
height does effect the line of sight or in this case radio line of
sight, (which is slightly greater than visual).

What Larry is talking about with the donut shaped vertical pattern is
a result of gain in the antenna. The higher the gain the flatter
(sharper) the vertical pattern of the antenna. Any tilt of the high
gain antenna will raise the pattern above the horizon or tilt it into
the sea. It will not be at the horizon where it does most good.

A lower gain antenna, like the 1/2 wave, has a much fatter pattern.
Like a fat doughnut or a ball. Tilting it one way or the other still
maintains about the same amount of radiation at the horizon. This is
because it normally radiates in a wide vertical pattern. With a wide
vertical pattern a great portion of the signal is wasted as it gets
radiated at high and low angles that are not useful. But the advantage
is that it can be moved (tilted) a long way and still maintain about
the same amount of useful signal.

To get gain, a gain antenna narrows the wide vertical pattern. It robs
some of the power normally radiated at high and low angles and places
more of it at the horizon. Thus the narrower vertical pattern.
The advantage is a stronger signal. The disadvantage is that if it is
tilted very far the signal drops off sharply because the vertical
angle is very narrow.

Regards
Gary

 

[Chuck Tribolet]

Larry, nobody's suggesting 9 db antennas. They are for shore stations
and real ships. 6 db, on the other hand, seems to work fine on my 17'
boat. I talk to dive buddies who are theoretically out of range, and
they come in fine, with no fade in/out as the boat rocks.

--
Chuck Tribolet
[email protected]
http://www.almaden.ibm.com/cs/people/triblet

Silicon Valley: STILL the best day job in the world.

 

[Bruce in Alaska]

Now, as I understand it, at VHF frequencies, the methodology of
providing energy to the antenna (loading/feed) is not as important to
the generation/reception of the signal as is height. In fact, if I
read the pattern charts correctly, the height of the antenna has more
to do with the lobe pattern (the donut you were discussing) than the
method of feeding the antenna.

Yes/No?

Later,

Tom
S. Woodstock, CT

snipped

Regards
Gary[/QUOTE]

As always a very excelent explanation, Gary.

Bruce in alaska

 

[Shortwave Sportfishing]

The method of feeding the antenna has nothing to do with the take off
angle of an antenna.

The stuff you are reading in the handbook about ground effect on
vertical angle are discussing HF antennas where due to the low height
compared to wavelength and the propagation method at hf the ground has
a great effect.

An antenna at VHF is usually several wavelengths above ground and
ground has little effect on take off angle. Given a 1/4 wave ground
plane antenna with radials at 90 degrees, the ground plane of that
antenna will be the controlling factor. It does tilt the angle up a
little from what a 1/2 wave dipole would be.

If you take the ground radials and bend them down and folded over the
coax feed line so that they are vertical you then have a half wave
center fed vertical dipole antenna.
This particular version would have the coax going up through the
center of the folded down ground radials. A tube or pipe could be
substituted for the folded over ground radials. In most cases that is
exactly what is done. A metal tube is used in place of the radials.
This is commonly referred to as a "coaxial sleeve antenna". Just about
all of the fiberglass type VHF antennas have some form of this type of
ground plane in them.

Another type of 1/2 wave vertical antenna that does not need a ground
plane is one that is fed at the end of the 1/2 wave length rather than
at the center as above. The METZ type half wave is one such type. It
uses a coil at the feed end to transform the low impedance of the coax
to the high impedance feed end of the antenna.
At a high impedance feed point the current is much less than it is at
a low impedance feed point (center of 1/2 wave) so not much of a
ground plane is needed. In this case the coax shield acts as the
ground plane for the 1/2 wave antenna. It is not the best ground plane
but again not much is needed in this case. The high current point is
in the center (quarter wave point) of the antenna.

Once you get a few wavelengths above ground additional height does not
much effect the radiation lobes (pattern) of the vhf antenna. But
height does effect the line of sight or in this case radio line of
sight, (which is slightly greater than visual).

What Larry is talking about with the donut shaped vertical pattern is
a result of gain in the antenna. The higher the gain the flatter
(sharper) the vertical pattern of the antenna. Any tilt of the high
gain antenna will raise the pattern above the horizon or tilt it into
the sea. It will not be at the horizon where it does most good.

A lower gain antenna, like the 1/2 wave, has a much fatter pattern.
Like a fat doughnut or a ball. Tilting it one way or the other still
maintains about the same amount of radiation at the horizon. This is
because it normally radiates in a wide vertical pattern. With a wide
vertical pattern a great portion of the signal is wasted as it gets
radiated at high and low angles that are not useful. But the advantage
is that it can be moved (tilted) a long way and still maintain about
the same amount of useful signal.

To get gain, a gain antenna narrows the wide vertical pattern. It robs
some of the power normally radiated at high and low angles and places
more of it at the horizon. Thus the narrower vertical pattern.
The advantage is a stronger signal. The disadvantage is that if it is
tilted very far the signal drops off sharply because the vertical
angle is very narrow.

Excellant Gary - that's what I was trying to get at by taking some
leading questions and seeing where the discussion went. There were
some details that I think were confusing and I just wanted to clear
some of it up without it sounding like I was a newbie big shot trying
to take over the subject.

Nice job.

Regards,

Tom, NM1Q
S. Woodstock, CT

 

[Gary Schafer]

All antennas are two terminal devices.
There is no such thing as a single point feed antenna.

Changing a horizontal antenna to a vertical antenna at the same height
does not improve the radiation angle.
If it did everyone would have their horizontal antennas in the
vertical position.

AM radio stations depend on ground wave signals not sky wave.

A horizontal antenna theoretically does not have a ground wave so
verticals are used for AM stations as they produce a ground wave
signal.

For sky wave signals height is important in order to produce better
lower angle of radiation. Lower angle radiation provides longer hops.

Adjusting the height to adjust the impedance of the antenna is not to
be worried about. That's what matching systems are for.
With multielement antennas (beams) the feed point impedance can be
very low even if the antenna is high in the air. Some other means of
matching the antenna to the line is required.

Regards
Gary

 

[Gary Schafer]

Hmm....I've been using horizontal antennas to transmit ground wave
communications since I was 11 years old in 1957!

All VHF and UHF TV stations use ground wave only signals and every one
of them in the USA are HORIZONTALLY POLARIZED. Kinda blows that
theory all to hell, doesn't it? Until very recently, all FM radio
stations were all horizontally polarized, too, but that was changed
because cars have vertically polarized antennas......or did when they
changed the rules. Embedded FM antennas in windshields are
horizontally polarized, a dipole.

Depends on how far you wish to talk. I use very high angles of
radiation on 3915 Khz to talk to my buddies around SC, NC and Georgia
on 75 meters. This is easily possible because my inverted-V dipole is
only up about 30', not much % of a wavelength when the wavelength is
240 feet long. Listen from 3.5 to 4.0 Mhz nights and 7.0-7.3 Mhz days
and hear lots of us "high-angle-radiators" shooting the breeze, ad
nauseum. Works great since 1957....(c;

Absolutely nothing radiates like a TUNED antenna, without the lossy
tuner between feedline and antenna. Damned Navy has tuners so
inefficient trying to load a flagpole whip they have to have BLOWERS
in them to keep from melting them. All that power ISN'T radiated,
obviously.

TV and FM stations do not depend on ground wave propagation. It is
line of sight. Nothing to do with polarization or ground wave.

FM stations have had dual polarization (both horizontal and vertical)
for many years. Only because cross polarization with the receiving
antenna greatly attenuates the signal.

Ground wave propagation as in AM stations is not a sky wave. It
follows the curvature of the earth staying close to the ground (ground
wave).

A horizontal antenna does not produce any significant ground wave.
(wave that stays close to the earth) The ground wave from a horizontal
antenna will fall off in just a few miles.

Yup that's what I said. "For sky wave signals height is important in
order to produce better lower angle of radiation. Lower angle
radiation provides longer hops."

The propagation that you experience on 80 and 40 meters with the guys
around the state is the result of high angle radiation and reflections
as you state.

If you want higher angle, shorter hops, keep the antenna low.


Tuned antenna? I assume you are referring to a "resonant antenna"?

A non resonant antenna radiates just as well as a resonant antenna.
Only difference is the impedance presented to the feed line. But even
a resonant antenna may not provide 50 ohms to your coax. Depends on
how high it is. :>)

Regards
Gary

 

[Shortwave Sportfishing]

TV and FM stations do not depend on ground wave propagation. It is
line of sight. Nothing to do with polarization or ground wave.

Gary, maybe you can help me out with something here, because either
I've forgotten or I was taught incorrectly.

Over short distances (Up to five to eight miles), the ground wave
component of any VHF signal is actually the primary method by which
VHF signals are received and not by Line Of Sight.

Not true?

Later,

Tom
S. Woodstock, CT

 

[Gary Schafer]

Gary, maybe you can help me out with something here, because either
I've forgotten or I was taught incorrectly.

Over short distances (Up to five to eight miles), the ground wave
component of any VHF signal is actually the primary method by which
VHF signals are received and not by Line Of Sight.

Not true?

Later,

Tom
S. Woodstock, CT

I don't think so. There is some bending, refraction of the signal that
makes the VHF radio horizon slightly longer than line of sight but it
is due to the atmosphere and not ground wave propagation.

If you have a VHF station on a high mountain peak or a very high tower
there could be no ground effect at all.

Regards
Gary

 

[Shortwave Sportfishing]

I don't think so. There is some bending, refraction of the signal that
makes the VHF radio horizon slightly longer than line of sight but it
is due to the atmosphere and not ground wave propagation.

If you have a VHF station on a high mountain peak or a very high tower
there could be no ground effect at all.

You are making me unpack my reference books now. :>)

Later,

Tom
S. Woodstock, CT

 

[Gary Schafer]

Some clarification of "ground wave" is in order here. The term ground
wave is sometimes lumped into several different modes that are not
"sky wave" modes.

SURFACE WAVES:

The ground wave that I am referring to here is sometimes called
"surface wave". It is in the ground wave family.
A surface wave travels close to the ground actually touching the
ground. AM radio stations depend on this method of propagation. At AM
frequencies it is good out to about 100 miles. The signal strength is
quite weak at that distance. But in the shorter distances it provides
a very reliable and predictable constant coverage. Unlike sky wave
signals that come and go between night and day.

At higher frequencies that distance is much shorter as the signal gets
highly attenuated. So "surface waves" are not very useful at
frequencies much above the broadcast band for any significant
distance.

The old 2 MHZ marine band used to depend on "surface wave"
communication. With good conditions range can be better than VHF but
usually quite noisy on those frequencies. Also a more complicated
antenna system was needed and higher power than what is used on VHF.

DIRECT WAVES:

Line of sight propagation is also a form of ground wave propagation in
that it does not become a sky wave and reflect off the upper
atmosphere. It may follow the earths curvature and may or may not
touch the earth. This type of propagation is commonly called "direct
wave".

As you asked about "the ground wave component of VHF signals being the
primary method by which VHF signals are received and not by line of
sight".

The primary method is by line of sight which is technically a form of
ground wave as explained above. But usually when someone mentions
"ground wave" they are usually referring to "surface wave".
Line of sight is usually used when referring to "direct wave" VHF
communications.


In line of sight communications there are reflections from the earth
or other objects that can add or subtract to the signal that is being
received directly. The reflected signal gets shifted 180 degrees as it
is reflected. At VHF the reflected wave (because it is rather short
compared to the distances involved) can shift rapidly in phase. This
is what causes flutter on the signal at times.

With horizontal polarization at low HF the wave length is long
compared to the distances involved in the direct and ground reflected
waves. They arrive almost always 180 degrees out of phase. That highly
attenuates the direct wave signal.

NO SURFACE WAVE FROM HORIZONTAL ANTENNAS: from ARRL antenna handbook.

With a vertical antenna the electric field is perpendicular to the
ground. It is a law of electromagnetics that electric lines must touch
a conductor (the earth in this case) perpendicular or else they would
have to generate infinite currents in the conductor. The earth is a
rather good conductor below about 10 mhz so waves at these frequencies
are mainly vertical when traveling close to the ground. If they were
horizontal they would try to induce large currents and be absorbed by
the earth. They get shorted out.

This is why a horizontal antenna has little ground wave or surface
wave. It dies out very rapidly. In addition horizontally polarized
direct waves at low HF get canceled by ground reflections. (see above)

Thus the reason for vertical AM broadcast antennas.

At VHF antenna heights are usually several wavelengths above ground
and polarization does not matter. Plus the earth is not a good
conductor as to short out the horizontal waves as it does at low HF.

The reason for vertical polarization being mostly used at VHF is it is
much easier to obtain omni coverage with simpler antennas. Especially
portable and mobile antennas.

Regards
Gary

 

[Short Wave Sportfishing]

Some clarification of "ground wave" is in order here. The term ground
wave is sometimes lumped into several different modes that are not
"sky wave" modes.

SURFACE WAVES:

The ground wave that I am referring to here is sometimes called
"surface wave". It is in the ground wave family.
A surface wave travels close to the ground actually touching the
ground. AM radio stations depend on this method of propagation. At AM
frequencies it is good out to about 100 miles. The signal strength is
quite weak at that distance. But in the shorter distances it provides
a very reliable and predictable constant coverage. Unlike sky wave
signals that come and go between night and day.

At higher frequencies that distance is much shorter as the signal gets
highly attenuated. So "surface waves" are not very useful at
frequencies much above the broadcast band for any significant
distance.

The old 2 MHZ marine band used to depend on "surface wave"
communication. With good conditions range can be better than VHF but
usually quite noisy on those frequencies. Also a more complicated
antenna system was needed and higher power than what is used on VHF.

DIRECT WAVES:

Line of sight propagation is also a form of ground wave propagation in
that it does not become a sky wave and reflect off the upper
atmosphere. It may follow the earths curvature and may or may not
touch the earth. This type of propagation is commonly called "direct
wave".

As you asked about "the ground wave component of VHF signals being the
primary method by which VHF signals are received and not by line of
sight".

The primary method is by line of sight which is technically a form of
ground wave as explained above. But usually when someone mentions
"ground wave" they are usually referring to "surface wave".
Line of sight is usually used when referring to "direct wave" VHF
communications.

~~ snippage ~~

Thank you. :>)

Later,

Tom
S. Woodstock, CT