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Author Topic: buried ground radial length?  (Read 3880 times)
N7IOH
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« on: February 14, 2013, 06:07:40 PM »

I read somewhere that buried radial that are longer then the height of the vertical are not needed (detuning maybe).  So a 43' vertical that operates on the low bands won't benefit from longer radials then 43'.  I did not believe it when I first read it but it always stuck in my mind.  Is there any truth to this? 

      Al, n7ioh
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WD8KNI
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« Reply #1 on: February 14, 2013, 06:34:23 PM »

Buried radials (not above ground) do not need to be resonate, 30 radials 20 feet long is about the best you can do.  Radial length has absolutely nothing to do with antenna height, antenna could be linearly loaded with a huge capacitance hat i.e. very short, and the radial length will not change.. 

Fred
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K0ZN
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« Reply #2 on: February 14, 2013, 07:14:22 PM »

All that in-ground radials do is to improve the conductivity of the ground system. I.e. reduce resistive ground loss and improve radiation efficiency.  Period. It is just Ohm's Law.

Restated: plain dirt is a poor conductor of radio frequency current, therefore transmitter output power is wasted simply heating the earth.... With radials more current flows in the low resistance wire.

In-ground radials are not resonant. Essentially, they can't be resonant because they are part of the Earth, so to speak. I guess if the soil was essentially a perfect insulator and non-conductive you could see resonance effects like in an elevated radial system.

The length of radials needed to maximize radiation efficiency is related to the ELECTRICAL length of the vertical radiator because the in-ground current maximum is related to radiator length. To wit: with a 1/4 wave radiator, the ground current is highest right at the base of the antenna, whereas, with a 1/2 wave vertical the current maximum occurs 1/4 wave out from the base and the current is nearly zero at the base. It is simply related to the sine wave current distribution.

73, K0ZN
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N7IOH
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« Reply #3 on: February 14, 2013, 07:31:07 PM »

OK, I got it.  That was simple.  Thanks. 

Al, n7ioh
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RFRY
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« Reply #4 on: February 15, 2013, 03:06:22 AM »

Below is a link to a useful graphic on this topic, taken from the experimental work of Brown, Lewis and Epstein of RCA Laboratories in Princeton, NJ (published in the Proceedings of the I.R.E in 1937).

It shows that when using 113 x 0.37-wave* buried radials with an 88-deg vertical, series-fed monopole, current distribution is not sinusoidal, but essentially uniform along the entire length of each radial.

The current distribution for fewer than 113 radials shows reduced current toward the far ends for the longer radial lengths, especially when only 15 or 30 radials are used.

The reason for this is that the r-f displacement/conduction currents induced in the earth by radiation from the monopole necessarily need to travel through longer paths just below the surface of the earth to reach the fewer number of wires, even though the wires extend beyond 1/4 wavelength.

For good system radiation efficiency, those currents need to be returned to the antenna r-f ground terminal at the feedpoint, so any loss in that process reduces radiation from the monopole.

The groundwave fields measured by BL&E showed that an 88-deg monopole using 113 x 0.37-wave buried radials produced over 90% of the maximum theoretical field possible for the applied power when radiated from a perfect monopole of that height, over a perfect ground plane.

The fields measured when using 15 and 30 radials up to 0.412 wavelength were significantly less.

These measurements were taken for a frequency of 3 MHz.  The conductivity of the earth in which the radials were buried was 2 mS/m.

* Graph shows the physical lengths of the radials in free space.

http://i62.photobucket.com/albums/h85/rfry-100/BLampEFig7_zps4bd19180.jpg
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N7IOH
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« Reply #5 on: February 15, 2013, 04:31:28 AM »

Richard, very interesting.  I do have a question though, how tall is an 88 degree vertical?  At 3 MHz I would think it would be quite long.  Do you know if the results have been verified since 1937.  Just curious because measuring equipment has become much more accurate since then.  I remember reading that after a certain number of radials there was not much improvement.  I don't recall the number but it was way less then the 113. 

Al, n7ioh
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W5DXP
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« Reply #6 on: February 15, 2013, 05:44:39 AM »

... how tall is an 88 degree vertical?

Assuming those are electrical degrees (not physical degrees) it is four degrees (4.44%) shorter than a 1/4WL (90 deg) vertical.

If we look at buried radials from a real-world quantum physics standpoint, what is happening is pretty clear. RF energy travels through the earth medium surrounding each buried radial wire as a cloud of photons, i.e. NOT INSIDE the wire conductor itself (surface effect). Most RF photons are attenuated by the lossy earth medium before they reach the ends of 1/4WL radials. Another way of saying the same thing is that sparsely spaced buried radials are lossy and the velocity factor is low. When we have a lot of closely spaced buried radials, some of the photons from one radial are absorbed by electrons in adjacent wire radials thus conserving that RF energy within the ground radial system. The so-called "displacement current" is not electron flow but is instead the transfer of energy by the ordinary flow of photons between the radiating element and the ground radial system. Although useful as a conceptual model, quantum physics has disproved the existence of "electron carrier displacement current" since electrons cannot travel at anywhere near the speed of light. (I can walk faster than most electrons travel through a wire.) Electrons energized by RF energy just vibrate in place while emitting and absorbing RF photons.

http://photontheory.com/Kemp/Kemp.html

Please note that electromagnetic fields owe their existence to quantized photon particles.
« Last Edit: February 15, 2013, 06:11:29 AM by W5DXP » Logged
N7IOH
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« Reply #7 on: February 15, 2013, 06:12:41 AM »

OK, that makes since to me now.  Now the buried radials, any difference if insulated wire is used vs bare wire?  I have a large amount of insulated military wire I thought I might use for a buried radial system.

Al, n7ioh
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RFRY
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« Reply #8 on: February 15, 2013, 06:22:59 AM »

I do have a question though, how tall is an 88 degree vertical?  At 3 MHz I would think it would be quite long.

From the BL&E paper, it was a galvanized steel pipe 80 feet in height x 2-1/2 inches OD.

Do you know if the results have been verified since 1937.  Just curious because measuring equipment has become much more accurate since then.

Yes, the nearby groundwave field intensity for a given r-f power radiated by a ~1/4-wave monopole driven against 120 x 1/4-wave buried radials has been accurately measured by broadcast engineers thousands of times since BL&E measured it in 1937, and those readings confirm the values measured by BL&E.

The test equipment used by BL&E was laboratory grade, and although it was not as compact as modern, professional level field intensity meters, it was no less accurate.

I remember reading that after a certain number of radials there was not much improvement.  I don't recall the number but it was way less then the 113.

There has been a lot material written and posted on this topic, but probably none has been as comprehensive, and as scientifically determined as in this BL&E study.

Now the buried radials, any difference if insulated wire is used vs bare wire?

No, because insulation has a negligible affect on the r-f currents passing between the earth and the wires.

However if one or more ground rods are not installed near the tower base and connected to the common point of the radials, the tower will not be well protected from static charge buildup, and will not have a low-impedance path to the earth for transients from nearby lightning strikes.

There are other means of providing this too -- using a static drain choke and a ball gap, for example.

AM broadcast stations typically use uninsulated buried radials, along with both a drain choke and a ball gap.
« Last Edit: February 15, 2013, 06:35:42 AM by RFRY » Logged
W5DXP
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« Reply #9 on: February 15, 2013, 06:28:29 AM »

Now the buried radials, any difference if insulated wire is used vs bare wire?

Insulated wire in free space has a somewhat lower velocity factor than bare wire but that effect is negligible when the wire is buried. Seems to me, the insulation might cause the buried radial to last longer but otherwise has little effect even though the insulation is less lossy than the soil.

I've heard that if you want a really efficient buried radial system, locate your antenna in the middle of the Sahara Desert.Smiley
« Last Edit: February 15, 2013, 06:33:33 AM by W5DXP » Logged
N7IOH
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« Reply #10 on: February 15, 2013, 06:38:54 AM »

Thanks guys, I have learned a lot from your posts.  The internet is a great source of information.  The biggest problem is separating fact from opinion, hearsay, and just plain guessing. 

  Al, n7ioh
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W5DXP
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« Reply #11 on: February 15, 2013, 06:50:25 AM »

Thanks guys, I have learned a lot from your posts.

Half the time, the response to one of my posts showing how RF energy is limited by the laws of quantum physics is:

"We don't need no stinkin' photons for no ham radio stuff."Smiley
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RFRY
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« Reply #12 on: February 15, 2013, 06:53:20 AM »

I've heard that if you want a really efficient buried radial system, locate your antenna in the middle of the Sahara Desert.

The BL&E paper and other research shows that a monopole of 50 degrees height or more will radiate >90% of the r-f power applied to it, when driven against a set of 120 x 1/2-wave radials buried in the earth -- even if that earth is the middle of the Sahara Desert.

Of course, those fields are attenuated faster with distance for those conditions, but the power that is actually radiated (launched) from the monopole is not much affected by such an installation, compared to one where soil conductivity is relatively high.

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W5DXP
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« Reply #13 on: February 15, 2013, 07:13:49 AM »

-- even if that earth is the middle of the Sahara Desert.

What I heard was that, located on top of dry sand, radials perform as if they are elevated since any moisture is so far below the surface. Of course, I never tried such.
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WA2ONH
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« Reply #14 on: February 15, 2013, 07:52:16 AM »

The internet is a great source of information. 

Al, if you haven't viewed this already, take a look at N6LF Rudy's site on verticals at...

LINK: http://www.antennasbyn6lf.com/

A Great source of information! Good luck.
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73 de WA2ONH   ...Charlie
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