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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

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. ;D

Fred

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

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.

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.

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.

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

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.

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

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

73,

Kevin, WB4AIO.

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

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

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.

190 degrees.

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.

Thanks Walt,

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

Fred

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

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

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?

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.

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

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

Don - A few comments to your post...


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.


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

RF

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.

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

It's been my experience that if you lay them on top of the ground, they will bury themselves in a year or two, especially if grass is allowed to grow up round them, and (carefully) mowed on a  regular basis.  First they get covered in thatch, then sod develops over the wire.  At least on the ground here, after a couple of years you would have a hard time pulling them up, as I have discovered from leaving scrap wire lying on the ground. But if you are counting on "natural burial" of the wire, each  radial has to be pulled tight and fixed to a stake at each end, and it helps to add "staples" made of stiff wire at intervals along the way wherever the ground is irregular enough for the wire to tend not to lie flat.

Walt, thanks for joining the discussion. I know you will have some significant facts to add.  Regarding the 0.4λ, you are correct.  Maybe the maximum current density is closer to 0.25λ, but as I recall, it's not exactly that figure. 0.3 maybe? I'll have to look it back up.

Right I am familiar with the various charts on height verses angle of radiation. I guess what I was asking is, did Brown or others actually build these antennas and measure resulting field strengths to determine optimum heights? This was before NEC and MoM calculations came into view, of course.

Are you familiar with Valentin Trainotti's skirted half wave grounded vertical monopoles, where he uses a skirt starting from a little less than 0.25 Lambda down to the ground, and then feeds the skirt at about the 0.205 Lambda point for a 50 ohm impedance? He claims an average bandwith of about 30kHz.

Thanks

Phil - AC0OB

Of course George Brown was able to calculate the elevation pattern shape for radiators of various heights  and predict its affect on groundwave signals -- but such measurements were made directly.

In the 1930s, RCA was hired by WCAU in Philadelphia to determine the reason for the large "fading zone" present in their nighttime groundwave signal.  George Brown was the lead engineer in the project for RCA.  WCAU was using a 400 foot Blaw-Knox tower popular in those early days, and the tower had a 100' steel pole attached to the top to extend its physical height to about 5/8 lambda.

The solution was to remove the steel pole from the top, making the radiator closer to 1/2 lambda.  Measurements made up to 75 miles from the tower site before and after the change showed that the fade-free, groundwave nighttime service area had increased by a factor of four.

Brown determined that part of the problem was the variable cross-section of this Blaw-Knox tower, which tapered from a few feet at the base and top to 26 feet at about 190 feet above the ground.  Of course that couldn't be changed readily.

RCA published a paper in 1935 showing why uniform cross-section monopoles were preferable for MW broadcast stations.  After that the use of "Blaw-Knox" type towers became less popular.  However a few are still in use today, such as at WLW and WSM.

(The above information about WCAU was taken from George Brown's autobiography.)


Yes, although it is not difficult to get 30 kHz bandwidth and more in the MW band with a conventional series-fed, base insulated monopole having a suitable height to width ratio.  The most common approach for AM broadcast stations is to supply an adjustable network at the feedpoint of an unskirted, series-fed monopole to match the impedance there to the transmission line back to the transmitter.

RF

That's amazing! It never occurred to me that a non-uniform tower diameter could have an effect like this.

I'd like to get a citation for Brown's autobiography, which I did not know existed.  I think it would be very interesting to read.  If you would provide the title, publisher, date of publication and Brown's full name I'd appreciate it.

Rob

It is "and part of which I was   Recollections of a Research Engineer" by George H. Brown.  It is a 342-page, hard cover book published privately in 1979, with a revised edition in 1982.  His recount of the development of the compatible color television standard that finally was accepted by the FCC in preference to the CBS field-sequential "color wheel" system is especially interesting.

Brown's son, George H. Brown, Jr is the only source for the book that I know of.  When I bought my copy he was living at 117 Hunt Drive, Princeton, NJ 08540.  His telephone number is/was 609 924-3358.  At that time the cost was about $35 per copy, including S&H.

George Brown Jr included a hand-written note to me, as I had asked him if it was OK to forward his contact information if anyone else asked me for it, which permission he gave.

RF

I had always believed that although the effect was measurable, it was negligible.  Walt pointed out some references to the contrary, which I checked out, and it makes a greater difference than I had thought.

From what I have read, the diamond-shaped Blaw-Knox tower was designed with mechanical engineering in mind, contrary to a common misbelief that the tower is made larger at the middle to accommodate the current  loop of a near half-wavelength monopole. Only one set of guys is needed, at the mid-point.  Blaw-Knox did make a few uniform cross-section towers with the same single guy set at the mid point, maybe after the 1935 paper.

One thing that has always puzzled me about that design, called a "cantilever" tower, is that the single guy point near the middle would act as a pivot, and instances of wind-shear where the wind velocity is much higher at the upper reaches of the tower than it is at ground level, would tend to make the tower want to rotate towards the horizontal about the guy point, putting a lot of lateral stress on the base insulator. The ceramic base insulator is designed to withstand downward compression, not lateral force, and I would think the latter would eventually result in failure of the brittle ceramic or the bonding between the ceramic and the metal end castings. This is especially true since those tall towers normally use two base insulators stacked one on top of the other, one inverted and attached to the concrete pier and the other to the base of the tower, the two linked together with a metal pin. But those towers have remained standing since the 1930's, so this is apparently not a problem.

A few years after the WSM tower was erected, about 80' was removed from the tubular mast at the top, to correct a problem of severe fading in Chattanooga, about 120 miles away. That mast was given to a school, which used it as a flag pole until the school building was torn down about 7 years ago.

Back when they had those Nashville tornadoes around 5 to 10 years ago (it was when there was one in or near downtown Nashville) one went right by the WSM tower.  It's supposed to be a bit crooked or slightly twisted in some way as a result, but as far as I know, they never did anything about it, probably because nothing could be done other than replace it which I doubt if they'll ever do until they have to.  Maybe the damage was partly due to the construction design.

R. Fry, thanks for the information on the Brown autobiography.  I did some checking and the addr. and phone number is for the publisher of the book.  I called and got an answering machine message that sounded like a residence.  There is a second hand copy available at ABEBooks.com, but the bookstore with it wants $150 for it, which is a lot more than my interest.  I am chiefly interested in the part of his life pertaining to his work with medium wave broadcast antennas.  I am fascinated by the sophistication of the analytical work at the time which was done with none of the modern computational equipment we have now, and perhaps not as much of the specialized measuring equipment we have today. 

Rob

I wonder if you know Dick Kessler from Harris. He is a mentor of mine.

C

Here is a list of some of the papers of George Brown on this topic.  Most public libraries have access to them.

General Considerations of Tower Antennas for Broadcast Use, Proc IRE, April 1935

Ground Systems as a Factor in Antenna Efficiency, Proc. IRE, 25:753, June, 1937.

Experimentally Determined Impedance Characteristics of Cylindrical Antennas, Proc. IRE, April 1945

A Critical Study of the Characteristics of Broadcast Antennas as Affected by Antenna Current Distribution, Proc. IRE, 24:48, January, 1936.

Directional Antennas, Proc. IRE, 25:78, January, 1937.

Consideration of the Radio Frequency Voltages Encountered by the Insulating Materials of Broadcast Towers, Proc. IRE, 27:566, September, 1939.

Adjusting Unequal Tower Broadcast Arrays, Electronics, Proc. IRE 16:118, December, 1943.

The RCA Antennalyzer - An Instrument Useful in the Design of Antenna Systems. Proc. IRE, 34:992, December, 1946.
//
To ke7trp:  No, I don't know Dick Kessler.  Maybe he was with a Harris group other than the Broadcast Division (Quincy, IL)?

Mr Fry, I notice that at one time you were associated with WJR.  Back in the 70's (I was a design engineer at Delco Radio/Delco Electronics - GM) we used to test AM front end designs at (among other places) near the WJR transmitter in Riverview.  In those days, I noticed that WJR had an auxillary antenna that looked sort of like a top loaded vertical using a long horizontal wire for top loading.  Now I see on Google earth that they are using a newly constructed "short" vertical for an auxillary antenna.  Is that how you remember it?

Yes - that's right.  Below is a scan of a 35mm slide I took of the WJR tx site in late 1964.  The two poles supporting the "tee" aux antenna are visible in back of, and to the right of the building.

The main antenna is a 700 foot, uniform cross-section monopole (195º on 760 kHz).  The tee was replaced by a conventional series-fed monopole some years later, I think within the last 15 years.

The main transmitter in the 1960s was a 50 kW Continental 317C, and the aux transmitter was a 50 kW Western Electric 407A.  Both were replaced later by much more efficient Harris DX-50 solid state transmitters, which use digital techniques to directly generate the AM waveform.  Overall a-c input to r-f output conversion efficiency of the DX-50 is about 85%, including the power needed to cool the transmitter in ambient air temps to 50ºC.

I am always amazed at all the homes that have been built near the WJR site since the 1960s, when that area was open country. The r-f field from WJR at some of those closer houses is over 4 V/m.

RF

The past decade or two I have noticed increasing encroachment of residential development into the immediate vicinity of AM broadcast towers, large and small. Sometimes not enough open space is left even for 90º radials, let alone 0.4 or 0.5λ. At WLW, residential housing has been built on the opposite side of the road, (maybe it's an optical illusion) but those houses appear closer to the base of the tower than the tower is tall.  Interestingly, the road goes round in a slight kink at one point to dodge one of the guy anchors for the famous Blaw-Knox tower.

A small AM station in Nashville has apparently sold or leased most of their radial field and now industrial buildings and an automobile junk yard lie within about 100 ft. of the tower.

I wonder if radials are still intact under the development in these cases.  Maybe the BC station still holds an easement to prohibit nearby property owners from disturbing the wires.  What about excavation for foundations and cellars? Or maybe the station's bean counters are willing to sacrifice field strength for the commercial value of their real estate.  But in that case, I would think the FCC would step in.

For a view of the WLW site, check out Tylersville Road, Mason Ohio on Google Maps, just east of I-75.

I am very amazed when I see a HAM tower anywhere near a 1-A site.   At the 670 site in Glendale Heights here I was non plused to see a short tower and yagi on it no more than a half mile from the tower.  An HF yagi no less.  What are these guys thinking?  I'd like to know what they go through to keep 250 KW at 125% positive out of their rx front ends, and broadcast audio out of their transmissions.   I have a friend a few miles from the 1070 site in Indianapolis, a DA 50 KW day and 5 KW night and he has to employ all kinds of high pass filters etc. to deal with the daytime signal. 

Regarding the development, WSB is in a shopping mall if I remember correctly:  http://www.fybush.com/sites/2005/site-050204.html

Rob

If you think there's a lot of RF near a 50 kW broadcast station, you should have visited the VOA at Bethany, Ohio.

2.5 megawatts ERP from their huge Sterba curtain, pointed directly at a row of houses directly across the road.

The RF field was so high, that you could hear the program standing near the 3' high steel fence right across the road from the houses (in the little arcs in the rusty fence joints.)

And yes, there were families living in those houses.

etc.

Never visited the Bethany, OH VOA site, but in 1961 I did visit the VOA SW site near Delano, CA.

I remember the signs in/around the parking lot next to the tx building there, advising drivers to park with their car bumpers touching the grounded metal plate at the end of each parking space.

That tended to reduce the zap that might be felt by people who were in physical contact both with the body of the car and the earth, when they exited and entered their vehicles.

That advice would not always be useful these days, given the plastic bumpers used in some modern vehicles.

RF