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Author Topic: Low-Angle Monopole Radiation vs. Earth Conductivity  (Read 1737 times)
RFRY
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« on: November 29, 2012, 04:08:15 AM »

After reading comments in various threads here...

Popular beliefs about monopoles are that their low-angle radiation is a function of earth conductivity, and that poorer conductivities result in higher "takeoff angles."

But when considering the fields actually launched by a vertical monopole of 5/8WL and less, maximum field always occurs in the horizontal plane regardless of earth conductivity at and near the monopole installation site.  Other things equal, better earth conductivity produces greater radiated fields, but has little affect on the shape of elevation pattern itself.

These low-angle fields will reach the ionosphere, and under the right conditions produce the greatest single-hop skywave range.

The link below illustrates this performance, using NEC4.2.

http://i62.photobucket.com/albums/h85/rfry-100/Monopole_Low_Angle_Radiation.jpg
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STAYVERTICAL
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« Reply #1 on: November 29, 2012, 07:45:56 PM »

Very interesting, a picture saves a thousand words.

Thanks for sharing your knowledge.

73 - Rob
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WX7G
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« Reply #2 on: November 30, 2012, 07:05:51 AM »

Now repeat with measures further out and your results will change. This was recently discussed at length on the top band reflector.
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RFRY
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« Reply #3 on: November 30, 2012, 08:53:24 AM »

"Now repeat with measures further out and your results will change."

The fields at and just above the surface of the earth (the ground wave) will be reduced as radiation travels further from the monopole. Beyond some horizontal distance from the monopole, such losses mean that the field in the horizontal plane no longer will be the greatest field in the elevation pattern of the radiator.  However this doesn't mean that those reduced fields resulted from the original elevation pattern of the monopole.

The radiation directed toward the low angles 0.1 km downrange in the linked NEC4 analysis remains as shown there. For example, the fields for an elevation angle of 5.7o would continue to propagate essentially along that radial path as space waves, until they reach the ionosphere.  The value of the groundwave further downrange has no affect on this.

These low-angle space waves will lose field intensity at a 1/r rate, but that is much less than the groundwave loses due to propagation along a terrestrial path (and earth curvature).
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W5WSS
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« Reply #4 on: November 30, 2012, 09:21:07 AM »

A vertical does however develop more than one lobe as the base height increase above Earth. Therefore power manifests in the lobes and a sum of the lobes and power manifestation occur simultaneously with skywave variation.

We can see the effects of more major and more minor lobes adding and subtracting from the sum of the total power envelope relative to refraction reflection and diffraction out and away from the antenna site downrange.
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W5WSS
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« Reply #5 on: November 30, 2012, 09:22:38 AM »

As an adjunct to my previous posting,Often times these effects are misconstrued as ground conductivity
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RFRY
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« Reply #6 on: November 30, 2012, 09:45:23 AM »

A vertical does however develop more than one lobe as the base height increase above Earth. Therefore power manifests in the lobes and a sum of the lobes and power manifestation occur simultaneously with skywave variation.

The link below leads to the elevation patterns "launched" by several monopoles from 90o to 225o in height.  Once the height exceeds 180o a high-angle lobe begins to develop, however the greatest field generated by all of these heights (and those below 90o as well) always occurs in the horizontal plane.

http://i62.photobucket.com/albums/h85/rfry-100/MWElPatComparison.jpg
« Last Edit: November 30, 2012, 09:48:18 AM by RFRY » Logged
W5WSS
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« Reply #7 on: November 30, 2012, 09:58:06 AM »

yes and is why a better explanation supporting the reinforcing effects as expressed as radiation increase out and downrange from a vertical that incorporates sloping elevated radials adds to the sum of Field strength somewhere in the pattern.

As compared to a vertical elevated ground plane where the 4 radials are horizontal they cancel each other and can not contribute to Field strength like sloping radials can.

Your explanation helps to support why that is the case.

We add radiation when the radials are sloped they are then proactive as opposed to just being a launch and feed point impedance found in purely horizontal radials and the singular gain expressed in dbd of a 1/4 wave with no help.

The slope angle of the radials that radiate and the vertical angle of the 1/4 monopole combine downrange and your graph shows why it is happening.73
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RFRY
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« Reply #8 on: November 30, 2012, 04:58:36 PM »

...The slope angle of the radials that radiate and the vertical angle of the 1/4 monopole combine downrange and your graph shows why it is happening.73

Thanks, but just to note that my plots included no far-field radiation applicable to sloping, elevated radials.

My plots are based on the use of 120 x 1/4WL, buried, symmetrically-arrayed horizontal radials.  The far-field, v-pol radiation directly produced by such a radial system is zero.
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W5DXP
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« Reply #9 on: December 02, 2012, 06:21:58 AM »

Could this question be resolved by performing A/B tests on the vertical vs e.g. a 1/2WL dipole at 1/2WL height (in the dipole's favored direction)? Would the NEC2 vs NEC4 discrepancy show up on the longest paths?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
RFRY
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« Reply #10 on: December 02, 2012, 08:20:21 AM »

Could this question be resolved by performing A/B tests on the vertical vs e.g. a 1/2WL dipole at 1/2WL height (in the dipole's favored direction)?

Probably better to compare two radiators not having such dissimilar polarization and radiation patterns (including the earth reflection).

But in any case there is no question that the elevation fields shown in the linked NEC comparison exist, and that the greatest field intensity launched by a vertical monopole of 5/8WL and less always occurs in the horizontal plane.

Would the NEC2 vs NEC4 discrepancy show up on the longest paths?

NEC2 shows the same effects for the conditions stated in that link as when using NEC4.  However the NEC2 model could not use buried conductors.  In NEC2 the monopole would have to be elevated a meter or so above the earth, and driven against a counterpoise of 6 or 8 horizontal, 1/4-wave, elevated wires.

Not all NEC software supports the full set of NEC engine "cards," and includes the graphic instructions/outputs needed to produce the data shown in that link.

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PA1ZP
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« Reply #11 on: December 03, 2012, 01:44:00 PM »

Hi All

I have and have had and build many verticals from 2 - 80 mtrs full size.

Good earth conductivety only reduces losses. as can be done by using enough (elevated) radials .
Take of angle is also made on a larger distance from the antenna as the end of your radials, so yo can not do a lot in that direction.
Good conductivety and or enough radials keeps ground losses low that is all you can do.
Experimenting with height or half waves or vertically stacking (colineair stacking)  can reduse take of angle , but take care if you do it wrongly it will make  sky-warmer out of your vertical.

I have build a 2 x 5/8 wave portable colineair vertical for 10 mtrs now, only have to test it and build the 50 ohms transformer to it.
But not everyone can handle or place or build a 16 mtr long antenna with mast.
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