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Author Topic: How can a ground mounted vertical ever work well?  (Read 5768 times)
WB4SPT
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« on: January 13, 2013, 11:28:12 AM »

OK, I have your attention.  Obviously, they can and do work well, as evidenced by all the AM broadcasters in the world.  But, I do have a better question.  If the "ground" side of the vertical carries essentially the same current as the vertical radiator element, where is that power going?  I have Kraus Antenna book, and there is good discussion of perfectly conductive soil, infinite ground planes and the like, but isn't half of the transmit energy ending up heating up dirt?  In fact, he talks of half of the energy delivered to the "image antenna".  What am I missing?  I feel I should know this.  You have lots of RF current in the ground radials; that must radiate, correct?  So, it's radiating at or just below ground level? 
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RFRY
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« Reply #1 on: January 13, 2013, 11:58:09 AM »

The displacement currents radiated by a monopole through its capacitance to the earth within a 1/2-wave radius become conduction currents that need to be returned to the r-f ground terminal of the transmit system.  The sum of all those earth currents is equal to the current flowing along the monopole.

A perfect ground plane produces no I2R loss to those earth currents; therefore the monopole radiates 100% of the r-f power applied to it.

A set of 120 x 1/4-wave buried radials has an r-f loss of 2 ohms or less (depending on the conductivity of the earth in which they are buried), and with a 2 ohm loss a 1/4-wave monopole radiates about 95% of the applied power.

« Last Edit: January 13, 2013, 12:58:36 PM by RFRY » Logged
WB4SPT
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« Reply #2 on: January 13, 2013, 02:14:19 PM »

I'm along as far as the radial and/or conductive soil currents add to be equal and opposite phase of the vertical element.  My lack of understanding is how these ground radial currents are told not to radiate.  How is this fundamentally different from a half wave dipole, except that half the antenna is at grade?  In other words, how are these supporting currents not radiating? If they are radiating, appears to be a soil warmer.
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RFRY
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« Reply #3 on: January 13, 2013, 04:01:41 PM »

Radial wires buried within 1/2-wavelength radius from the base of a monopole do not produce useful, far-field radiation.

Rather those wires improve the conductivity of the ground plane against which the monopole is driven -- which reduces the I2R losses to which those ground currents are subjected. This tends to maximize the r-f current flowing along the monopole (other things equal).

An "image" of a monopole antenna below the surface of the earth does not exist.  That is merely a convenient concept/graphic to account for the total gain/radiation of a monopole antenna system, including its reflection from the surface of the earth.

All of the useful radiation launched by a vertical monopole is produced by the r-f current flowing along the vertical monopole, itself.  So maximizing that current maximizes the radiation efficiency of that antenna system (again, other things equal).

A perfect, 1/4-wave monopole driven against a perfect ground plane has the intrinsic peak directivity of a 1/2-wave dipole in free space, e.g., 2.15 dBi.  The reason for this is that the radiation resistance of that 1/4-wave, base-fed monopole is 1/2 that of a 1/2-wave, center-fed, free-space dipole.  So for the same applied power, twice the r-f current flows along that 1/4-wave monopole as along each arm of that free space dipole.

But in the case of a monopole with its base at/near the earth, all of its radiation is confined to one hemisphere, by the adjacent ground plane.  If that ground plane has perfect conductivity, then the reflection from its surface adds 3.01 dB to the intrinsic directivity of the monopole, yielding an effective directivity of 5.16 dBi for that system.  This reality has been proven by thousands of accurate field intensity measurements of AM broadcast stations going back many decades.

For example, a perfect 1/4-wave monopole driven with 1 kW against a perfect ground plane, and having a peak directivity of 5.16 dBi  produces a groundwave field of about 313.7 mV/m at a distance of 1 km.

A typical AM broadcast station using a 1/4-wave monopole driven against a set of 120 x 1/4-wave buried radials produces a groundwave field of about 306 mV/m at 1 km, other things equal.

This difference in field is equivalent to a reduction in radiated power of about 5% (less than 0.5 dB) from the theoretical maximum field of a perfect system.

R. Fry, CPBE
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WB6BYU
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« Reply #4 on: January 13, 2013, 04:13:49 PM »

Consider for a moment a typical elevated quarter wave ground plane antenna with
horizontal radials - let's say they run N, S, E and W.  All the radials carry equal
currents, each 1/4 of the current in the main radiator.

If we stand to the West and look at the current distribution on the N and S radials,
they have equal currents flowing in opposite directions.  While either would
radiate by itself, the net radiation from the two radials is ZERO, or very close to it,
because any radiation from one radial is canceled by equal out-of-phase radiation from
the other radial.  The same applies to the E and W radials.

So the net effect is that there is no significant radiation from the radial system,
even though there is current flowing on each of them.


If the radials are sloping downwards, then the horizontal components of current still
cancel on each, but the vertical components are in phase, so they do radiate.  That's
why the radiation resistance increases when the radials are sloped downwards,
because the radials now contribute somewhat to the total radiation.
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WD4ELG
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« Reply #5 on: January 13, 2013, 06:31:25 PM »

BEST. TECHNICAL. THREAD. EVER.
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KQ6Q
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« Reply #6 on: January 14, 2013, 01:44:46 AM »

Another way to think of the situation - the currents in the monopole and the radials don't radiate, the E and H fields generated between the monopole and the radials or ground plane radiate, or propagate, away from the antenna. The better the radials or ground plane, the stronger the field, the more efficient the radiation.
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WB4SPT
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« Reply #7 on: January 14, 2013, 07:27:48 AM »

I think I got it.  The ground radials DO radiate, but being in the near antenna field the magnetic/electric fields CANCEL and net-net, don't effect a far field radiation. 
So, it still demands that soil heating doesn't occur by the radials, and I'm left with the idea that ground radials should not be BURIED for any appreciable distance under grade; does this make sense?  I don't recall reading issues with deeply buried radials, with this new knowledge, it would seem one could still create a problem even with good radials, that have low I2R losses.  You do need good SURFACE conductivity, true? 
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W8JX
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« Reply #8 on: January 14, 2013, 09:31:34 AM »

This is good reading but it leaves out the point that you can get good performance without 120 radials and you can do well with far less.
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RFRY
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« Reply #9 on: January 14, 2013, 10:35:20 AM »

I think I got it.  The ground radials DO radiate, but being in the near antenna field the magnetic/electric fields CANCEL and net-net, don't effect a far field radiation.  So, it still demands that soil heating doesn't occur by the radials

Heating of the earth by r-f current actually is reduced when buried radials are used.  Those currents enter the earth as displacement currents flowing through space between the vertical monopole, and earth.  They are not radiated into the earth from/by the buried radial wires, themselves.

Without the buried radial wires those currents would need to travel back and forth to/from the r-f ground terminal of the antenna system entirely through the earth -- a very lossy path.  Instead they naturally take the much lower resistance paths provided by the buried radial wires.
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RFRY
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« Reply #10 on: January 14, 2013, 10:53:45 AM »

This is good reading but it leaves out the point that you can get good performance without 120 radials and you can do well with far less.

Using fewer/shorter buried radials than the typical set of 120 x 1/4-wave wires used by AM broadcast stations is not a serious limitation as long as the conductivity of the earth in which they are buried is very good.

The link below shows how that earth conductivity affects the radiation efficiency of a short monopole system using 32 x 10-meter radials, on 160 meters.

http://i62.photobucket.com/albums/h85/rfry-100/10m_Vert32Buried_Radials.jpg
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AD4U
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« Reply #11 on: January 14, 2013, 11:14:59 AM »

Hook the center conductor of the coax to the radial system and the shield to the vertical part and see if it works.  I am curious what you find.

Dick   AD4U
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WB4SPT
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Posts: 160




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« Reply #12 on: January 14, 2013, 11:53:41 AM »

I think I got it.  The ground radials DO radiate, but being in the near antenna field the magnetic/electric fields CANCEL and net-net, don't effect a far field radiation.  So, it still demands that soil heating doesn't occur by the radials

Heating of the earth by r-f current actually is reduced when buried radials are used.  Those currents enter the earth as displacement currents flowing through space between the vertical monopole, and earth.  They are not radiated into the earth from/by the buried radial wires, themselves. Is there an experiment, or even a model to suggest that this is true? That is, a set of radials buried some depth work better than either the same at the surface, or even a giant copper ground plane on the surface?

Without the buried radial wires those currents would need to travel back and forth to/from the r-f ground terminal of the antenna system entirely through the earth -- a very lossy path.  Instead they naturally take the much lower resistance paths provided by the buried radial wires.  Makes perfect sense to me.

I suppose I'm still compelled to think that you want to keep the radials close to the surface and NOT bury them, especially if you are dealing with lossy soil, which all soil is.
« Last Edit: January 14, 2013, 11:56:41 AM by WB4SPT » Logged
WA0CRI
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« Reply #13 on: January 14, 2013, 12:14:22 PM »

Another way to understand what radials do - they are to radio frequency radiation as a mirror is to light frequency radiation.  Viewing a light bulb suspended above a large mirror at a distance would appear as two light sources.  Without the mirror half the bulb's output would be lost as heat in the ground.  Because the wavelength at radio frequencies is so much longer, radials can be spaced apart at much greater distances than the reflecting particles of a mirror's surface.  I know this seems like a simple concept, but I'm always amazed how many Hams fail to make the analogy between RF and light frequency radiation.
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W8JX
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« Reply #14 on: January 14, 2013, 01:01:17 PM »

I want to add a few pictures of my vertical. It was originally meant as a temporary antenna that turned permanent. This first picture is of my 5BTV in a side field near house.

http://files.qrz.com/x/w8jx/Antenna_02.JPG

This next picture is of how it is mounted. You will notice no radials at all. The only ground it has is a 7 foot field fence pole driven nearly 6 feet into ground and antenna is bolted too it. This is not to say radials do not help but this antenna works surprisingly well on 40 and 80 and okay on 20 and above but no barn burner on upper bands. I spend most of my time on 40 when active and many are amazed at how well it works. 

http://files.qrz.com/x/w8jx/Ant_base01.JPG

Point of all this is that you do not need a commercial grade radial field to do well on 40 and 80 with a vertical.
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