Monopole Configuration and Performance

[R. Fry SWL]

For discussion...

All monopoles need an electrical reference point to be "driven against."

Using a symmetrical arrangement of two or more 1/4-wave-resonant,
horizontal wires elevated sufficiently above the earth provides that,
by acting at their junction under the base of the monopole as a point
with constant electrical characteristics with respect to the current
flowing in the antenna system.

NEC shows the peak free-space gain of such a system using a 1/4-wave
monopole to be the same as that of a 1/2-wave dipole in free space,
e.g., 2.15 dBi.  When that system is operating within a few electrical
degrees above a perfect ground plane then the net, peak gain rises to
5.16 dBi, because all of the radiation is re-directed/confined to one
hemisphere, producing an additional 3 dB gain for the antenna system.

Horizontal wires lying on, or buried several inches below the surface
of the earth do not have the same electrical characteristics or
function as when they are elevated.  Instead, they serve to collect
the r-f currents generated by the displacement field radiation of the
monopole -- which currents flow in the earth out to about 1/2
wavelength from the base of the monopole.

The conductivity of that circular area at and just below the surface of
the earth to about 1/2-wavelength from a monopole is, in fact, a circuital
element of such an antenna system.

If the earth was a perfect conductor then those currents could travel
through the earth without loss, and a single, short ground rod would
serve as an electrical reference point for the r-f current flowing in the
antenna system. Those currents need to be returned to the r-f ground
terminal/chassis of the transmitter in order to complete the series
path needed for r-f current to flow in the antenna system.

The sum of those r-f currents flowing in the earth around the monopole,
and collected by that ground rod will be equal to the base current in a
1/4-wave, series-fed monopole.

The gain of this configuration is 5.16 dBi, the same as when using a few
elevated, 1/4-wave resonant wires as a counterpoise.

But the earth is not a perfect conductor.  For that reason it is
necessary, when using buried radials, to install enough of them in the
surface area out to about 1/2 wavelength to collect those r-f currents
before they have traveled through much of the lossy earth to reach
those wires.

The benchmark 1937 I.R.E. paper of RCA's Brown, Lewis and Epstein
showed that 113 x 0.412-lambda buried radial wires used with monopoles
of about 45 to 90 degrees in height produced a radiated groundwave
field when measured at 3/10 of a mile that was within several percent
of the theoretical maximum for a perfect monopole radiator with a zero-
ohm connection to a perfect ground plane -- and this despite the fact
that earth conductivity at/near their test site was not better than 4 mS/m.

As an aside: NEC analyses for far-field conditions show an elevation
gain of zero in the horizontal plane for a monopole over real earth,
and peak relative field gain at some elevation angle above the
horizontal plane.  However the radiated, relative fields that exist
at, and relatively close to the edge of the near-field boundary of the
radiation hemisphere of all monopoles of 1/4 wavelength and less are
very nearly equal to the cosine of the elevation angle -- which value
is 1.0 at zero degrees (the horizon), and zero at the zenith.  If they
were not, then the fields measured by BL&E would be much different
than they recorded in their 1937 paper, and MW AM broadcast stations
would not have the groundwave coverage areas that they do.

It is only after those fields propagate a significant distance over a
real earth path that they depart significantly, and progressively more
significantly with distance, from the relative fields described by the
cosine value of the elevation angle.

R. Fry

[KA2DZT]

R. Fry,

Welcome to the AM Forum.

Could you have at least kept your first post a little easier for us old radio men to digest.

It's Sunday,  this subject is way too hard for Sundays and may take a few days to figure out.

Fred

[W7TFO]

Verticals are always welcome, discussion always ensues, techniques abound.

There's a lot of current and retired broadcast engineering types lurking here, where that type of skyhook is ubiquitous. 

Not to mention Don KYV, our resident guru and go-to guy.  Well, OK, I mentioned him anyway...

The RCA papers are legend, and some more recent modeling data is indeed useful.

73DG

[WB4AIO]

As a broadcast engineer, I noticed that during proofs of performance (extensive measurements of ground wave field strength in various directions on a directional antenna system), snow and rain on the ground would definitely increase the signal intensity as measured in the far field.

Kevin, WB4AIO.

[R. Fry SWL]

...snow and rain on the ground would definitely increase the signal intensity as measured in the far field.

Somewhat - because the effective conductivity along those groundwave paths is affected to some extent by the moisture content present at, and just below the surface of the earth.  

This also can reduce the losses in the r-f ground connection used by a monopole antenna system.

R. Fry

• Staff Engineer, WJR/760 kHz, Detroit (early 1960s)
• RF Systems/Field Engineer, RCA Broadcast Div (1965-1980) and Harris Corp Broadcast Div (1980-1999); now retired.

[WB4AIO]

...snow and rain on the ground would definitely increase the signal intensity as measured in the far field.

Somewhat - because the effective conductivity along those groundwave paths is affected to some extent by the moisture content present at, and just below the surface of the earth. 

This also can reduce the losses in the r-f ground connection used by a monopole antenna system.

R. Fry

[...]

It seemed to be only a far field effect, so not related to losses in the ground connection. It was interesting, though -- especially the surprising fact that dead frozen ice and snow also helped. I wasn't expecting that. But that's what I observed.

73,


Kevin, WB4AIO.

[R. Fry SWL]

...It seemed to be only a far field effect, so not related to losses in the ground connection...

Agree that any reduction in the loss of a typical r-f ground connection used by an AM broadcast station as a result of rain/snow would be small, however I included it for the sake of accuracy.

But just to note that in practice, the radius to the start of the far field of a directional MW broadcast station array is 10 times the greatest h-plane spacing of the towers used in the array.  For a single monopole it is 2*H^2  / lambda, where H is the physical height of the monopole (in the same units as lambda).

The physical locations for the field intensity monitoring points used to confirm that a directional AM broadcast system/array meets its licensed parameters normally are not much beyond that distance, so as to minimize seasonal and meteorological variations in the groundwave signals measured at those points.

OTOH, NEC software defines the far field to exist only at an infinite distance from the radiator/array, and over a flat ground plane, at that.  Such radiation patterns show zero relative field in the horizontal plane for real earth paths, and not much more for small elevation angles above that.

Thus the elevation plane radiation patterns produced by NEC for the "far field" are not very useful at distances where a real-world groundwave exists, and lead some to believe (mistakenly) that monopoles launch little to no radiation near/in the horizontal plane.

R. Fry

[WB4AIO]

When we did our "full proofs" at WEAM 1390, which were years apart, we went out beyond 20 miles along each significant radial. I forget the exact distance.

The reason I knew the snow/rain effect was not related to local ground losses was because the tuning of the four-tower array and the loading of the transmitter were utterly unaffected.

Thanks for the insight into NEC modeling. I always knew that real world experience with the Standard Broadcast band proved some discrepancy between reality and the model, and now I know why.

Thanks,


Kevin, WB4AIO.

[R. Fry SWL]

When we did our "full proofs" at WEAM 1390, which were years apart, we went out beyond 20 miles along each significant radial. I forget the exact distance.

Measuring the groundwave field intensities at various distances along radials of various defined compass bearings is used to determine the r.m.s. value of the h-plane pattern of a directional AM broadcast array -- which is important in establishing its "efficiency" to the FCC.

But the monitoring point locations used to prove that the directional array of an AM broadcast station meets its licensed parameters typically are located within several miles of the geographic center of the array.

RF

[WB4AIO]

Yes, the "monitoring points," which we measured weekly, were within a mile or so of the station.

73,

Kevin, WB4AIO.

[DMOD]

Welcome Mr. Fry.

Question: It was mentioned in one of the IRE papers that 0.544 Lambda appeard to be the best compromise vertical radiator for BC.

However, they never went on to fully explain this statement.

Do you have any background info?

Thanks

Phil

[KA2DZT]

I always thought a 5/8 wave vertical radiator worked the best.  I think producing the greater amount of low angle radiation.

Maybe we can get a few comments on this.

Fred

[W2DU]

The problem with the 5/8wl height is that a minor lobe of opposite polarity is generated that causes a fading ring well inside the service area. To avoid this problem the heights have been shortened to eliminate the minor lobe. At the moment I've forgotten what the standard height is now, but I'm sure Richard Fry will come in and give it. The shortening from 5/8 to the present standard height results in only a significant loss in radiation.

Walt, W2DU

PS--sorry, DMOD already gave the height, .544 lambda, my bad.

[K5UJ]

190 degrees.

[W0BTU]

... Horizontal wires lying on, or buried several inches below the surface of the earth do not have the same electrical characteristics or function as when they are elevated. ...

Hello Richard,

I don't fully understand this statement. (Maybe if I was more awake, I would. :-)  In any case, whether radials are on the ground or 10' in the air, the pattern does not change.

Good to see you here. Lots of knowledgeable folks on Amfone. We should get Dan to join this thread.

[KA2DZT]

The problem with the 5/8wl height is that a minor lobe of opposite polarity is generated that causes a fading ring well inside the service area. To avoid this problem the heights have been shortened to eliminate the minor lobe. At the moment I've forgotten what the standard height is now, but I'm sure Richard Fry will come in and give it. The shortening from 5/8 to the present standard height results in only a significant loss in radiation.

Walt, W2DU

PS--sorry, DMOD already gave the height, .544 lambda, my bad.

Thanks Walt,

I guess that's why DMOD mentioned that .544lambda was a compromise length.

Fred

[R. Fry SWL]

It was mentioned in one of the IRE papers that 0.544 Lambda appeard to be the best compromise vertical radiator for BC.  Do you have any background info?

The reason that ~0.544-lambda verticals (~195º) are used by most 50 kW, 24/7 AM broadcast stations is that this height minimizes the ground area where the nighttime skywave from the radiator causes interference to the groundwave.  Stations with lower powers and/or using higher AM broadcast frequencies and/or in areas where earth conductivity is poor have less of a problem with this, because the groundwave has fallen below the ambient r-f noise floor before the skywave can interfere with it.

A plot is attached showing the elevation patterns for monopoles of various physical heights, including the development of the high-angle lobe for a 5/8-wave monopole that Walt mentioned.

Also attached is a clip from Terman's Radio Engineers' Handbook, 1st Edition, showing how this self-interference zone is produced.

RF

[R. Fry SWL]

... Horizontal wires lying on, or buried several inches below the surface of the earth do not have the same electrical characteristics or function as when they are elevated. ...

I don't fully understand this statement. (Maybe if I was more awake, I would. :-)

Thanks to you and others for the welcome given me.

To expand on my statement quoted above -- two, self-resonant, 1/4-wave horizontal wires oriented in opposite directions and used as an elevated counterpoise for a vertical monopole produce a free space elevation pattern and directivity about equal to a perfect monopole driven against a perfect ground plane. There is no far-field radiation from the two horizontal conductors because the r-f currents in them are equal, and flow in opposite directions at every instant, thus their fields cancel each other.  All of the far-field radiation is produced by the vertical conductor.

If those same two wires are re-located to the surface of the earth or buried in it, then they are no longer resonant, because the velocity of propagation along them is much slower than when they are elevated above the earth.  Also they do not radiate much, because of their physical location on/in the earth.

Instead those conductors serve to collect r-f currents present in the earth as a result of radiation by the monopole, as discussed in my opening post in this thread, and many more than two of them are needed to do this efficiently.

Buried radials do not function as the other half of a dipole, or to resonate the monopole, or as a shield to prevent r-f current from penetrating the earth.  They are just a resistive, circuital element of the monopole antenna system configuration that uses them.

RF

[W0BTU]

Hi Rich,

I don't think two (or any number of) resonant elevated counterpoise wires radiate any more than radial wires laying on the earth, as long as they are balanced (180 degrees apart, same length, and same distance above the earth).

I thought you might be implying that they might radiate when we elevate them. But I see what you're saying now. Thanks.

If we add more elevated radials, it increases the efficiency by reducing the ground losses?

[k4kyv]

Buried radials do not function as the other half of a dipole, or to resonate the monopole, or as a shield to prevent r-f current from penetrating the earth.  They are just a resistive, circuital element of the monopole antenna system configuration that uses them.

Rich,

Welcome to the board; glad to see you have joined the discussion here.

Agreed the buried radials do not "function" as the other half of a dipole; perhaps a better word would be "replace".  Both the 1/4λ monopole and one leg of a half-wave dipole have to have something to "work against". The dipole leg works against the other leg.  The monopole works against ground. So the "function" of the ground plane in this sense is that of giving the 1/4λ piece of wire (or tower) a circuit element to work against. It does not do any radiating.

The ground pane does not resonate the monopole, but a quarter-wave piece of wire cannot be self resonant on its own. It must have something to work against to display resonance.  Standing alone, it becomes self-resonant as a  half-wave, at twice the monopole frequency.

The radial system collects the displacement currents from the monopole, and returns them to the common point at  the base of the antenna.  With a sufficient number of radials, minimal rf current passes through the lossy earth, since it finds a better conductive path through the radials. Isn't that identical to the function of a shield?

The shield function is more clearly evident in the case of a half-wave monopole.  In that case, there is nearly zero current at the base of the vertical.  IIRC, the displacement current in the soil is at a maximum somewhere around 0.4λ away from the base. Again, the radials provide a better conductive path than does the soil, effectively shielding the radiator from the lossy earth.  The half-wave monopole could be tuned perfectly to resonance to fully accept power from the transmitter, by feeding it as a half-wave end-fed Zepp (rotated to a vertical position), or by feeding it with a network consisting of a parallel tuned circuit working against a single 8' ground rod, but much if not most of the rf would be wasted as resistive losses in the soil.  Hence the fallacy of the commonly-heard Hammy Hambone belief that "radials are not necessary" for a half-wave vertical.  If the half-wave vertical is raised high enough (several wavelengths) off the ground, earth  losses become inconsequential. But as the base of the antenna approaches the ground, earth losses increase, to a maximum when the base is sitting right on the ground, mounted on a base insulator. Therefore, one of the functions of the radial system can be thought of as a form of shielding that isolates the monopole from the poorly-conductive soil.

Ground radials (as opposed to elevated radials) are more effective when lying right on the surface. The only reason they are buried is to protect the wires from foot traffic, mowing equipment, etc.  It is counter productive (and a tremendous waste of effort) to bury the radials too deeply in the ground.  I recall a pre-WWII ARRL publication that recommends burying radials about two feet in the soil! Hopefully, that was just a mis-print, but I suspect the writer actually believed they would be more effective because they would make better electrical contact with the moist earth when buried deeply, totally untrue. By burying them that deeply, you are inserting a layer of lossy earth between the monopole and the radial system, which is counter to the very purpose of the ground plane. Likewise, there is no advantage to using bare wire for radials instead of insulated, other than cost of the wire.  Insulated wire may be worth the extra expense, since it helps protect the wire from corrosive minerals in the soil. The purpose of radial wires is to collect displacement currents from the radiator, not from ground currents in the soil.

[K5UJ]

Somewhere I heard or read that you can get away with radials buried a few feet down if your station is on a frequency in the lower part of the broadcast band.    Supposedly the RF penetration is deeper as the transmit frequency goes down.  Hams and broadcast stations up in the top 3rd of the bc band can't go deep.  The how deep can I get away with has been a topic of slight interest over the past couple of years because going deep has been seen as a way to guard against copper theft.   

Laying them on the surface which some hams do (and which I have done) is a mixed blessing from what I have seen because ground heave and/or other forces slowly move them around and after a few years they are no longer nice and neatly straight. 

Rob

[R. Fry SWL]

If we add more elevated radials, it increases the efficiency by reducing the ground losses?

Strictly speaking, a monopole using 1/4-wave, resonant, elevated horizontal wires as a counterpoise has no ground losses associated with the physics of radiation, because the r-f currents flowing in the vertical and horizontal conductors have not had to pass through the earth (ground) in that process. The radiation efficiency of such an antenna system can be limited only by the ohmic losses of the radiating conductors, and the loss across any insulators used in the system.

Once the radiated fields beyond 1/2-wavelength from the monopole encounter the earth, they are subject to the same groundwave propagation losses for an elevated counterpoise monopole system as for a buried radial monopole system.  Both NEC analyses, and data measured for such elevated systems by broadcast engineering consultants have shown that the groundwave fields they produce are at least the equal to those from a monopole using a conventional set of 120 x 1/4-wave buried radials (other things equal).

Several such systems using a counterpoise are in use by AM broadcast stations at sites where rocky ground at the tower site made the use of buried radials impractical.

Using more than two 1/4-wave, self-resonant, elevated horizontal wires in a symmetric physical orientation in a counterpoise may help the system impedance bandwidth somewhat, but otherwise just two such wires are enough to achieve close to 100% radiation efficiency.

Dr. George H. Brown of RCA¹ was the inventor of the ground plane antenna, which is comprised of a single vertical conductor driven against a set of horizontal conductors -- an analog for the topic here.  In his autobiography he wrote that while only two horizontal conductors are required for omni, h-plane radiation from that configuration, early customers thought that the h-plane radiation pattern with only two horizontal conductors must be bi-directional.  He wrote that finally he bowed to RCA Marketing types, and supplied them with four horizontal conductors arrayed in space quadrature.

¹ The same George Brown of the famous 1937 IRE paper about buried radials with monopoles

RF

[R. Fry SWL]

Don - A few comments to your post...

Ground radials (as opposed to elevated radials) are more effective when lying right on the surface. ...  It is counter productive (and a tremendous waste of effort) to bury the radials too deeply in the ground.

Just to note that even for the 3 MHz frequency used in the BL&E tests, they buried their ground radials to a depth of about six inches, using a plow they designed.  Their field measurements at 3/10 of a mile from the monopole showed system radiation efficiencies of about 95% for monopoles of 45º to 90º --- and this where the conductivity at/near the site was not better than 4 mS/m.

The purpose of radial wires is to collect displacement currents from the radiator, not from ground currents in the soil.

Slight clarification in that the displacement currents become conduction currents once they enter the earth.

RF

[W2DU]

Don, you mentioned that you believe the maximum displacement current occurs at 0.4 lambda. According to the BLE experments, they found that at 0.4 lambda the displacement currents had reached a diminishing-returns value, and therefore, adding further radial length would be worthless.

Walt

PS--I too, welcome Richard Fry to our Forum--he's an expert guru in this field.

[KD6VXI]

I've read that a 1/2 wave monopole doesn't NEED an extensive ground radial network:  More like .05 to .1 wavelength is about ideal on the voltage fed antenna.

I tried it, and yeah.....  Increasing the radials from a 2 foot (10 meter) to 9 foot made ZILCH in the way of increased signal level on either rx or tx (as would be expected).

Did the same experiment with 1/4 wave and 5/8 wave monopoles (ground planes, actually), and the radial length made a BIG difference.

The problem I ran into was the high feedpoint of the halfwave....  > 1Kohm.  The only way I found to decrease that was to run a piece of quarter wave transmission line made out of aluminum and flared on one end....  The flares actually became the ground 'plane'.


This was all elevated, of course.  On a 36 foot tiltover tower extended fully.


--Shane
KD6VXI