Monopole Configuration and Performance

[k4kyv]

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. 

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.

[DMOD]

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.

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

[R. Fry SWL]

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.

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.)

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.

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

[W0BTU]

...Brown determined that part of the problem was the variable cross-section of this Blaw-Knox tower, which tapered ... RCA published a paper in 1935 showing why uniform cross-section monopoles were preferable for MW broadcast stations.  ...

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

[K5UJ]

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

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

[R. Fry SWL]

I'd like to get a citation for Brown's autobiography, which I did not know existed.

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

[k4kyv]

...Brown determined that part of the problem was the variable cross-section of this Blaw-Knox tower, which tapered ... RCA published a paper in 1935 showing why uniform cross-section monopoles were preferable for MW broadcast stations.  ...

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

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.

[K5UJ]

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

[ke7trp]

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

C

...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.

[R. Fry SWL]

I am chiefly interested in the part of his life pertaining to his work with medium wave broadcast antennas.

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)?

[Jeff W9GY]

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?

[R. Fry SWL]

... 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.  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

[k4kyv]

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.

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.

[K5UJ]

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.

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

[W0BTU]

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.

[R. Fry SWL]

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

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