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Author Topic: Highest Gain Mid-VHF Omni Design?  (Read 1186 times)
KC2NLT
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Posts: 92




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« on: November 04, 2017, 01:42:23 PM »

I realize that with higher gain, the directivity of a regular antenna will usually go up, meaning that there is an inverse relationship.

But, is there a way to get the best omnidirectional capabilities while pushing the gain higher without sacrificing directivity?

Ideally, I'm looking for a theoretical 360 horizontal coverage and 180 vertical coverage (from the horizon and up).

I should mention that the signal is vertically polarized.

What kind of antenna design would work best?

If a single element/antenna is not enough, can this be achieved using some type of array?

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K1ZJH
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Posts: 3308




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« Reply #1 on: November 04, 2017, 01:57:29 PM »

It depends on the antenna's vertical length.

For vertically polarized coverage, 360 degrees, at best you will get around 5.8 dB
using a 20 foot long vertical array.  Common configurations are either vertical
colinear arrays or an array of folded dipoles. Gain is achieved by compressing
the radiation pattern into a flattened donut shape.

Expect to pay close to a grand for a commercial quality antenna cut for the
 ham band.  Otherwise one of the imports will probably work,
but remember most of the gain figures they show are pure hogwash.
A ten dB gain omni vertical takes up almost 40 feet of tower space at 2 meters.

Pete
 
« Last Edit: November 04, 2017, 01:59:37 PM by K1ZJH » Logged
KC2NLT
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Posts: 92




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« Reply #2 on: November 04, 2017, 02:13:54 PM »

I'm not trying to be funny, but a donut shaped radiation pattern is going to have a hole in the middle, so that would basically be a null?

Just to reiterate, ideally a radiation pattern that comes close to a dome shape would be great http://www.structuralice.com/images/01_pykrete%20dome.jpg
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KB1GMX
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Posts: 1496




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« Reply #3 on: November 04, 2017, 02:22:10 PM »

around 1.75db...  or less.

That would be for perfect hemispherical coverage.  A full 360 azimuth with 0 to 90 degree elevation
is not easy to achieve with simple vertical of any kind.   There is a null off the end.

FYI a 5.8db vertical would be typically colinear for vhf.   Also the stated gain is achieved by
directivity in the vertical plane, so high angles would be lost.  Antenna gain is not an amplifier,
it's simply focusing the total energy in a direction or plane that is deemed more useful.  Since
its focused there will be no or little energy in other directions.


Allison
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K1ZJH
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« Reply #4 on: November 04, 2017, 02:29:20 PM »

I'm not trying to be funny, but a donut shaped radiation pattern is going to have a hole in the middle, so that would basically be a null?

Just to reiterate, ideally a radiation pattern that comes close to a dome shape would be great http://www.structuralice.com/images/01_pykrete%20dome.jpg

We are not talking ground mounted verticals.  The pattern should be donut shaped for an omnidirectional gain vertically polarized antenna.

https://d1jiktx90t87hr.cloudfront.net/185/wp-content/uploads/2017/06/ANT400F2-Omnidirectional-Collinear-Antenna-360-455MHz-PATTERN.jpg

https://www.cisco.com/c/dam/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.doc/_jcr_content/renditions/0900aecd806a1a3e_null_null_null_08_07_07-05.jpg
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G8HQP
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« Reply #5 on: November 04, 2017, 03:25:57 PM »

Quote from: KC2NLT
I realize that with higher gain, the directivity of a regular antenna will usually go up, meaning that there is an inverse relationship.

But, is there a way to get the best omnidirectional capabilities while pushing the gain higher without sacrificing directivity?
No. The only way to get more power in one direction is to have less power in some other direction. That is, with higher gain the directivity does not "usually go up"; it must necessarily always go up.
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K1ZJH
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« Reply #6 on: November 04, 2017, 03:51:59 PM »

The gain comes by maximizing radiation to the horizon.
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KC2NLT
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Posts: 92




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« Reply #7 on: November 04, 2017, 04:26:14 PM »

So something like this, can't be achieved in VHF?

http://www.antennamagus.com/images/Newsletter4-2/conical_sinuous_radiation_patterns.png

How about this?

http://files.maartenbaert.be/quadcopters/pagoda2-rad-3d-l80-wp-30db.png?
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K1ZJH
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« Reply #8 on: November 04, 2017, 04:41:08 PM »


That is a very low gain antenna.  As gain is increased that fat donut pattern would flatten and expand more towards the horizon.  A very high gain antenna would look like a pancake pattern.
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KB1GMX
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Posts: 1496




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« Reply #9 on: November 04, 2017, 04:48:20 PM »

>>>So something like this, can't be achieved in VHF?

http://www.antennamagus.com/images/Newsletter4-2/conical_sinuous_radiation_patterns.png<<<<

That would be a vertical dipole, plenty on the horizon, and nothing ove head.

>>>How about this?

http://files.maartenbaert.be/quadcopters/pagoda2-rad-3d-l80-wp-30db.png?<<<

A conical quadrature antenna.  Doable but low gain and also low gain on the horizon, good overhead.


Allison
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WB6BYU
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Posts: 17061




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« Reply #10 on: November 04, 2017, 05:22:01 PM »

Here's the basic principle:  there is a direct correlation between the beamwidth of the antenna
and the gain.  Imagine if you have a beam antenna with a beam that is 30 degrees wide in azimuth
and 25 degrees in elevation.  That's 750 square degrees, and, assuming 100% efficiency, would
correspond to a particular gain over isotropic (which sprays power evenly in every direction.)  The
more "square degrees" in the pattern, the wider it is, and the lower the antenna gain.

With a hemispherical pattern, that's half of "everywhere", which is how Allison came up with the number
of 1.75 dBi (or less).  By specifying the beamwidth of the pattern, you have specified the maximum
possible gain.


In order to increase the gain you have to narrow the pattern somewhere.  Most "omnidirectional gain
antennas" do this by assuming that overhead radiation is useless, so they focus all your power only
at the horizon.  That works well for a lot of terrestial work, though sometimes if you are on a mountain
top you actually have to aim the pattern downwards to cover nearby stations.

So if you want high gain, you have to limit the beamwidth of the pattern  If you want a wide pattern,
you have to live with lower gain.
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N8EKT
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Posts: 588




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« Reply #11 on: November 04, 2017, 05:33:04 PM »

For terrestrial communications, any rf above the horizon is wasted rf if you are talking about a repeater antenna high on a hill.
But it may be of some advantage if you are a valley dweller trying to use that repeater on the hill.
Your second example appears to show the pattern produced by a right hand circular polarized directional antenna pointed toward the sky which would be a poor antenna for terrestrial communications.
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W9IQ
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Posts: 1706




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« Reply #12 on: November 04, 2017, 06:12:19 PM »

A couple of quick points.

Gain is not strictly due to directivity. Gain is also dependent on efficiency (Gain = Directivity * Efficiency). Some coaxial cable collinear designs do not have great efficiency so gain is reduced as a result (although in many cases it is due to improper phasing/separation of the collinear elements).

If an antenna is 100% efficient and it radiates in a perfect hemisphere, it has a 3 dBi gain. This should be somewhat intuitive since all power that would have been in the other hemisphere in the case of an isotropic antenna is now in the other hemisphere. As a result, the power is doubled. However, a 1/4 wave ground plane antenna, well removed from ground effects, will have a 1.25 dBi gain as it will still radiate equally well below the ground plane.

Increasing the gain of a vertical antenna does not ensure the gain is aimed at the horizon nor does it ensure a single lobe. For example, commercial antennas can be ordered with up-tilt or down-tilt relative to the horizon. This is typically doen by adjusting element phasing.

- Glenn W9IQ
« Last Edit: November 04, 2017, 06:23:30 PM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KC2NLT
Member

Posts: 92




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« Reply #13 on: November 04, 2017, 10:09:14 PM »

Thanks for all the input. I now understand the theory and the limitations involved.

So it looks like gain will have to be about 1.5dBi.

Am I building a 1/4 wave ground plane antenna, or a collinear antenna?
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WB6BYU
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Posts: 17061




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« Reply #14 on: November 05, 2017, 07:28:35 AM »

Neither, if you want to avoid the null directly overhead.

Though mobile whips that may need to talk to aircraft often have a bend in
them to help fill in the null.  It still isn't as strong there as in other directions,
but an overhead aircraft should be a direct line-of-sight path so high gain
isn't needed.  (Note that "equal signal in all directions" is not the same as
"equal signal strength to stations in all directions", because (assuming a
reasonable altitude limit) stations close to the horizon are often further
away than those overhead.  This is particularly true for working satellites.)


A ground plane or a vertical dipole will give a reasonable approximation of
your desired pattern if you don't care about the overhead null.  A collinear
array is 2 or more such antennas stacked vertically, which increases the
gain at the horizon at the expense of higher radiation angles.  For coverage
at high vertical angles, the simple dipole or ground plane is probably
better, but it depends on your specific application.
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