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Author Topic: 3/8-wave Vertical Height Advantage?  (Read 1474 times)
KM1H
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« Reply #30 on: December 07, 2018, 12:15:12 PM »

... I am not seeing the data and/or math in regard to the statements above claiming surface waves somehow detaching and becoming standard propagating EM waves.

The radiation pattern of a vertical monopole radiator over real Earth is a continuous field.

Only the part of that radiation that is ~tangent to the surface of the earth suffers greater than 1/r losses due the relatively poor conductivity of the Earth.

Radiation toward elevation angles greater than about 1 degree don't "detach" from the rest of the elevation field, but do decay at a 1/r rate.

I just can not fathom why a small few have such a hard time understanding that Richard? There is nothing in ham literature that even suggests a detachment that Im aware of but I didnt follow 73 Magazine which tended to stretch reality.
Trying to reinvent the wheel it appears.

There is also nothing to support a claim that hams are only interested in 3 to 30 degree elevation radiation on 160.

Carl
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NO9E
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« Reply #31 on: December 07, 2018, 12:18:45 PM »

I tried 3 antennas for 20m. One was ground mounted vertical with two 25x2 chicken wire. It had resistance of about 30 Ohm, indicating very good return path. The other antennas were a sloping vertical 15m up and a dipole 15m up.

Tests by WSPR indicated that the ground vertical was the weakest, the vertical dipole was 5 db up, a dipole was 15 db up.  More less across the board in all directions. This was all in GA with clay ground.

Judging by prior experience, the performance of a vertical depends strongly on the ground, even with excellent radials, and I am not sure modeling fully reflects this. That is why anyone asking the same questions gets many different answers.

I am a great fan of verticals next to salt water. On a bridge over salt water, in lousy conditions just few weeks ago, I worked 5 continents in an hr using KX3. Heard the 6th strongly but nobody was calling CQ.

Ignacy, NO9E
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KD6RF
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« Reply #32 on: December 07, 2018, 01:05:46 PM »

All very interesting - will study the plots.

Re vert vs. horizontal dipole - tests I did recently using WSPR and DXPlorer shows good agreement between the standard modeling results (EZNec or whatever) for two stations - mine using a fairly well characterized Inverted-L, and a station 100 miles away or so using a dipole.  Data for both is compared for WSPR spots out at closse-in and out to long low-angle distances.

This test was at the request of some folks who want to do a regular sked between USA and South Africa on 40 M, and I keep a fairly active presence on WSPR bands.

DXPlorer, part of WSPRLite system, makes reading the data very easy.  EZNec says Inverted-L should be 1.2 dB or so up, and the measurement showed about 3 dB up.  Write-up here on the zed ===> https://forums.qrz.com/index.php?threads/quicky-example-of-why-we-do-trust-eznec-and-correctly-made-measurements.631861/#post-4838861



« Last Edit: December 07, 2018, 01:14:38 PM by KD6RF » Logged

VTenn Antennas
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KD6RF
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« Reply #33 on: December 07, 2018, 03:02:19 PM »

Note on above - the data does not include the approx 1/2 or 3/4 dB of unaccounted for line loss on the other fella's side - this means that the measured WSPR data is within about 1 dB or EZNec prediction for my Inverted-L vs. the medium height dipole - not bad!




Ok, re the graph - I guess I'm slow today... what I see is a red 1/r line for a regular propagating EM wave.  And the green and blue curves showing greater than 1/r decay with distance.

This seems to be exactly as expected and would seem to be evidence that the ground wave, or whatever one wold call these very low angle waves, contribute very little to ionospheric propagation.  The further away, the more rapid the departure from expected 1/r....... No?

What would a plot out at say 1 degree elev and at the height of the ionosphere look like in magnitude compared to the regular old 1/r field component?


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VTenn Antennas
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KM1H
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« Reply #34 on: December 07, 2018, 05:20:41 PM »

Quote
What would a plot out at say 1 degree elev and at the height of the ionosphere look like in magnitude compared to the regular old 1/r field component?

Describe the conditions of this ionosphere at 1680 kHz as well as the slope and conductivity of the earth at poor, medium, good, and excellent conductivity and ending up with salt water.
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KD6RF
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« Reply #35 on: December 07, 2018, 06:29:14 PM »

I can imagine that those items would make a difference.... but it still remains to show that these low angle surface-ish waves actually provide substantially more low angle magnitude than predicted by NEC...... whatever the numbers you choose to use.

I am not seeing how a plot out to 10 kM, and a height of a few meters, and that shows greater than 1/r decay, proves anything other than it's a relatively unimportant-to-ionospheric-propagation surface wave.  Of course I might be wrong, so.....

....so whatever y'all may wish to set these numbers to is fine - as long as you can show this performance enhancement for the above plotted surface waves, but out at 4000 kM range (or whatever number you come up with as the correct distance) and at a height of 60 kM (or whatever you determine to be the correct ionosphere height)........  

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VTenn Antennas
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WB6BYU
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« Reply #36 on: December 07, 2018, 08:15:31 PM »

The big difference, of course, is the ground/surface wave component.  That's the
primary desired coverage mode for most AM BC stations, but contributes much less
to ham contacts using higher frequencies, longer distances, and very often lower
power power levels.

Jasik's Antenna Engineering Handbook gives a formula for the approximate
surface wave attenuation factor as a function of ground characteristics and
wavelength.  The accompanying table 33-12 shows field intensity vs distance for
different frequencies over "poor" ground (and another chart for salt water).  If I
am reading the table correctly, the field strength with a 1kW transmitter is
1 uV/m at about 180 miles at 1 MHz vs. 35 miles at 15 MHz.

So you really can't say that the antenna radiation patterns are exactly the same
in the two cases:  the 1MHz surface wave signal is 40 dB stronger than the 15 MHz
one at 35 miles from the antenna due to the difference in attenuation.

A Field strength measurement relatively close to the antenna then doesn't really
tell the whole story:  the surface wave component that follows the Earth attenuates
relatively quickly with distance, and will contribute virtually nothing to received
signal strengths over the path lengths we associate with DX.  Only that portion of
the signal that is not trapped in the surface wave will contribute to ionospheric
propagation.  By contrast, at MF the ground wave is the desired mode for AM BC
coverage, and the skywave is minimized (where possible) to reduce fading.


That's why hams usually ignore the ground wave components: yes, they are present,
and may contribute to relatively local contacts, especially on 80m or 160m, but
it simply represents wasted power when your target is several thousand miles away.
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RFRY
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« Reply #37 on: December 08, 2018, 03:06:29 AM »

... What would a plot out at say 1 degree elev and at the height of the ionosphere look like in magnitude compared to the regular old 1/r field component?

A useful concept when considering whether a radiated e-m wave is a space wave subject to a decay rate of 1/r, or a "ground" or "surface" wave subject to a decay rate exceeding 1/r is given by the decay rate, itself.  If that rate = 1/r, then the associated e-m wave along that propagation path is a space wave.

One way to learn that a radiated wave is a space wave can be determined by a suitable model* using M-o-M software, such as NEC4.x.  If its fields at the opposite ends of a path where the two ends are separated by a linear distance factor of 2 also are related by a factor of either 2 or 1/2, then that radiation is defined as a space wave.

Below is a graphic illustrating that the radiation leaving a vertical monopole at a departure angle of 5 degrees elevation is decaying at a 1/r rate (85.3/170.6, in this case). Therefore that field is a space wave, by definition. If a clear propagation path exists, that field will continue to propagate along that elevation angle at a 1/r decay rate, until it reaches the ionosphere.

* The usual NEC "far field" modeling approach used by many hams will not show this result.  The NEC-defined surface wave must be included in the analysis.  The means of doing that vary with the software application, which are left for their users to discover and implement.

« Last Edit: December 08, 2018, 03:12:30 AM by RFRY » Logged
KD6RF
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« Reply #38 on: December 08, 2018, 05:35:45 AM »

Thanks for the plots.  Still, it looks like exactly what I would expect - a major propagating wave lobe zooming out toward the ionosphere at the medium and higher angles, a rapidly decaying wave stuck to the ground at low angles, and ground bounce cancellation in between...

But you know, it's been a while since I've concentrated on the math details like this.  I got good grades in E/M classes Grin, but have really concentrated on hands-on implementation of ultra-wideband comm and radar hardware and antennas ever since.  Sooooo, I am going to watch a while and let y'all who fiddle with the knobs and dials of Maxwell's equations carry on.
« Last Edit: December 08, 2018, 05:39:08 AM by KD6RF » Logged

VTenn Antennas
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WB6BYU
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« Reply #39 on: December 08, 2018, 08:05:35 AM »

Getting back to the original topic, there is no particular advantage to having
the vertical precisely 1/8 wavelength tall.  As you increase the length from
about 1/8 wavelength to 1/2 wavelength or so there is a monotonic increase
in signal strength  due to a number of factors, along with an increase (at
least over the latter part of the range) in the height of the current maximum.

That may mean that you don't need as many traps to have a useful antenna.

If you do try your trapped approach, remember that a trap provides inductive
loading on lower bands:  if you cut the wire to 3/8 wavelength on 40m to
start with then it will effectively be longer than that on 40m after adding
the two traps for the lower bands.  The exact amount depends on the L and C
values used in the traps.
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KM1H
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« Reply #40 on: December 08, 2018, 08:58:31 AM »

Quote
That's why hams usually ignore the ground wave components: yes, they are present,
and may contribute to relatively local contacts, especially on 80m or 160m, but
it simply represents wasted power when your target is several thousand miles away.

There is very little wasted power if the target area requires a very low elevation angle in the low single digit ranges and that is for flat ground. If that ham lives on a hilltop, such as I do, with a nice roll off and a minus 3 degree horizon nothing is wasted. Run a N6BV Terrain Analysis on my site to see why I do so well on 160/80 as well as the rest of HF thru microwave. AM BC abandoned hills/mountains back in the 1920's as unproductive for surface wave.
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WA7ARK
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« Reply #41 on: December 08, 2018, 10:31:30 AM »

... AM BC abandoned hills/mountains back in the 1920's as unproductive for surface wave.
So you agree that surface wave is not useful for ionospheric communication for hams...

If so, you can appreciate why we think that Richard's postings have little to teach hams.
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RFRY
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« Reply #42 on: December 08, 2018, 11:15:01 AM »

... AM BC abandoned hills/mountains back in the 1920's as unproductive for surface wave.
So you agree that surface wave is not useful for ionospheric communication for hams... If so, you can appreciate why we think that Richard's postings have little to teach hams.

WA7ARK:  Please study the graphic and text in my Reply 37.

Radiation at a 5° elevation angle from a vertical monopole does not have the characteristics of a surface wave.

Such radiation is not accurately shown by the NEC far-field calculations used by most hams.
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KM1H
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« Reply #43 on: December 08, 2018, 12:46:59 PM »

... AM BC abandoned hills/mountains back in the 1920's as unproductive for surface wave.
So you agree that surface wave is not useful for ionospheric communication for hams...

If so, you can appreciate why we think that Richard's postings have little to teach hams.

I cant agree with much of anything you say since you twist things around to suit your own view of reality. And isnt it a little presumptuous to include "we" when so few, if any, agree with you?
What have YOU done on 160? At least Richard is both a ham and BC engineer

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KD6RF
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« Reply #44 on: December 08, 2018, 02:05:34 PM »

Dang... seems to be quite the grumpy topic...  Come on, y'all are the "good guys", each on the side of truth, justice, and the reality-based American way!.

Anyway, another request.  As I've said, the plots thus far don't really tell the story of enhanced low angle radiation that is under-reported by NEC far-field-only codes.

Here's what I would find believable and easily understandable - take this plot:



a) Show it both with and without "ground wave"
b) march each pair of plots (w and w/o gnd wave) out to 10,000 kM - say 1kM, 10kM, 100kM, 1000kM, 10,000kM.

That way it should be clear how much the NEC far-field only approach underestimates low angle performance.  Make sense?


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