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Author Topic: why no-gain antennas?  (Read 993 times)
KE5BCG
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Posts: 39




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« on: June 19, 2004, 12:31:10 AM »

I'm a newly frocked technician, so pardon my ignorance. I'm looking forward to some antenna building projects, and wonder why there are no-gain antennas in use? You want max gain within the limits of space or whatever, right? It doesn't seem to make sense.
73
Pete
KE5BCG
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W8CAR
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Posts: 109


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« Reply #1 on: June 19, 2004, 01:51:34 AM »

You have to remember that in reality every antenna must be compared to something else. A unity gain antenna is usually a basis for comparison. Why would you want one? For several reasons. Maybe you want coverage over a 360 degree area (like mobile or portable) so a small low or no gain antenna is a good bet. Other factors come into play. Low or no gain antennas are simple to construct and use while gain antennas usually have bandwidth, physical construction ($) and feed considerations. My 80 meter dipole at 40 feet is pretty omnidirectional and has a high angle of radiation- but that is just what I want for morning ragchews with stations within a few hundred miles. For DX I use my 80 meter vertical that has basically no gain BUT it radiates at a very low angle for DX AND it is a pretty crummy antenna for local stuff compared to the 80 meter dipole. If you look at the uses of antennas that determines what type of gain, polarization (horizontal or vertical) and height above ground. I have a multi band vertical that I use to see which way 20,15 or 10 is open too and then I switch to the beam for the gain in the direction I want. Pick up a copy of the ARRL antenna book and it gives several good chapters of explanations of how antennas function.

In general remember, if your antenna stays up -it's too small, higher is better and you should always put antennas up in bad weather. (okay that last part is tongue in cheek) Good luck in you ham career.

Dan W8CAR
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KC0ARF
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Posts: 1




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« Reply #2 on: June 19, 2004, 02:41:59 AM »

Hello,

Welcome to Ham Radio.  It is true that you will learn a lot more about the practical art of ham radio after you receive your license.

Another point to remember about high-gain antennas is that they may sacrifice parts of the spectrum in order to favor other parts.

I have a gain antenna for 2m / 70cm that works really well for that band, but the coils in the antenna filter out signals on HF (for simple listening -- remember you do not need a gain antenna to listen).  If I remove that antenna, and place a "unity" 2/70 antenna on the mount, HF / Short wave signals come in weak, but are not tuned/filtered out.

As a ham, you will collect a lot of antennas for mobile and fixed use.  Don't sell them!  You might find yourself needing it in the future.  Also, standardize quickly on the mobile ones if you would like NMO or PL-259 style mounts.  

Enjoy!

Christian KC0ARF
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WB8JKR
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Posts: 45




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« Reply #3 on: June 19, 2004, 08:12:08 AM »


 Actually, the only "no-gain" antenna would be an isotropic model. Unless an antenna radiates equally
in ALL diections, its going to have loss or gain in
one direction or another.

Mark  WB8JKR
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W2NSF
Member

Posts: 122




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« Reply #4 on: June 19, 2004, 08:33:28 AM »

Congratulations and welcome!
I think Dan (W8CAR) said it all very well and very accurately.
I think the most important thing he said was that you should chose your antenna (if you have the luxury of having more than one) based on your communications needs.
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NI0C
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Posts: 2422




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« Reply #5 on: June 19, 2004, 08:58:51 AM »

Recall that antenna size is proportional to wavelength (inversely proportional to frequency).  Thus high-gain yagis are routine for VHF/UHF whereas extremely difficult for the bands below (in frequency) 20 meters.  Other types of antennas, such as fixed wire beams and vertical arrays are employed on the lower frequency bands by those who have the real estate.  Those of us who live in urban or suburban areas are usually lucky to be able to deploy a dipole or single vertical antenna for HF.

73 de Chuck  NI0C
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AA4PB
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Posts: 12990




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« Reply #6 on: June 19, 2004, 11:01:43 AM »

Remember too that antenna gain is a relative thing. It must be compared to some other antenna to mean anything. If some manufacturer says their antenna has 3dB gain and doesn't tell you 3dB over what then their number is meaningless.

Sometimes mfgs use "isotropic" as a reference. Isotropic is an imaginary antenna the radiates signal equally in every direction around a point in space. A dipole or 1/4-vertical has about 2dB gain over isotropic. Sometimes mfgs use a dipole or 1/4-wave vertical as their reference. You can see that an antenna could be specified as having say 3dB gain or 5dB gain, depending on the reference used. Let the buyer beware!

With the exception of very lossy antennas like a 9-foot mobile whip on 75M, the only way to get gain from an antenna is to take energy from one direction and concentrate it in another direction. It's like a flashlight that is very bright in one direction. If you take the bulb out of the reflector and let it radiate in all directions then it is not nearly so bright in any one direction.
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X-WB1AUW
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Posts: 559




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« Reply #7 on: June 19, 2004, 02:28:36 PM »

ARRL Antenna book, and ON4UN's book, "Low Band Dxing" will fill in the missing information, as will a read thru of Cebik's site.

73
Bob
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K1CJS
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Posts: 6055




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« Reply #8 on: June 19, 2004, 08:47:48 PM »

Another reason for the use of a unity gain antenna is the question of range.  I note that you have a Tech class license.  On 2 meters, here in New England, a gain antenna will increase the range of a rig to the point that if you are trying to use a local repeater, you may also be getting into a repeater some distance away.  More is not always better.  May I suggest both antennae and a switch to change between them depending on how much range you need?  If possible for your budget, that is.  That way you won't be interfering with any other repeater if you are trying to use your local repeater, and if you're going simplex, you'll have the greater range you need.

Good luck in this most satisfying hobby.  One last thing, HAVE FUN!!
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AA4PB
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Posts: 12990




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« Reply #9 on: June 19, 2004, 09:28:46 PM »

Of course you can also limit your range by reducing the transmitter power ouput. It's not good to run 75 watts to access your local repeater when 5 watts is full quieting.
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N6AJR
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Posts: 9921




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« Reply #10 on: June 19, 2004, 10:43:54 PM »

sometimes it is more important to match an antenna than to have gain...  check this out

http://www.hamuniverse.com/multidipole.html
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WB6BYU
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Posts: 13486




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« Reply #11 on: June 21, 2004, 06:13:37 PM »

As the others have said, gain is relative to something,
often an isotropic antenna or dipole.  (Except, of course,
the gains quoted my some antenna manufacturers in their
sales literature - this is often unrelated to the laws
of physics.)

If you are using a dipole antenna, and are comparing
it with a dipole reference, it will have no gain, or
0dBd.  But the dipole is quite an adequate antenna in
many circumstances, both on HF and VHF.  Many antennas
are not even as good as a dipole, but are still used
regularly to make contacts.

There are two practical points to consider (which have
been touched on in previous posts):  directivity and
size.  The only way you can get gain over a dipole is
to compress the radiation pattern so more power goes
in some directions and less goes in others.  A long
beam has a lot of gain, but you will have to point it
in the direction of the station/repeater  you are working.
Simplex conversations among several stations become
inconvenient if you have to move the beam between each
over.

And, yes, there are antennas that provide omnidirectional
gain - but that brings us to the second point:  size.
Generally speaking, the higher the gain the larger the
antenna.  That is another one of those laws of physics.
Added size often also means more weight and wind loading,
requiring a more sturdy mount.  This all increases cost.

For 2m I have an omnidirectional antenna with 6db of
gain over a dipole, and a beam with 10dBd gain.  They
are sitting unused in the garage.  My primary base
antennas are a 3dBd vertical on a TV push-up mast for
the radio on my desk, and a 0dBd dipole in the barn
attic for the radio at my workbench.  Either can hit
all the local repeaters - and some 60 miles away - on
5 watts, so I don't need anything bigger for my personal
operating preferences.  Besides, the beam would require
me to install a rotator and control box, and wouldn't
fit in the barn rafters.  The omnidirectional antenna
is 20' tall (you can't get omnidirectional gain without
that kind of height) and would require a much heavier
support to mount it than the TV push-up mast.  (Did I
mention we have a lot of wind?)

On HF I have a rotatable dipole.  Actually it is the
driven element of a 3-element yagi that clearly would
have more gain if I installed the other elements as
well.  But then I'd have to hook up the rotator and
control box, and point it in the direction I am looking
for contacts.  With the dipole I don't have to rotate
it to make contacts in many directions (though I do
have a rope tied to it so I can change the direction
from the ground if needed.)  Yes, some day maybe I'll
put up a big yagi - but first I have to get the tower
bracketed to the barn so it doesn't blow over.  (Did I
mention we have a lot of wind?)


Every antenna installation is a compromise between
performance, convenience, cost, space, etc.  It is NOT
true that we always want maximum gain:  we want to be
able to communicate over one or more paths.  Certainly
there are many paths that require high gain antennas at
each end - along with high power and a good receiver -
to maintain communications.  On the other hand, if a
rubber duck antenna on an HT provides adequate coverage
through the local repeater, it doesn't make sense to
use a bigger antenna just because it has higher gain.
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